3-1

The en route phase of flight has seen some of the mostdramatic improvements in the way pilots navigatefrom departure to destination. Developments in tech-nology have played a significant role in most of theseimprovements. Computerized avionics and advancednavigation systems are commonplace in both generaland commercial aviation.

The procedures employed in the en route phase of flightare governed by a set of specific flight standards estab-lished by Title 14 of the Code of Federal Regulations(14 CFR), Federal Aviation Administration (FAA)Order 8260.3, United States Standard for TerminalInstrument Procedures (TERPS), and related publica-tions. These standards establish courses to be flown,obstacle clearance criteria, minimum altitudes, naviga-tion performance, and communications requirements.For the purposes of this discussion, the en route phase offlight is defined as that segment of flight from the termi-nation point of a departure procedure to the originationpoint of an arrival procedure.

EN ROUTE NAVIGATIONPart 91.181 is the basis for the course to be flown. Tooperate an aircraft within controlled airspace underinstrument flight rules (IFR), pilots must either flyalong the centerline when on a Federal airway or, onroutes other than Federal airways, along the directcourse between navigational aids or fixes defining theroute. The regulation allows maneuvering to pass wellclear of other air traffic or, if in visual flight rules(VFR) conditions, to clear the flight path both beforeand during climb or descent.

En route IFR navigation is evolving from the groundbased navigational aid (NAVAID) airway system to asophisticated satellite and computer-based system thatcan generate courses to suit the operational require-ments of almost any flight. Although the promise ofthe new navigation systems is immense, the presentsystem of navigation serves a valuable function and isexpected to remain for a number of years.

The procedures pilots employ in the en route phase offlight take place in the structure of the NationalAirspace System (NAS) consisting of three strata. Thefirst, or lower stratum is an airway structure thatextends from the base of controlled airspace up to but

not including 18,000 feet mean sea level (MSL). Thesecond stratum is an area containing identifiable jetroutes as opposed to designated airways, and extendsfrom 18,000 feet MSL to Flight Level (FL) 450. Thethird stratum, above FL 450 is intended for random,point-to-point navigation.

AIR ROUTE TRAFFIC CONTROL CENTERSThe Air Route Traffic Control Center (ARTCC)encompasses the en route air traffic control systemair/ground radio communications, that provides safeand expeditious movement of aircraft operating on IFRwithin the controlled airspace of the Center. ARTCCsprovide the central authority for issuing IFR clearancesand nationwide monitoring of each IFR flight. Thisapplies primarily to the en route phase of flight, andincludes weather information and other inflight serv-ices. There are 20 ARTCCs in the conterminous UnitedStates (U.S.), and each Center contains between 20 to80 sectors, with their size, shape, and altitudes deter-mined by traffic flow, airway structure, and workload.Appropriate radar and communication sites are con-nected to the Centers by microwave links and telephonelines. [Figure 3-1 on page 3-2]

The CFRs require the pilot in command under IFR incontrolled airspace to continuously monitor an appropri-ate Center or control frequency. When climbing aftertakeoff, an IFR flight is either in contact with a radar-equipped local departure control or, in some areas, anARTCC facility. As a flight transitions to the en routephase, pilots typically expect a handoff from departurecontrol to a Center frequency if not already in contactwith the Center. The FAA National AeronauticalCharting Office (NACO) publishes en route chartsdepicting Centers and sector frequencies, as shown inFigure 3-2 on page 3-2. During handoff from one Centerto another, the previous controller assigns a new fre-quency. In cases where flights may be still out of range,the Center frequencies on the face of the chart are veryhelpful. In Figure 3-2 on page 3-2, notice the boundarybetween Memphis and Atlanta Centers, and theremoted sites with discrete very high frequency (VHF)and ultra high frequency (UHF) for communicatingwith the appropriate ARTCC. These Center frequencyboxes can be used for finding the nearest frequencywithin the aircraft range. They also can be used

3-2

for making initial contact with the Center for clearances.The exact location for the Center transmitter is notshown, although the frequency box is placed as close aspossible to the known location.

During the en route phase, as a flight transitions fromone Center facility to the next, a handoff or transfer ofcontrol is required as previously described. The hand-off procedure is similar to the handoff between other

Cleveland Center

Albuquerque Center

Seattle Center

Atlanta Center

Chicago Center

Boston Center

Washington Center (DC)

Denver Center

Fort Worth Center

Houston Center

Indianapolis Center

Jacksonville Center

Kansas City Center

Los Angeles Center

Salt Lake City Center

Miami Center

Memphis Center

Minneapolis Center

New York Center

ZID

ZMP

ZOB

ZBW

ZNYZNY

ZDC

ZAU

ZKC

ZMEZTL

ZJX

ZMA

ZHU

ZFW

ZAB

ZDV

ZLA

ZOA

ZLC

ZSE

Oakland Center

HonoluluCenter

ZHN

AnchorageCenter

ZAN

Figure 3-1. Air Route Traffic Control Centers.

Figure 3-2. ARTCC Centers and Sector Frequencies.

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radar facilities, such as departure or approach control.During the handoff, the controller whose airspace isbeing vacated issues instructions that include the nameof the facility to contact, appropriate frequency, andother pertinent remarks.

Accepting radar vectors from controllers does not relievepilots of their responsibility for safety of flight. Pilotsmust maintain a safe altitude and keep track of their posi-tion, and it is their obligation to question controllers,request an amended clearance, or, in an emergency, devi-ate from their instructions if they believe that the safetyof flight is in doubt. Keeping track of altitude and posi-tion when climbing, and during all other phases of flight,are basic elements of situational awareness. Aircraftequipped with an enhanced ground proximity warningsystem (EGPWS) or terrain awareness and warning sys-tem (TAWS) and traffic alert and collision avoidancesystem (TCAS) help pilots detect and correct unsafe alti-tudes and traffic conflicts. Regardless of equipment,pilots must always maintain situational awarenessregarding their location and the location of traffic in theirvicinity.

PREFERRED IFR ROUTESA system of preferred IFR routes helps pilots, flightcrews, and dispatchers plan a route of flight to mini-mize route changes, and to aid in the efficient, orderlymanagement of air traffic using Federal airways.Preferred IFR routes are designed to serve the needs ofairspace users and to provide for a systematic flow ofair traffic in the major terminal and en route flight envi-ronments. Cooperation by all pilots in filing preferredroutes results in fewer air traffic delays and better effi-ciency for departure, en route, and arrival air trafficservice. [Figure 3-3]

Preferred IFR routes are published in theAirport/Facility Directory for the low and high altitudestratum. If they begin or end with an airway number, itindicates that the airway essentially overlies the airportand flights normally are cleared directly on the airway.Preferred IFR routes beginning or ending with a fixindicate that pilots may be routed to or from these fixesvia a standard instrument departure (SID) route, radarvectors, or a standard terminal arrival route (STAR).Routes for major terminals are listed alphabeticallyunder the name of the departure airport. Where severalairports are in proximity they are listed under the prin-cipal airport and categorized as a metropolitan area;e.g., New York Metro Area. One way preferred IFRroutes are listed numerically showing the segment fixesand the direction and times effective. Where more thanone route is listed, the routes have equal priority foruse. Official location identifiers are used in the routedescription for very high frequency omnidirectionalranges (VORs) and very high frequency omnidirec-tional ranges/tactical air navigation (VORTACs), andintersection names are spelled out. The route is directwhere two NAVAIDs, an intersection and a NAVAID, aNAVAID and a NAVAID radial and distance point, orany navigable combination of these route descriptionsfollow in succession.

SUBSTITUTE EN ROUTE FLIGHT PROCEDURESAir route traffic control centers are responsible for speci-fying essential substitute airway and route segments andfixes for use during VOR/VORTAC shutdowns.Scheduled shutdowns of navigational facilities requireplanning and coordination to ensure an uninterruptedflow of air traffic. A schedule of proposed facility shut-downs within the region is maintained and forwarded asfar in advance as possible to enable the substitute routesto be published. Substitute routes are normally based on

VOR/VORTAC facilities estab-lished and published for use inthe appropriate altitude strata. Inthe case of substitute routes inthe upper airspace stratum, it maybe necessary to establish routesby reference to VOR/VORTACfacilities used in the low altitudesystem. Nondirectional radio bea-con (NDB) facilities may only beused where VOR/VORTAC cov-erage is inadequate and ATCrequirements necessitate useof such NAVAIDs. Whereoperational necessity dictates,navigational aids may be usedbeyond their standard servicevolume (SSV) limits, pro-vided that the routes can begiven adequate frequency pro-tection.Figure 3-3. Preferred IFR Routes.

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The centerline of substitute routes must be containedwithin controlled airspace, although substitute routesfor off-airway routes may not be in controlled air-space. Substitute routes are flight inspected to verifyclearance of controlling obstacles and to check forsatisfactory facility performance. To provide pilotswith necessary lead time, the substitute routes are sub-mitted in advance of the en route chart effective date. Ifthe lead time cannot be provided, the shutdown may bedelayed or a special graphic NOTAM may be consid-ered. Normally, shutdown of facilities scheduled for 28days (half the life of the en route chart) or less will notbe charted. The format for describing substitute routesis from navigational fix to navigational fix. A minimumen route altitude (MEA) and a maximum authorizedaltitude (MAA) is provided for each route segment.Temporary reporting points may be substituted for theout-of-service facility and only those other reportingpoints that are essential for air traffic control. Normally,temporary reporting points over intersections are notnecessary where Center radar coverage exists. A mini-mum reception altitude (MRA) is established for eachtemporary reporting point.

TOWER EN ROUTE CONTROLWithin the NAS it is possible to fly an IFR flight with-out leaving approach control airspace, using tower enroute control (TEC) service. This helps expedite airtraffic and reduces air traffic control and pilot commu-nication requirements. TEC is referred to as “tower enroute,” or “tower-to-tower,” and allows flight beneaththe en route structure. Tower en route control reallo-cates airspace both vertically and geographically toallow flight planning between city pairs while remain-ing with approach control airspace. All users areencouraged to use the TEC route descriptions in theAirport/Facility Directory when filing flight plans. Allpublished TEC routes are designed to avoid en routeairspace, and the majority are within radar coverage.[Figure 3-4]

The graphic depiction of TEC routes is not to be usedfor navigation or for detailed flight planning. Not allcity pairs are depicted. It is intended to show geo-graphic areas connected by tower en route control.Pilots should refer to route descriptions for specificflight planning. The word “DIRECT” appears as theroute when radar vectors are used or no airway exists.Also, this indicates that a SID or STAR may beassigned by ATC. When a NAVAID or intersectionidentifier appears with no airway immediately preced-ing or following the identifier, the routing is understoodto be direct to or from that point unless otherwisecleared by ATC. Routes beginning and ending with anairway indicate that the airway essentially overflies

the airport, or radar vectors will be issued. Wheremore than one route is listed to the same destination,ensure that the correct route for the type of aircraftclassification has been filed. These are denoted afterthe route in the altitude column using J (jet powered),M (turbo props/special, cruise speed 190 knots orgreater), P (non-jet, cruise speed 190 knots orgreater), or Q (non-jet, cruise speed 189 knots or less).Although all airports are not listed under the destina-tion column, IFR flights may be planned to satelliteairports in the proximity of major airports via thesame routing. When filing flight plans, the codedroute identifier, i.e., BURL1, VTUL4, or POML3,may be used in lieu of the route of flight.

AIRWAY AND ROUTE SYSTEMThe present en route system is based on the VHF air-way/route navigation system. Low frequency (LF) andintegrated LF/VHF airways and routes have graduallybeen phased out in the conterminous U.S., with someremaining in Alaska.

MONITORING OF NAVIGATION FACILITIESVOR, VORTAC, and instrument landing system (ILS)facilities, as well as most nondirectional radio bea-cons (NDBs) and marker beacons installed by theFAA, are provided with an internal monitoring fea-ture. Internal monitoring is provided at the facilitythrough the use of equipment that causes a facilityshutdown if performance deteriorates below estab-lished tolerances. A remote status indicator also maybe provided through the use of a signal-samplingreceiver, microwave link, or telephone circuit. OlderFAA NDBs and some non-Federal NDBs do not havethe internal feature and monitoring is accomplishedby manually checking the operation at least once eachhour. FAA facilities such as automated flight servicestations (AFSSs) and ARTCCs/sectors are usually thecontrol point for NAVAID facility status. Pilots canquery the appropriate FAA facility if they have ques-tions in flight regarding NAVAID status, in addition tochecking notices to airmen (NOTAMs) prior to flight,since NAVAIDs and associated monitoring equipmentare continuously changing.

LF AIRWAYS/ROUTESNumerous low frequency airways still exist in Alaska,as depicted in this NACO en route low altitude chartexcerpt near Nome, Alaska. [Figure 3-5] Colored LFeast and west airways G7, G212 (green), and R35 (red),are shown, along with north and south airways B2, B27(blue), and A1 (amber), all based upon the Fort DavisNDB en route NAVAID. The nearby Nome VORTACVHF en route NAVAID is used with victor airwaysV452, V333, V507, V506, and V440.

3-5

Figure 3-4.Tower En Route Control.

Figure 3-5. LF and VHF Airways — Alaska.

3-6

VHF AIRWAYS/ROUTESFigure 3-6 depicts numerous arrowed, single directionjet routes on this excerpt from a NACO en route highaltitude chart, effective at and above 18,000 feet MSLup to and including FL 450. Notice the MAAs of 41,000and 29,000 associated with J24 and J193, respectively.Additionally, note the BAATT, NAGGI, FUMES, andMEYRA area navigation (RNAV) waypoints.Waypoints are discussed in detail later in this chapter.

VHF EN ROUTE OBSTACLE CLEARANCE AREASAll published routes in the NAS are based on specificobstacle clearance criteria. An understanding of enroute obstacle clearance areas helps with situationalawareness and may help avoid controlled flight into ter-rain (CFIT). Obstacle clearance areas for the en routephase of flight are identified as primary, secondary, andturning areas.

The primary and secondary area obstacle clearance cri-teria, airway and route widths, and the ATC separationprocedures for en route segments are a function ofsafety and practicality in flight procedures. These flightprocedures are dependent upon the pilot, the aircraft,and the navigation system being used, resulting in atotal VOR system accuracy factor, along with an asso-ciated probability factor. The pilot/aircraft informationcomponent of these criteria includes pilot ability totrack the radial and the flight track resulting from turnsat various speeds and altitudes under different wind

conditions. The navigation system informationincludes navigation facility radial alignment displace-ment, transmitter monitor tolerance, and receiveraccuracy. All of these factors were considered duringdevelopment of en route criteria. From this analysis,the computations resulted in a total system accuracyof ±4.5° 95 percent of the time and ±6.7° 99 percentof the time. The 4.5° figure became the basis for pri-mary area obstacle clearance criteria, airway androute widths, and the ATC separation procedures. The6.7° value provides secondary obstacle clearance areadimensions. Figure 3-7 depicts the primary and sec-ondary obstacle clearance areas.

PRIMARY AREAThe primary obstacle clearance area has a protectedwidth of 8 nautical miles (NM) with 4 NM on eachside of the centerline. The primary area has widths ofroute protection based upon system accuracy of a±4.5° angle from the NAVAID. These 4.5° linesextend out from the NAVAID and intersect the bound-aries of the primary area at a point approximately 51NM from the NAVAID. Ideally, the 51 NM point iswhere pilots would change over from navigating awayfrom the facility, to navigating toward the next facil-ity, although this ideal is rarely achieved.

If the distance from the NAVAID to the changeoverpoint (COP) is more than 51 NM, the outer boundaryof the primary area extends beyond the 4 NM widthalong the 4.5° line when the COP is at midpoint. This

Figure 3-6. VHF Jet Routes.

3-7

means the primary area, along with its obstacle clear-ance criteria, is extended out into what would havebeen the secondary area. Additional differences in theobstacle clearance area result in the case of the effect ofan offset COP or dogleg segment. For protected enroute areas the minimum obstacle clearance in the pri-mary area, not designated as mountainous under Part95 — IFR altitude is 1,000 feet over the highest obsta-cle. [Figure 3-8]

Mountainous areas for the Eastern and Western U.S.are designated in Part 95, as shown in Figure 3-9 onpage 3-8. Additional mountainous areas are desig-nated for Alaska, Hawaii, and Puerto Rico. With someexceptions, the protected en route area minimumobstacle clearance over terrain and manmade obsta-cles in mountainous areas is 2,000 feet. Obstacleclearance is sometimes reduced to not less than 1,500feet above terrain in the designated mountainous areasof the Eastern U.S., Puerto Rico, and Hawaii, and maybe reduced to not less than 1,700 feet in mountainousareas of the Western U.S. and Alaska. Consideration isgiven to the following points before any altitudes pro-viding less than 2,000 feet of terrain clearance areauthorized:

• Areas characterized by precipitous terrain.

• Weather phenomena peculiar to the area.

• Phenomena conducive to marked pressure differ-entials.

• Type of and distance between navigational facilities.

• Availability of weather services throughout thearea.

• Availability and reliability of altimeter resettingpoints along airways and routes in the area.

Altitudes providing at least 1,000 feet of obstacle clear-ance over towers and/or other manmade obstacles maybe authorized within designated mountainous areas ifthe obstacles are not located on precipitous terrain whereBernoulli Effect is known or suspected to exist.

4.5

4.551

51 4.5

4.5

4 NM

4 NM

4 NM

4 NM

2 NM

2 NM

6.7

6.7

6.7

6.7 51

51

Primary Obstacle Clearance Area

Secondary Obstacle Clearance Area

Figure 3-7. VHF En Route Obstacle Clearance Areas.

1,000 Feet AboveHighest Obstacle

Primary En RouteObstacle Clearance Area

Nonmountainous Area

Figure 3-8. Obstacle Clearance - Primary Area.

3-8

Bernoulli Effect, atmospheric eddies, vortices, waves,and other phenomena that occur in conjunction with dis-turbed airflow associated with the passage of strongwinds over mountains can result in pressure deficienciesmanifested as very steep horizontal pressure gradients.Since downdrafts and turbulence are prevalent underthese conditions, potential hazards may be multiplied.

SECONDARY AREAThe secondary obstacle clearance area extends along aline 2 NM on each side of the primary area. Navigationsystem accuracy in the secondary area has widths ofroute protection of a ±6.7° angle from the NAVAID.These 6.7° lines intersect the outer boundaries of the sec-ondary areas at the same point as primary lines, 51 NMfrom the NAVAID. If the distance from the NAVAID tothe COP is more than 51 NM, the secondary areaextends along the 6.7° line when the COP is at mid-point. In all areas, mountainous and nonmountainous,obstacles that are located in secondary areas are con-sidered as obstacles to air navigation if they extendabove the secondary obstacle clearance plane. Thisplane begins at a point 500 feet above the obstaclesupon which the primary obstacle clearance area isbased, and slants upward at an angle that causes it tointersect the outer edge of the secondary area at a point500 feet higher. [Figure 3-10]

The obstacle clearance areas for LF airways and routesare different than VHF, with the primary and secondaryarea route widths both being 4.34 NM. The accuracylines are 5.0° in the primary obstacle clearance area and7.5° in the secondary area. Obstacle clearance in theprimary area of LF airways and routes is the same asthat required for VHF, although the secondary areaobstacle clearance requirements are based upon dis-tance from the facility and location of the obstaclerelative to the inside boundary of the secondary area.

Figure 3-9. Designated Mountainous Areas.

Puerto RicoMountainous Area

67 6630' 66

20

25

67 6630' 66

30

35

40

45

758595105

115125

45

40

35

30

25

20

125

115

105

9585

75

LEGEND

WASHINGTON

OREGON

MONTANA

IDAHO

NEVADA

WYOMING

UTAH COLORADO

CALIFORNIA

ARIZONA NEW MEXICO

NORTHDAKOTA

SOUTHDAKOTA

NEBRASKA

KANSAS

OKLAHOMA

TEXASLA

MS

FL

GEORGIA

SC

MD

DE

RI

ME

VTNHMA

CTNEW YORK

PA

WV

VIRGINA

MI

IN OHIO

WISCONSIN

MINNESOTA

IOWA

ILLINOIS

MISSOURI

ARKANSAS

KENTUCKY

NJ

NO CAROLINATENNESSEE

AL

(NOT DRAWN TO SCALE)

Mountainous Areas

Nonmountainous Area

MountainousArea

Primary Area

Secondary Area

500 Feet

Total Widthof SecondaryArea

Distance fromObstacle toOuter Edge ofSecondary Area

Figure 3-10. Obstacle Clearance - Secondary Area.

When a VHF airway or route terminates at a NAVAID orfix, the primary area extends beyond that terminationpoint. Figure 3-11 and its inset show the construction ofthe primary and secondary areas at the terminationpoint. When a change of course on VHF airways androutes is necessary, the en route obstacle clearanceturning area extends the primary and secondaryobstacle clearance areas to accommodate the turnradius of the aircraft. Since turns at or after fix pas-sage may exceed airway and route boundaries, pilotsare expected to adhere to airway and route protectedairspace by leading turns early before a fix. Theturn area provides obstacle clearance for both turnanticipation (turning prior to the fix) and flyoverprotection (turning after crossing the fix). This doesnot violate the requirement to fly the centerline of theairway. Many factors enter into the construction andapplication of the turning area to provide pilots withadequate obstacle clearance protection. These mayinclude aircraft speed, the amount of turn versusNAVAID distance, flight track, curve radii, MEAs,and minimum turning altitude (MTA). A typical pro-tected airspace is shown in Figure 3-11. Turning areasystem accuracy factors must be applied to the mostadverse displacement of the NAVAID or fix and theairway or route boundaries at which the turn is made.If applying nonmountainous en route turning area cri-teria graphically, depicting the vertical obstructionclearance in a typical application, the template mightappear as in Figure 3-12 on page 3-10.

Turns that begin at or after fix passage may exceed theprotected en route turning area obstruction clearance.

Figure 3-13 on page 3-10 contains an example of aflight track depicting a turn at or after fix passage,together with an example of an early turn. Withoutleading a turn, an aircraft operating in excess of 290knots true airspeed (TAS) can exceed the normal air-way or route boundaries depending on the amount ofcourse change required, wind direction and velocity,the character of the turn fix (DME, overhead naviga-tion aid, or intersection), and pilot technique in makinga course change. For example, a flight operating at17,000 feet MSL with a TAS of 400 knots, a 25° bank,and a course change of more than 40° would exceed thewidth of the airway or route; i.e., 4 NM each side ofcenterline. Due to the high airspeeds used at 18,000feet MSL and above, additional IFR separation protec-tion for course changes is provided.

NAVAID SERVICE VOLUMEEach class of VHF NAVAID (VOR/DME/TACAN)has an established operational service volume toensure adequate signal coverage and frequency pro-tection from other NAVAIDs on the same frequency.The maximum distance at which NAVAIDs are usablevaries with altitude and the class of the facility. Whenusing VORs for direct route navigation, the followingguidelines apply:

• For operations above FL 450, use aids not morethan 200 NM apart. These are High Altitude (H)class facilities and are depicted on en route highaltitude charts.

Figure 3-11.Turning Area, Intersection Fix, NAVAID Distance less than 51 NM.

RadiiCenter

SecondaryArcs

PrimaryArcs

PrimaryIndexes"Outside"

"Inside"

TerminationAreas

CENTERLINE

CENTERLI

NE

En RouteFacility Facility

Providing IntersectionRadial

FixDisplacement

Area

4.5 3.6

3-9

3-10

• For operations that are off established airwaysfrom 18,000 feet MSL to FL 450, use aids notmore than 260 NM apart. These are High Altitude(H) class facilities and are depicted on en routehigh altitude charts.

• For operations that are off established airwaysbelow 18,000 feet MSL, use aids not more than80 NM apart. These are Low Altitude (L) classfacilities and are shown on en route low altitudecharts.

• For operations that are off established airwaysbetween 14,500 feet MSL and 17,999 feet MSLin the conterminous United States, use H-classfacilities not more than 200 NM apart.

The use of satellite based navigation systems hasincreased pilot requests for direct routes that take theaircraft outside ground based NAVAID service volumelimits. These direct route requests are approved only ina radar environment, and approval is based on pilotresponsibility for staying on the authorized direct route.ATC uses radar flight following for the purpose of air-craft separation. On the other hand, if ATC initiates adirect route that exceeds NAVAID service volume lim-its, ATC also provides radar navigational assistance asnecessary. More information on direct route navigationis located in the En Route RNAV Procedures sectionlater in this chapter.

NAVIGATIONAL GAPSWhere a navigational course guidance gap exists,referred to as an MEA gap, the airway or route segmentmay still be approved for navigation. The navigationalgap may not exceed a specific distance that variesdirectly with altitude, from zero NM at sea level to 65NM at 45,000 feet MSL and not more than one gap mayexist in the airspace structure for the airway or routesegment. Additionally, a gap usually does not occur atany airway or route turning point. To help ensure themaximum amount of continuous positive course guid-ance available when flying, there are established enroute criteria for both straight and turning segments.Where large gaps exist that require altitude changes,MEA “steps” may be established at increments of notless than 2,000 feet below 18,000 feet MSL, or not lessthan 4,000 feet at 18,000 MSL and above, provided thata total gap does not exist for the entire segment withinthe airspace structure. MEA steps are limited to onestep between any two facilities to eliminate continuousor repeated changes of altitude in problem areas. Theallowable navigational gaps pilots can expect to see

Secondary AreaPrimary Area

500 Feet

500 Feet

Figure 3-12.Turning Area Obstruction Clearance.

Airway Route Boundary

Airway Route Boundary

TurningFix

Early Turn

Turn at or afterFix Passage

Figure 3-13. Adhering to Airway/Route Turning Area.

3-11

are determined, in part, by reference to the graphdepicted in Figure 3-14. Notice the en route chartexcerpt depicting that the MEA is established with agap in navigation signal coverage northwest of theCarbon VOR/DME on V134. At the MEA of 13,000,the allowable navigation course guidance gap isapproximately 18.5 NM, as depicted by Sample 2. Thenavigation gap area is not identified on the chart bydistances from the navigation facilities.

CHANGEOVER POINTSWhen flying airways, pilots normally change frequen-cies midway between navigation aids, although thereare times when this is not practical. If the navigationsignals cannot be received from the second VOR at themidpoint of the route, a changeover point (COP) isdepicted and shows the distance in NM to each NAVAID,as depicted in Figure 3-15 on page 3-12. COPs indicatethe point where a frequency change is necessary toreceive course guidance from the facility ahead of theaircraft instead of the one behind. These changeover

points divide an airway or route segment and ensurecontinuous reception of navigation signals at the pre-scribed minimum en route IFR altitude. They alsoensure that other aircraft operating within the same por-tion of an airway or route segment receive consistentazimuth signals from the same navigation facilitiesregardless of the direction of flight.

Where signal coverage from two VORs overlaps at theMEA, the changeover point normally is designated atthe midpoint. Where radio frequency interference orother navigation signal problems exist, the COP isplaced at the optimum location, taking into considera-tion the signal strength, alignment error, or any otherknown condition that affects reception. The changeoverpoint has an effect on the primary and secondary obsta-cle clearance areas. On long airway or route segments,if the distance between two facilities is over 102 NMand the changeover point is placed at the midpoint,the system accuracy lines extend beyond the mini-mum widths of 8 and 12 NM, and a flare or spreading

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90

MEA OF AIRWAY ORROUTE SEGMENT(THOUSANDS OF FEET)

Sample 1: Enter with MEA of 27,000 Feet.

Read Allowable Gap 39 NM

Sample 2: Enter with MEA of 13,000 Feet.

Read Allowable Gap 18.5 NM

ALLOWABLE NAVIGATION COURSE GUIDANCE GAP (NM)

Figure 3-14. Navigational Course Guidance Gaps.

3-12

outward results at the COP, as shown in Figure 3-16.Offset changeover points and dogleg segments on air-ways or routes can also result in a flare at the COP.

IFR EN ROUTE ALTITUDESMinimum en route altitudes, minimum reception alti-tudes, maximum authorized altitudes, minimumobstruction clearance altitudes, minimum crossingaltitudes, and changeover points are established bythe FAA for instrument flight along Federal airways,as well as some off-airway routes. The altitudes areestablished after it has been determined that the nav-igation aids to be used are adequate and so orientedon the airways or routes that signal coverage isacceptable, and that flight can be maintained withinprescribed route widths.

For IFR operations, regulations require that pilots oper-ate their aircraft at or above minimum altitudes. Exceptwhen necessary for takeoff or landing, pilots may notoperate an aircraft under IFR below applicable mini-mum altitudes, or if no applicable minimum altitude isprescribed, in the case of operations over an area desig-nated as mountainous, an altitude of 2,000 feet above

the highest obstacle within a horizontal distance of 4 NMfrom the course to be flown. In any other case, an altitudeof 1,000 feet above the highest obstacle within a horizon-tal distance of 4 NM from the course to be flown must bemaintained as a minimum altitude. If both a MEA and aminimum obstruction clearance altitude (MOCA) areprescribed for a particular route or route segment, pilotsmay operate an aircraft below the MEA down to, but notbelow, the MOCA, only when within 22 NM of the VOR.When climbing to a higher minimum IFR altitude (MIA),pilots must begin climbing immediately after passing thepoint beyond which that minimum altitude applies,except when ground obstructions intervene, the pointbeyond which that higher minimum altitude applies mustbe crossed at or above the applicable minimum crossingaltitude (MCA) for the VOR.

If on an IFR flight plan, but cleared by ATC to maintainVFR conditions on top, pilots may not fly below mini-mum en route IFR altitudes. Minimum altitude rulesare designed to ensure safe vertical separation betweenthe aircraft and the terrain. These minimum altituderules apply to all IFR flights, whether in IFR or VFR

Figure 3-16. Changeover Point Effect on Long Airway or Route Segment.

4 NM

4 NM

2 NM

2 NM

6.7

4.5

4.5

6.7 70

70

Secondary Areas

Primary Area Flare

Flare

Figure 3-15. Changeover Points.

22

45

13,000 ft

13,000V 344

3-13

weather conditions, and whether assigned a specificaltitude or VFR conditions on top.

MINIMUM EN ROUTE ALTITUDEThe minimum enroute altitude (MEA) is the lowestpublished altitude between radio fixes that assuresacceptable navigational signal coverage and meetsobstacle clearance requirements between those fixes.The MEA prescribed for a Federal airway or segment,RNAV low or high route, or other direct route appliesto the entire width of the airway, segment, or routebetween the radio fixes defining the airway, segment,or route. MEAs for routes wholly contained withincontrolled airspace normally provide a buffer above thefloor of controlled airspace consisting of at least 300feet within transition areas and 500 feet within controlareas. MEAs are established based upon obstacle clear-ance over terrain and manmade objects, adequacy ofnavigation facility performance, and communicationsrequirements, although adequate communication at theMEA is not guaranteed.

MINIMUM OBSTRUCTION CLEARANCE ALTITUDEThe minimum obstruction clearance altitude (MOCA)is the lowest published altitude in effect between fixeson VOR airways, off-airway routes, or route segmentsthat meets obstacle clearance requirements for the entireroute segment. This altitude also assures acceptable nav-igational signal coverage only within 22 NM of a VOR.The MOCA seen on the NACO en route chart, may havebeen computed by adding the required obstacle clear-ance (ROC) to the controlling obstacle in the primaryarea or computed by using a TERPS chart if the control-ling obstacle is located in the secondary area. This figureis then rounded to the nearest 100 - foot increment, i.e.,2,049 feet becomes 2,000, and 2,050 feet becomes 2,100feet. An extra 1,000 feet is added in mountainous areas,in most cases. The MOCA is based upon obstacle clear-ance over the terrain or over manmade objects, adequacyof navigation facility performance, and communicationsrequirements.

ATC controllers have an important role in helping pilotsremain clear of obstructions. Controllers are instructedto issue a safety alert if the aircraft is in a position that, intheir judgment, places the pilot in unsafe proximity toterrain, obstructions, or other aircraft. Once pilots informATC of action being taken to resolve the situation, thecontroller may discontinue the issuance of further alerts.A typical terrain/obstruction alert may sound like this:“Low altitude alert. Check your altitude immediately.The MOCA in your area is 12,000.”

MINIMUM VECTORING ALTITUDESMinimum vectoring altitudes (MVAs) are establishedfor use by ATC when radar ATC is exercised. The MVAprovides 1,000 feet of clearance above the highestobstacle in nonmountainous areas and 2,000 feet above

the highest obstacle in designated mountainous areas.Because of the ability to isolate specific obstacles, someMVAs may be lower than MEAs, MOCAs, or otherminimum altitudes depicted on charts for a givenlocation. While being radar vectored, IFR altitudeassignments by ATC are normally at or above theMVA.

Controllers use MVAs only when they are assured anadequate radar return is being received from the air-craft. Charts depicting minimum vectoring altitudesare normally available to controllers but not availableto pilots. Situational awareness is always important,especially when being radar vectored during a climbinto an area with progressively higher MVA sectors,similar to the concept of minimum crossing altitude.Except where diverse vector areas have been estab-lished, when climbing, pilots should not be vectoredinto a sector with a higher MVA unless at or above thenext sector’s MVA. Where lower MVAs are requiredin designated mountainous areas to achieve compati-bility with terminal routes or to permit vectoring to aninstrument approach procedure, 1,000 feet of obstacleclearance may be authorized with the use of AirportSurveillance Radar (ASR). The MVA will provide atleast 300 feet above the floor of controlled airspace.The MVA charts are developed to the maximum radarrange. Sectors provide separation from terrain andobstructions. Each MVA chart has sectors largeenough to accommodate vectoring of aircraft withinthe sector at the MVA. [Figure 3-17 on page 3-14]

MINIMUM RECEPTION ALTITUDEMinimum reception altitudes (MRAs) are deter-mined by FAA flight inspection traversing an entireroute of flight to establish the minimum altitude thenavigation signal can be received for the route andfor off-course NAVAID facilities that determine a fix.When the MRA at the fix is higher than the MEA, anMRA is established for the fix, and is the lowest alti-tude at which an intersection can be determined.

MINIMUM CROSSING ALTITUDEA minimum crossing altitude (MCA) is the lowest alti-tude at certain fixes at which the aircraft must cross whenproceeding in the direction of a higher minimum en routeIFR altitude, as depicted in Figure 3-18 on page 3-14.MCAs are established in all cases where obstacles inter-vene to prevent pilots from maintaining obstacle clearanceduring a normal climb to a higher MEA after passing apoint beyond which the higher MEA applies. The sameprotected en route area vertical obstacle clearance require-ments for the primary and secondary areas are consideredin the determination of the MCA. The standard for deter-mining the MCA is based upon the following climbgradients, and is computed from the flight altitude:

• Sea level through 5,000 feet MSL—150 feet perNM

3-14

Figure 3-17. Example of an Air Route Traffic Control Center MVA Chart.

10,000

RIW 15,800

14,500 12,000

13,700 10,700

11,000

12,400 12,000

15,500

14,200

11,000 13,800

13,300

12,300

CKW

VEL

FBR

RKS

RK

S 325

RK

S 0

03

070

065

243 142 RKS 269

211

RKS 080

270

00

1

065

110

18

0

RK

S 1

60

20

40

60

80

100

120

14,500

00027070

000

22,2, 00

14214

0655

C

000

X

V 3615900

V 3615200

V 361 5900 ESQWID

Figure 3-18. Minimum Crossing Altitude.

3-15

• 5000 feet through 10,000 feet MSL — 120 feetper NM

• 10,000 feet MSL and over — 100 feet per NM

To determine the MCA seen on a NACO en routechart, the distance from the obstacle to the fix is com-puted from the point where the centerline of the enroute course in the direction of flight intersects thefarthest displacement from the fix, as shown in Figure3-19. When a change of altitude is involved with acourse change, course guidance must be provided ifthe change of altitude is more than 1,500 feet and/or ifthe course change is more than 45 degrees, althoughthere is an exception to this rule. In some cases, course

changes of up to 90 degrees may be approved withoutcourse guidance provided that no obstacles penetratethe established MEA requirement of the previous air-way or route segment. Outside U. S. airspace, pilotsmay encounter different flight procedures regardingMCA and transitioning from one MEA to a higherMEA. In this case, pilots are expected to be at the higherMEA crossing the fix, similar to an MCA. Pilots mustthoroughly review flight procedure differences when fly-ing outside U.S. airspace. On NACO en route charts,routes and associated data outside the conterminous U.S.are shown for transitional purposes only and are not partof the high altitude jet route and RNAV route systems.[Figure 3-20]

Figure 3-19. MCA Determination Point.

3200'

700'

2000'

2000'

6 NM

4620' MSL120' per NM Required

Multiply by 6 NM -720 Ft.

Maximum Displacement

MSL

Obstacle Line

Obstruction Height 4620' Required Clearance +2000' MOCA At Obstruction = 6620' Climb Value* - 720'MCA Required = 5900' * Based upon 6 NM @ 120 Feet Per NM

X

MEA 5200'

MCA 5900' E

Figure 3-20. Crossing a Fix to a Higher MEA in Canada.

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MAXIMUM AUTHORIZED ALTITUDEA maximum authorized altitude (MAA) is a publishedaltitude representing the maximum usable altitude orflight level for an airspace structure or route segment. Itis the highest altitude on a Federal airway, jet route,RNAV low or high route, or other direct route for whichan MEA is designated at which adequate reception ofnavigation signals is assured. MAAs represent proce-dural limits determined by technical limitations or otherfactors such as limited airspace or frequency interfer-ence of ground based facilities.

IFR CRUISING ALTITUDE OR FLIGHT LEVELIn controlled airspace, pilots must maintain the altitudeor flight level assigned by ATC, although if the ATCclearance assigns “VFR conditions on-top,” an altitudeor flight level as prescribed by Part 91.159 must bemaintained. In uncontrolled airspace (except while in aholding pattern of 2 minutes or less or while turning) ifoperating an aircraft under IFR in level cruising flight,an appropriate altitude as depicted in the legend ofNACO IFR en route high and low altitude charts mustbe maintained. [Figure 3-21]

When operating on an IFR flight plan below 18,000 feetMSL in accordance with a VFR-on-top clearance, anyVFR cruising altitude appropriate to the direction offlight between the MEA and 18,000 feet MSL may beselected that allows the flight to remain in VFR condi-tions. Any change in altitude must bereported to ATC and pilots must complywith all other IFR reporting procedures.VFR-on-top is not authorized in Class Aairspace. When cruising below 18,000feet MSL, the altimeter must be adjustedto the current setting, as reported by astation within 100 NM of your position.In areas where weather-reporting sta-tions are more than 100 NM from theroute, the altimeter setting of a stationthat is closest may be used. During IFRflight, ATC advises flights periodicallyof the current altimeter setting, but itremains the responsibility of the pilot orflight crew to update altimeter settingsin a timely manner. Altimeter settingsand weather information are availablefrom weather reporting facilities oper-ated or approved by the U.S. NationalWeather Service, or a source approvedby the FAA. Some commercial oper-ators have the authority to act as agovernment-approved source ofweather information, includingaltimeter settings, through certifica-tion under the FAA’s EnhancedWeather Information System.

Flight level operations at or above 18,000 feet MSLrequire the altimeter to be set to 29.92. A flight level(FL) is defined as a level of constant atmospheric pres-sure related to a reference datum of 29.92 in. Hg. Eachflight level is stated in three digits that represent hun-dreds of feet. For example, FL 250 represents analtimeter indication of 25,000 feet. Conflicts withtraffic operating below 18,000 feet MSL may arisewhen actual altimeter settings along the route of flightare lower than 29.92. Therefore, Part 91.121 specifiesthe lowest usable flight levels for a given altimetersetting range.

LOWEST USABLE FLIGHT LEVELWhen the barometric pressure is 31.00 inches of mer-cury or less and pilots are flying below 18,000 feetMSL, use the current reported altimeter setting. This isimportant because the true altitude of an aircraft islower than indicated when sea level pressure is lowerthan standard. When an aircraft is en route on an instru-ment flight plan, air traffic controllers furnish thisinformation at least once while the aircraft is in the con-troller’s area of jurisdiction. According to Part 91.144,when the barometric pressure exceeds 31.00 inchesHg., the following procedures are placed in effect byNOTAM defining the geographic area affected: Set31.00 inches for en route operations below 18,000 feetMSL and maintain this setting until beyond the affectedarea. Air traffic control issues actual altimeter settings

Figure 3-21. IFR Cruising Altitude or Flight Level.

3-17

and advises pilots to set 31.00 inches in their altimeter,for en route operations below 18,000 feet MSL inaffected areas. If an aircraft has the capability of settingthe current altimeter setting and operating into airportswith the capability of measuring the current altimetersetting, no additional restrictions apply. At or above18,000 feet MSL, altimeters should be set to 29.92inches of mercury (standard setting). Additional proce-dures exist beyond the en route phase of flight.

The lowest usable flight level is determined by theatmospheric pressure in the area of operation. As localaltimeter settings fall below 29.92, pilots operating inClass A airspace must cruise at progressively higherindicated altitudes to ensure separation from aircraftoperating in the low altitude structure as follows:

Current Altimeter Setting Lowest Usable Flight Level

• 29.92 or higher 180

• 29.91 to 29.42 185

• 29.41 to 28.92 190

• 28.91 to 28.42 195

• 28.41 to 27.92 200

When the minimum altitude, as prescribed in Parts91.159 and 91.177, is above 18,000 feet MSL, the low-est usable flight level is the flight level equivalent ofthe minimum altitude plus the number of feet specifiedaccording to the lowest flight level correction factor asfollows:

Altimeter Setting Correction Factor

• 29.92 or higher none

• 29.91 to 29.42 500 Feet

• 29.41 to 28.92 1000 Feet

• 28.91 to 28.42 1500 Feet

• 28.41 to 27.92 2000 Feet

• 27.91 to 27.42 2500 Feet

OPERATIONS IN OTHER COUNTRIESWhen flight crews transition from the U.S. NAS toanother country’s airspace, they should be aware of dif-ferences not only in procedures but also airspace. Forexample, when flying into Canada regarding altimetersetting changes, as depicted in Figure 3-22 on page 3-18,notice the change from QNE to QNH when flying north-bound into the Moncton flight information region(FIR), an airspace of defined dimensions where flight

information service and alerting service are provided.Transition altitude (QNH) is the altitude in the vicinityof an airport at or below which the vertical position ofthe aircraft is controlled by reference to altitudes (MSL).The transition level (QNE) is the lowest flight levelavailable for use above the transition altitude. Transitionheight (QFE) is the height in the vicinity of an airport ator below which the vertical position of the aircraft isexpressed in height above the airport reference datum.The transition layer is the airspace between the transi-tion altitude and the transition level. If descendingthrough the transition layer, set the altimeter to local sta-tion pressure. When departing and climbing through thetransition layer, use the standard altimeter setting (QNE)of 29.92 inches of Mercury, 1013.2 millibars, or 1013.2hectopascals. Remember that most pressure altimetersare subject to mechanical, elastic, temperature, andinstallation errors. Extreme cold temperature differencesalso may require a correction factor.

REPORTING PROCEDURESIn addition to acknowledging a handoff to anotherCenter en route controller, there are reports that shouldbe made without a specific request from ATC. Certainreports should be made at all times regardless ofwhether a flight is in radar contact with ATC, whileothers are necessary only if radar contact has been lostor terminated. Refer to Figure 3-23 on page 3-19 for areview of these reports.

NONRADAR POSITION REPORTSIf radar contact has been lost or radar service ter-minated, the CFRs require pilots to provide ATCwith position reports over designated VORs andintersections along their route of flight. Thesecompulsory reporting points are depicted onNACO IFR en route charts by solid triangles.Position reports over fixes indicated by open trian-gles are noncompulsory reporting points, and areonly necessary when requested by ATC. If on adirect course that is not on an established airway,report over the fixes used in the flight plan thatdefine the route, since they automatically becomecompulsory reporting points. Compulsory report-ing points also apply when conducting an IFRflight in accordance with a VFR-on-top clearance.Whether a route is on airways or direct, positionreports are mandatory in a nonradar environment,and they must include specific information. A typicalposition report includes information pertaining to air-craft position, expected route, and estimated timeof arrival (ETA). Time may be stated in minutesonly when no misunderstanding is likely to occur.[Figure 3-24 on page 3-20]

3-18

COMMUNICATION FAILURETwo-way radio communication failure procedures forIFR operations are outlined in Part 91.185. Unless other-wise authorized by ATC, pilots operating under IFR areexpected to comply with this regulation. Expanded pro-cedures for communication failures are found in theAIM. Pilots can use the transponder to alert ATC to aradio communication failure by squawking code 7600.[Figure 3-25 on page 3-20] If only the transmitter isinoperative, listen for ATC instructions on any opera-tional receiver, including the navigation receivers. It ispossible ATC may try to make contact with pilots over aVOR, VORTAC, NDB, or localizer frequency. In additionto monitoring NAVAID receivers, attempt to reestablishcommunications by contacting ATC on a previouslyassigned frequency, calling a FSS or Aeronautical RadioIncorporated (ARINC).

The primary objective of the regulations governing com-munication failures is to preclude extended IFR no-radiooperations within the ATC system since these operations

may adversely affect other users of the airspace. If theradio fails while operating on an IFR clearance, but inVFR conditions, or if encountering VFR conditions atany time after the failure, continue the flight under VFRconditions, if possible, and land as soon as practicable.The requirement to land as soon as practicable shouldnot be construed to mean as soon as possible. Pilotsretain the prerogative of exercising their best judgmentand are not required to land at an unauthorized airport, atan airport unsuitable for the type of aircraft flown, or toland only minutes short of their intended destination.However, if IFR conditions prevail, pilots must complywith procedures designated in the CFRs to ensure air-craft separation.

If pilots must continue their flight under IFR after expe-riencing two-way radio communication failure, theyshould fly one of the following routes:

• The route assigned by ATC in the last clearancereceived.

Figure 3-22. Altimeter Setting Changes.

3-19

• If being radar vectored, the direct route from thepoint of radio failure to the fix, route, or airwayspecified in the radar vector clearance.

• In the absence of an assigned route, the route ATChas advised to expect in a further clearance.

• In the absence of an assigned or expected route,the route filed in the flight plan.

It is also important to fly a specific altitude shouldtwo-way radio communications be lost. The altitudeto fly after a communication failure can be found inPart 91.185 and must be the highest of the followingaltitudes for each route segment flown.

• The altitude or flight level assigned in the lastATC clearance.

• The minimum altitude or flight level for IFRoperations.

• The altitude or flight level ATC has advised toexpect in a further clearance.

In some cases, the assigned or expected altitude maynot be as high as the MEA on the next route segment.In this situation, pilots normally begin a climb to thehigher MEA when they reach the fix where the MEArises. If the fix also has a published minimum cross-ing altitude, they start the climb so they will be at orabove the MCA when reaching the fix. If the nextsucceeding route segment has a lower MEA, descendto the applicable altitude ⎯ either the last assignedaltitude or the altitude expected in a further clearance⎯ when reaching the fix where the MEA decreases.

Figure 3-23. ATC Reporting Procedure Examples.

Leaving one assigned flight altitude or flight level for another

VFR-on-top change in altitude

Leaving any assigned holding fix or point

Missed approach

Unable to climb or descend at least 500 feet per minute

TAS variation from filed speed of 5% or 10 knots, whicheveris greater

Time and altitude or flight level upon reaching a holding fixor clearance limit

Loss of nav/comm capability (required by Part 91.187)

Unforecast weather conditions or other information relatingto the safety of flight (required by Part 91.183)

"Marathon 564, leaving 8,000, climb to 10,000."

"Marathon 564, VFR-on-top, climbing to 10,500."

"Marathon 564, leaving FARGO Intersection."

"Marathon 564, missed approach, request clearance to Chicago."

"Marathon 564, maximum climb rate 400 feet per minute."

"Marathon 564, advises TAS decrease to140 knots."

"Marathon 564, FARGO Intersection at 05, 10,000, holding east."

"Marathon 564, ILS receiver inoperative."

"Marathon 564, experiencing moderate turbulence at 10,000."

Leaving FAF or OM inbound on final approach

Revised ETA of more than three minutes

Position reporting at compulsory reporting points (requiredby Part 91.183)

"Marathon 564, outer marker inbound, leaving 2,000."

"Marathon 564, revising SCURRY estimate to 55."

See Figure 3-24 on page 3-20 for position report items.

RADAR/NONRADAR REPORTS

These reports should be made at all times without a specific ATC request.

NONRADAR REPORTS

When you are not in radar contact, these reports should be made without a specific request from ATC.

REPORTS EXAMPLE:

REPORTS EXAMPLE:

3-20

CLIMBING AND DESCENDING EN ROUTEBefore the days of nationwide radar coverage, en routeaircraft were separated from each other primarily byspecific altitude assignments and position reportingprocedures. Much of the pilot’s time was devoted toinflight calculations, revising ETAs, and relaying

position reports to ATC. Today, pilots and air trafficcontrollers have far more information and better toolsto make inflight computations and, with the expan-sion of radar, including the use of an en route flightprogress strip shown in Figure 3-26, position reportsmay only be necessary as a backup in case of radarfailure or for RNAV random route navigation. Figure3-26 also depicts the numerous en route data entriesused on a flight progress strip, generated by theARTCC computer. Climbing, level flight, anddescending during the en route phase of IFR flightinvolves staying in communication with ATC, mak-ing necessary reports, responding to clearances,

monitoring position, and staying abreast of anychanges to the airplane’s equipment status or weather.

PILOT/CONTROLLER EXPECTATIONSWhen ATC issues a clearance or instruction, pilots areexpected to execute its provisions upon receipt. In somecases, ATC includes words that modify their expectation.For example, the word “immediately” in a clearance orinstruction is used to impress urgency to avoid an immi-nent situation, and expeditious compliance is expectedand necessary for safety. The addition of a climb pointor time restriction, for example, does not authorizepilots to deviate from the route of flight or any otherprovision of the ATC clearance. If you receive a term

Identification

Position

Time

Altitude/Flight Level

IFR or VFR (in a report to an FSS only)

ETA over the next reporting fix

Following reporting point

Pertinent remarks

"Marathon 564,

Sidney

15, (minutes after the hour)

9,000,

IFR,

Akron 35, (minutes after the hour)

Thurman next."

(If necessary)

Figure 3-24. Nonradar Position Reports.

Figure 3-25.Two-Way Radio Communication FailureTransponder Code.

When an aircraft squawks code 7600 during a two-way radio communication failure, the information block on the radar screen flashes RDOF (radio failure) to alert the controller.

3-21

“climb at pilot’s discretion” in the altitude informationof an ATC clearance, it means that you have the optionto start a climb when you wish, that you are authorizedto climb at any rate, and to temporarily level off at anyintermediate altitude as desired, although once youvacate an altitude, you may not return to that altitude.

When ATC has not used the term “at pilot’s discretion”nor imposed any climb restrictions, you should climbpromptly on acknowledgment of the clearance. Climbat an optimum rate consistent with the operating char-acteristics of your aircraft to 1,000 feet below theassigned altitude, and then attempt to climb at a rate of

Figure 3-26. En Route Flight Progress Strip and Data Entries.

3-22

between 500 and 1,500 feet per minute until you reachyour assigned altitude. If at anytime you are unable toclimb at a rate of at least 500 feet a minute, advise ATC.If it is necessary to level off at an intermediate altitudeduring climb, advise ATC.

“Expedite climb” normally indicates you should usethe approximate best rate of climb without an excep-tional change in aircraft handling characteristics.Normally controllers will inform you of the reason foran instruction to expedite. If you fly a turbojet airplaneequipped with afterburner engines, such as a militaryaircraft, you should advise ATC prior to takeoff if youintend to use afterburning during your climb to the enroute altitude. Often, the controller may be able to plantraffic to accommodate a high performance climb andallow you to climb to the planned altitude withoutrestriction. If you receive an “expedite” clearance fromATC, and your altitude to maintain is subsequentlychanged or restated without an expedite instruction, theexpedite instruction is canceled.

During en route climb, as in any other phase offlight, it is essential that you clearly communicatewith ATC regarding clearances. In the followingexample, a flight crew experienced an apparentclearance readback/hearback error, that resulted inconfusion about the clearance, and ultimately, toinadequate separation from another aircraft.“Departing IFR, clearance was to maintain 5,000feet, expect 12,000 in ten minutes. After handoff toCenter, we understood and read back, ‘Leaving5,000 turn left heading 240° for vector on course.’The First Officer turned to the assigned headingclimbing through 5,000 feet. At 5,300 feet Centeradvised assigned altitude was 5,000 feet. We imme-diately descended to 5,000. Center then informed uswe had traffic at 12 o’clock and a mile at 6,000. Afterpassing traffic, a higher altitude was assigned andclimb resumed. We now believe the clearance wasprobably ‘reaching’ 5,000, etc. Even our readback to thecontroller with ‘leaving’ didn’t catch the different word-ing.” “Reaching” and “leaving” are commonly used ATCterms having different usages. They may be used inclearances involving climbs, descents, turns, or speedchanges. In the cockpit, the words “reaching” and “leav-ing” sound much alike.

For altitude awareness during climb, professional pilotsoften call out altitudes on the flight deck. The pilotmonitoring may call 2,000 and 1,000 feet prior toreaching an assigned altitude. The callout may be, “twoto go” and “one to go.” Climbing through the transi-tion altitude (QNH), both pilots set their altimeters to29.92 inches of mercury and announce “2992 inches”(or ‘standard,’ on some airplanes) and the flight level

passing. For example, “2992 inches” (‘standard’),flight level one eight zero.” The second officer on threepilot crews may ensure that both pilots have insertedthe proper altimeter setting. On international flights,pilots must be prepared to differentiate, if necessary,between barometric pressure equivalents with inchesof mercury, and millibars or hectopascals, to elimi-nate any potential for error, for example, 996 millibarserroneously being set as 2996.

For a typical IFR flight, the majority of inflight timeoften is flown in level flight at cruising altitude, from topof climb to top of descent (TOD). Generally, TOD isused in airplanes with a flight management system(FMS), and represents the point at which descent is firstinitiated from cruise altitude. FMSs also assist in levelflight by cruising at the most fuel saving speed, provid-ing continuing guidance along the flight plan route,including great circle direct routes, and continuous eval-uation and prediction of fuel consumption along withchanging clearance data. Descent planning is discussedin more detail in the next chapter, “Arrivals.”

AIRCRAFT SPEED AND ALTITUDEDuring the en route descent phase of flight, an addi-tional benefit of flight management systems is that theFMS provides fuel saving idle thrust descent to yourdestination airport. This allows an uninterrupted profiledescent from level cruising altitude to an appropri-ate minimum IFR altitude (MIA), except wherelevel flight is required for speed adjustment.Controllers anticipate and plan that you may level off at10,000 feet MSL on descent to comply with the Part 91indicated airspeed limit of 250 knots. Leveling off atany other time on descent may seriously affect air traf-fic handling by ATC. It is imperative that you makeevery effort to fulfill ATC expected actions on descentto aid in safely handling and expediting air traffic.

ATC issues speed adjustments if you are being radarcontrolled to achieve or maintain required or desiredspacing. They express speed adjustments in terms ofknots based on indicated airspeed in 10 knot incre-ments except that at or above FL 240 speeds may beexpressed in terms of Mach numbers in 0.01 incre-ments. The use of Mach numbers by ATC is restrictedto turbojets. If complying with speed adjustments,pilots are expected to maintain that speed within plusor minus 10 knots or 0.02 Mach.

Speed and altitude restrictions in clearances are subjectto misinterpretation, as evidenced in this case where acorporate flight crew treated instructions in a publishedprocedure as a clearance. “…We were at FL 310 andhad already programmed the ‘expect-crossing altitude’

3-23

of 17,000 feet at the VOR. When the altitude alertersounded, I advised Center that we were leaving FL 310.ATC acknowledged with a ‘Roger.’ At FL 270, Centerquizzed us about our descent. I told the controller wewere descending so as to cross the VOR at 17,000 feet.ATC advised us that we did not have clearance todescend. What we thought was a clearance was in factan ‘expect’ clearance. We are both experiencedpilots…which just means that experience is no substi-tute for a direct question to Center when you are indoubt about a clearance. Also, the term ‘Roger’ onlymeans that ATC received the transmission, not that theyunderstood the transmission. The AIM indicates that‘expect’ altitudes are published for planning purposes.‘Expect’ altitudes are not considered crossing restric-tions until verbally issued by ATC.”

HOLDING PROCEDURESThe criteria for holding pattern airspace is developedboth to provide separation of aircraft, as well as obstacleclearance The alignment of holding patterns typicallycoincides with the flight course you fly after leaving theholding fix. For level holding, a minimum of 1,000 feetobstacle clearance is provided throughout the primaryarea. In the secondary area 500 feet of obstacle clearanceis provided at the inner edge, tapering to zero feet at the

outer edge. Allowance for precipitous terrain is consid-ered, and the altitudes selected for obstacle clearancemay be rounded to the nearest 100 feet. When criteria fora climb in hold are applied, no obstacle penetrates theholding surface. [Figure 3-27]

There are many factors that affect aircraft during hold-ing maneuvers, including navigational aid ground andairborne tolerance, effect of wind, flight procedures,application of air traffic control, outbound leg length,maximum holding airspeeds, fix to NAVAID distance,DME slant range effect, holding airspace size, andaltitude holding levels. In order to allow for these fac-tors when establishing holding patterns, procedurespecialists must apply complex criteria contained inOrder 7130.3, Holding Pattern Criteria.

ATC HOLDING INSTRUCTIONSWhen controllers anticipate a delay at a clearance limitor fix, pilots will usually be issued a holding clearanceat least five minutes before the ETA at the clearancelimit or fix. If the holding pattern assigned by ATC isdepicted on the appropriate aeronautical chart, pilotsare expected to hold as published. In this situation, thecontroller will issue a holding clearance which includesthe name of the fix, directs you to hold as published,

Figure 3-27.Typical Holding Pattern Design Criteria Template.

Fix Displacement Area

Facility

Facility

Secondary Area

Primary Area Holding Pattern Airspace Area

3-24

and includes an expect further clearance (EFC) time.An example of such a clearance is: “Marathon fivesixty four, hold east of MIKEY Intersection as pub-lished, expect further clearance at 1521.” When ATCissues a clearance requiring you to hold at a fix where aholding pattern is not charted, you will be issued com-plete holding instructions. This information includesthe direction from the fix, name of the fix, course, leglength, if appropriate, direction of turns (if left turnsare required), and the EFC time. You are required tomaintain your last assigned altitude unless a new alti-tude is specifically included in the holding clearance,and you should fly right turns unless left turns areassigned. Note that all holding instructions shouldinclude an EFC time. If you lose two-way radio com-munication, the EFC allows you to depart the holdingfix at a definite time. Plan the last lap of your holdingpattern to leave the fix as close as possible to the exacttime. [Figure 3-28]

If you are approaching your clearance limit and havenot received holding instructions from ATC, you areexpected to follow certain procedures. First, call ATCand request further clearance before you reach the fix.If you cannot obtain further clearance, you are expectedto hold at the fix in compliance with the publishedholding pattern. If a holding pattern is not charted at

the fix, you are expected to hold on the inbound courseusing right turns. This procedure ensures that ATC willprovide adequate separation. [Figure 3-29] Assume youare eastbound on V214 and the Cherrelyn VORTAC isyour clearance limit. If you have not been able to obtainfurther clearance and have not received holding instruc-tions, you should plan to hold southwest on the 221degrees radial using left-hand turns, as depicted. If thisholding pattern was not charted, you would hold westof the VOR on V214 using right-hand turns.

Where required for aircraft separation, ATC mayrequest that you hold at any designated reporting pointin a standard holding pattern at the MEA or the MRA,whichever altitude is the higher at locations where aminimum holding altitude has not been established.Unplanned holding at en route fixes may be expectedon airway or route radials, bearings, or courses. If thefix is a facility, unplanned holding could be on anyradial or bearing. There may be holding limitationsrequired if standard holding cannot be accomplished atthe MEA or MRA.

MAXIMUM HOLDING SPEEDAs you have seen, the size of the holding pattern isdirectly proportional to the speed of the airplane. Inorder to limit the amount of airspace that must be pro-tected by ATC, maximum holding speeds in knots

Figure 3-28. ATC Holding Instructions.

A clearance for an uncharted holding pattern contains additional information:

There are at least three items in a clearance for a charted holding pattern:

• Direction to hold from the holding fix

• Holding fix

• Expect further clearance time

"...Hold southeast

of PINNE Intersection as published.

Expect further clearance at 1645."

• Direction to hold from holding fix

• Holding fix

• The holding course (a specified radial, magnetic bearing, airway or route number)

• The outbound leg length in minutes or nautical miles when DME is used

• Nonstandard pattern, if used

• Expect further clearance time

"...Hold west

of Horst Intersection

on Victor 8

5 mile legs

left turns

expect further clearance at 1430."

3-25

indicated airspeed (KIAS) have been designated forspecific altitude ranges [Figure 3-30]. Even so, someholding patterns may have additional speed restrictionsto keep faster airplanes from flying out of the protectedarea. If a holding pattern has a nonstandard speedrestriction, it will be depicted by an icon with the limit-ing airspeed. If the holding speed limit is less than youfeel is necessary, you should advise ATC of yourrevised holding speed. Also, if your indicated airspeedexceeds the applicable maximum holding speed, ATCexpects you to slow to the speed limit within three min-utes of your ETA at the holding fix. Often pilots canavoid flying a holding pattern, or reduce the length oftime spent in the holding pattern, by slowing down onthe way to the holding fix.

HIGH PERFORMANCEHOLDINGCertain limitations come intoplay when you operate at higherspeeds; for instance, aircraft donot make standard rate turns inholding patterns if the bankangle will exceed 30 degrees. Ifyour aircraft is using a flightdirector system, the bank angleis limited to 25 degrees. Sinceany aircraft must be traveling atover 210 knots TAS for the bankangle in a standard rate turn toexceed 30 degrees, this limitapplies to relatively fast air-planes. An aircraft using a flightdirector would have to be hold-ing at more than 170 knots TASto come up against the 25degrees limit. These true air-speeds correspond to indicatedairspeeds of about 183 and 156knots, respectively, at 6,000feet in a standard atmosphere[Figure 3-31 on page 3-26].Since some military airplanes

need to hold at higher speeds than thecivilian limits, the maximum at militaryairfields is higher. For example, themaximum holding airspeed at USAFairfields is 310 KIAS.

FUEL STATE AWARENESSIn order to increase fuel state awareness,commercial operators and other profes-sional flight crews are required to recordthe time and fuel remaining during IFRflight. For example, on a flight scheduledfor one hour or less, the flight crew mayrecord the time and fuel remaining at thetop of climb (TOC) and at one additionalwaypoint listed in the flight plan.

Generally, TOC is used in airplanes with a flight man-agement system, and represents the point at whichcruise altitude is first reached. TOC is calculated basedon current airplane altitude, climb speed, and cruisealtitude. The captain may elect to delete the additionalwaypoint recording requirement if the flight is so shortthat the record will not assist in the management of theflight. For flights scheduled for more than one hour, theflight crew may record the time and fuel remainingshortly after the top of climb and at selected waypointslisted in the flight plan, conveniently spaced approxi-mately one hour apart. The flight crew comparesactual fuel burn to planned fuel burn. Each fuel tankmust be monitored to verify proper burn off andappropriate fuel remaining. On two pilot airplanes,

CHERRELYND

( H )117.2 CHL

V214

331°

269°

221°

126°

Figure 3-29. Clearance Limit Holding.

Figure 3-30. Maximum Holding Speed Examples.

Maximum Holding Airspeed: 200 KIAS

14,000'MSL

6,000'MSL

Maximum Holding Airspeed: 265 KIAS

Maximum Holding Airspeed: 230 KIAS

MinimumHoldingAltitude(MHA)

6,001'MSL

14,001'MSL

3-26

the pilot monitoring (PM) keeps the flight plan record.On three pilot airplanes, the second officer and PMcoordinate recording and keeping the flight planrecord. In all cases, the pilot making the recordingcommunicates the information to the pilot flying.

DIVERSION PROCEDURESOperations Specifications (OpsSpecs) for commercialoperators include provisions for en route emergencydiversion airport requirements. Operators are expectedto develop a sufficient set of emergency diversion air-ports, such that one or more can be reasonablyexpected to be available in varying weather condi-tions. The flight must be able to make a safe landing,and the airplane maneuvered off of the runway at theselected diversion airport. In the event of a disabledairplane following landing, the capability to move thedisabled airplane must exist so as not to block theoperation of any recovery airplane. In addition, thoseairports designated for use must be capable of protect-ing the safety of all personnel by being able to:

• Offload the passengers and flight crew in a safemanner during possible adverse weather condi-tions.

• Provide for the physiological needs of the pas-sengers and flight crew for the duration until safeevacuation.

• Be able to safely extract passengers and flightcrew as soon as possible. Execution and comple-tion of the recovery is expected within 12 to 48hours following diversion.

Part 91 operators also need to be prepared for a diver-sion. Designation of an alternate in the IFR flightplan is a good first step; although, changing weatherconditions or equipment issues may require pilots toconsider other options.

EN ROUTE RNAV PROCEDURESRNAV is a method of navigation that permits aircraftoperations on any desired course within the coverage

of station-referenced signals, or within the limits ofself-contained system capability. The continued growthin aviation creates increasing demands on airspacecapacity and emphasizes the need for optimum utiliza-tion of available airspace. These factors, allied with therequirement for NAS operational efficiency, along withthe enhanced accuracy of current navigation systems,resulted in the required navigation performance (RNP)concept. RNAV is incorporated into RNP requirements.

OFF AIRWAY ROUTESPart 95 prescribes altitudes governing the operation ofyour aircraft under IFR on Federal airways, jet routes,RNAV low or high altitude routes, and other directroutes for which an MEA is designated in this regula-tion. In addition, it designates mountainous areas andchangeover points. Off-airway routes are establishedin the same manner, and in accordance with the samecriteria as airways and jet routes. If you fly for a sched-uled air carrier or operator for compensation or hire,any requests for the establishment of off-airway routesare initiated by your company through your principaloperations inspector (POI) who works directly withyour company and coordinates FAA approval. Air car-rier authorized routes are contained in the company’sOpsSpecs under the auspices of the air carrier operat-ing certificate. [Figure 3-32]

Off-airway routes predicated on public navigationfacilities and wholly contained within controlled air-space are published as direct Part 95 routes. Off-airwayroutes predicated on privately owned navigation facili-ties or not contained wholly within controlled airspaceare published as off-airway non-Part 95 routes. In eval-uating the adequacy of off-airway routes, the followingitems are considered; the type of aircraft and naviga-tion systems used; proximity to military bases, trainingareas, low level military routes; and the adequacy ofcommunications along the route. If you are a commer-cial operator, and you plan to fly off-airway routes,your OpsSpecs will likely address en route limitationsand provisions regarding en route authorizations to usethe global positioning system (GPS) or other RNAVsystems in the NAS. Your POI must ensure that yourlong-range navigation program incorporates therequired practices and procedures. These proceduresmust be in your manuals and in checklists, as appro-priate. Training on the use of long range navigationequipment and procedures must be included in yourtraining curriculums, and your minimum equipmentlists (MELs) and maintenance programs must addressthe long range navigation equipment. Examples ofother selected areas requiring specialized en routeauthorization include the following:

Figure 3-31. High Performance Holding.

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• Class I navigation in the U.S. Class A airspaceusing area or long range navigation systems.

• Class II navigation using multiple long rangenavigation systems.

• Operations in central East Pacific airspace.

• North Pacific operations.

• Operations within North Atlantic (NAT) mini-mum navigation performance specifications(MNPS) airspace.

• Operations in areas of magnetic unreliability.

• North Atlantic operation (NAT/OPS) with twoengine airplanes under Part 121.

• Extended range operations (ER-OPS) with twoengine airplanes under Part 121.

• Special fuel reserves in international operations.

• Planned inflight redispatch or rerelease en route.

• Extended over water operations using a singlelong-range communication system.

• Operations in reduced vertical separation mini-mum (RVSM) airspace.

DIRECT FLIGHTSThere are a number of ways to create shorter routes andfly off the airways. You can use NACO low and highaltitude en route charts to create routes for directflights, although many of the charts do not share thesame scale as the adjacent chart, so a straight line isvirtually impossible to use as a direct route for longdistances. Generally speaking, NACO charts are plot-ted accurately enough to draw a direct route that canbe flown. A straight line drawn on a NACO en routechart can be used to determine if a direct route willavoid airspace such as Class B airspace, restricted

areas, prohibited areas, etc. BecauseNACO en route charts use theLambert Conformal Conic projec-tion, a straight line is as close aspossible to a geodesic line (betterthan a great circle route). The closerthat your route is to the two stan-dard parallels of 33 degrees and 45degrees on the chart, the better yourstraight line. There are cautions,however. Placing our round earth ona flat piece of paper causes distor-tions, particularly on long east-westroutes. If your route is 180 degreesor 360 degrees, there is virtually nodistortion in the course line.

About the only way you can confi-dently avoid protected airspace is by the use of sometype of airborne database, including a graphic displayof the airspace on the long-range navigation systemmoving map, for example. When not using an airbornedatabase, leaving a few miles as a buffer helps ensurethat you stay away from protected airspace.

In Figure 3-33 on page 3-28, a straight line on a mag-netic course from SCRAN intersection of 270 degreesdirect to the Fort Smith Regional Airport in Arkansaswill pass just north of restricted area R-2401A and B,and R-2402. Since the airport and the restricted areasare precisely plotted, there is an assurance that you willstay north of the restricted areas. From a practicalstandpoint, it might be better to fly direct to the WizerNDB. This route goes even further north of therestricted areas and places you over the final approachfix to Runway 25 at Fort Smith.

One of the most common means for you to fly directroutes is to use conventional navigation such as VORs.When flying direct off-airway routes, remember toapply the FAA distance limitations, based uponNAVAID service volume.

RANDOM RNAV ROUTESRandom RNAV routes may be an integral solution inmeeting the worldwide demand for increased air traf-fic system capacity and safety. Random RNAV routesare direct routes, based on RNAV capability. They aretypically flown between waypoints defined in terms oflatitude and longitude coordinates, degree and distancefixes, or offsets from established routes and airways ata specified distance and direction. Radar monitoring byATC is required on all random RNAV routes.

With IFR certified RNAV units (GPS or FMS), there areseveral questions to be answered, including “Should Ifly airways or should I fly RNAV direct?” One of theconsiderations is the determination of the MIA. In most

Note 3 - Only B-747 and DC-10 operations authorized in these areas.

AUTHORIZED AREAS OF EN ROUTE OPERATION

LIMITATIONS, PROVISIONS, AND REFERENCE PARAGRAPHS

The 48 contiguous United States and the District of Columbia

Note 1

Canada, excluding Canadian MNPS airspace and the areas of magnetic unreliability as established in the Canadian AIP

Note 3

SPECIAL REQUIREMENTS:

Note 1 - B-737 Class II navigation operations with a single long-range system is authorized only within this area of en route operation.

oteote 3

au

3

hthorize- B-7

zed-73

d o37 C

QU

C

UIR

ass

EMEENME

e

NNotote 33

RNSGR

S, AAPPH

Figure 3-32. Excerpt of Authorized Areas of En Route Operation.

3-28

places in the world at FL 180 and above, the MIA is notsignificant since you are well above any terrain or obsta-cles. On the other hand, a direct route at 18,000 feet fromSalt Lake City, Utah to Denver, Colorado, means terrainand obstacles are very important. This RNAV directroute across the Rocky Mountains reduces your distanceby about 17 NM, but radar coverage over the Rockies atlower altitudes is pretty spotty. This raises numerousquestions. What will air traffic control allow on directflights? What will they do if radar coverage is lost? Whataltitudes will they allow when they can’t see you onradar? Do they have altitudes for direct routes? The easyanswer is to file the airways, and then all the airwayMIAs become usable. But with RNAV equipment, adirect route is more efficient. Even though on someroutes the mileage difference may be negligible, thereare many other cases where the difference in distance issignificant. ATC is required to provide radar separationon random RNAV routes at FL 450 and below. It is logi-cal to assume that ATC will clear you at an altitude thatallows it to maintain radar contact along the entire route,which could mean spending additional time and fuelclimbing to an altitude that gives full radar coverage.

All air route traffic control centers have MIAs for their areasof coverage. Although these altitudes are not publishedanywhere, they are available when airborne from ATC.

OFF ROUTE OBSTRUCTIONCLEARANCE ALTITUDEAn off-route obstruction clearance altitude(OROCA) is an off-route altitude that providesobstruction clearance with a 1,000-foot buffer in non-mountainous terrain areas and a 2,000-foot buffer indesignated mountainous areas within the U.S. Thisaltitude may not provide signal coverage fromground-based navigational aids, air traffic controlradar, or communications coverage. OROCAs areintended primarily as a pilot tool for emergencies andsituational awareness. OROCAs depicted on NACOen route charts do not provide you with an acceptablealtitude for terrain and obstruction clearance for thepurposes of off-route, random RNAV direct flights ineither controlled or uncontrolled airspace. OROCAsare not subject to the same scrutiny as MEAs, MVAs,MOCAs, and other minimum IFR altitudes. Sincethey do not undergo the same obstruction evaluation,

Figure 3-33. Direct Route Navigation.

3-29

airport airspace analysis procedures, or flight inspec-tion, they cannot provide the same level of confidenceas the other minimum IFR altitudes.

If you depart an airport VFR intending to or needing toobtain an IFR clearance en route, you must be aware ofthe position of your aircraft relative to terrain andobstructions. When accepting a clearance below theMEA, MIA, MVA, or the OROCA, you are responsi-ble for your own terrain/obstruction clearance untilreaching the MEA, MIA, or MVA. If you are unableto visually maintain terrain/obstruction clearance, youshould advise the controller and state your intentions.[Figure 3-34]

For all random RNAV flights, there needs to be at leastone waypoint in each ARTCC area through which youintend to fly. One of the biggest problems in creatingan RNAV direct route is determining if the route goesthrough special use airspace. For most direct routes,the chances of going through prohibited, restricted,

or special use airspace are good. In the U.S., all directroutes should be planned to avoid prohibited orrestricted airspace by at least 3 NM. If a bend in adirect route is required to avoid special use airspace,the turning point needs to be part of the flight plan.Two of the most prominent long range navigationsystems today include FMS with integrated GPSand stand-alone GPS. The following example is asimplified overview showing how the RNAV systemsmight be used to fly a random RNAV route:

In Figure 3-35 on page 3-30, you are northeast ofTuba City VORTAC at FL 200 using RNAV (showingboth GPS and FMS), RNAV direct on a southwesterlyheading to Lindbergh Regional Airport in Winslow.As you monitor your position and cross check youravionics against the high altitude en route chart, youreceive a company message instructing you to divertto Las Vegas, requiring a change in your flight plan ashighlighted on the depicted chart excerpt.

Figure 3-34. Off-Route Obstacle Clearance Altitude.

3-30

During your cockpit review of the high and low altitudeen route charts, you determine that your best course ofaction is to fly direct to the MIRAJ waypoint, 28 DMEnortheast of the Las Vegas VORTAC on the 045° radial.This places you 193 NM out on a 259° magnetic courseinbound, and may help you avoid diverting north,

allowing you to bypass the more distant originatingand intermediate fixes feeding into Las Vegas. Yourequest an RNAV random route clearance directMIRAJ to expedite your flight. Denver Center comesback with the following amended flight plan and initialclearance into Las Vegas:

Figure 3-35. Random RNAV Route.

3-31

“Marathon five sixty four, turn right heading two sixzero, descend and maintain one six thousand, clearedpresent position direct MIRAJ.”

The latitude and longitude coordinates of your presentposition on the high altitude chart are N36 19.10, andW110 40.24 as you change your course. Notice yourGPS moving map (upper left) and the FMS controldisplay unit (below the GPS), and FMS map modenavigation displays (to the right of the GPS) as youreroute your flight to Las Vegas. For situationalawareness, you note that your altitude is well aboveany of the OROCAs on your direct route as you arrivein the Las Vegas area using the low altitude chart.

PUBLISHED RNAV ROUTESAlthough RNAV systems allow you to select any num-ber of routes that may or may not be published on achart, en route charts are still crucial and required forRNAV flight. They assist you with both flight planningand inflight navigation. NACO en route charts are veryhelpful in the context of your RNAV flights. PublishedRNAV routes are fixed, permanent routes that can beflight planned and flown by aircraft with RNAV capa-bility. These are being expanded worldwide as newRNAV routes are developed, and existing charted,conventional routes are being designated for RNAVuse. It is important to be alert to the rapidly changingapplication of RNAV techniques being applied to con-ventional en route airways. Published RNAV routes maypotentially be found on any NACO en route chart. Thepublished RNAV route designation may be obvious, or,on the other hand, RNAV route designations may beless obvious, as in the case where a published routeshares a common flight track with a conventional air-way. Note: Since the use of RNAV is dynamic andrapidly changing, NACO en route charts are continu-ously being updated for information changes and youmay find some differences between charts.

According to the International Civil AviationOrganization (ICAO), who develops standard princi-ples and techniques for international air navigation,basic designators for air traffic service (ATS) routesand their use in voice communications have been estab-lished in Annex 11. ATS is a generic ICAO term forflight information service, alerting service, air trafficadvisory service, and air traffic control service. One ofthe main purposes of a system of route designators is toallow both pilots and ATC to make unambiguous refer-ence to RNAV airways and routes. Many countries haveadopted ICAO recommendations with regard to ATSroute designations. Basic designators for ATS routesconsist of a maximum of five, and in no case exceedsix, alpha/numeric characters in order to be usable byboth ground and airborne automation systems. The des-ignator indicates the type of the route such as high/low

altitude, specific airborne navigation equipmentrequirements such as RNAV, and the aircraft type usingthe route primarily and exclusively. The basic routedesignator consists of one or two letter(s) followed by anumber from 1 to 999.

COMPOSITION OF DESIGNATORSThe prefix letters that pertain specifically to RNAV des-ignations are included in the following list:

1. The basic designator consists of one letter of thealphabet followed by a number from 1 to 999.The letters may be:

a) A, B, G, R — for routes that form part ofthe regional networks of ATS routes and arenot RNAV routes;

b) L, M, N, P — for RNAV routes that formpart of the regional networks of ATS routes;

c) H, J, V, W — for routes that do not formpart of the regional networks of ATS routesand are not RNAV routes;

d) Q, T, Y, Z — for RNAV routes that do notform part of the regional networks of ATSroutes.

2. Where applicable, one supplementary letter mustbe added as a prefix to the basic designator asfollows:

a) K — to indicate a low level route estab-lished for use primarily by helicopters.

b) U — to indicate that the route or portionthereof is established in the upper airspace;

c) S — to indicate a route established exclu-sively for use by supersonic airplanesduring acceleration/deceleration andwhile in supersonic flight.

3. Where applicable, a supplementary letter may beadded after the basic designator of the ATS routeas a suffix as follows:

a) F — to indicate that on the route or portionthereof advisory service only is provided;

b) G — to indicate that on the route or portionthereof flight information service only isprovided;

c) Y — for RNP 1 routes at and above FL 200to indicate that all turns on the routebetween 30° and 90° must be made withinthe tolerance of a tangential arc between thestraight leg segments defined with a radiusof 22.5 NM.

3-32

d) Z — for RNP 1 routes at and below FL 190to indicate that all turns on the routebetween 30° and 90° shall be made withinthe tolerance of a tangential arc between thestraight leg segments defined with a radiusof 15 NM.

USE OF DESIGNATORS IN COMMUNICATIONSIn voice communications, the basic letter of a designa-tor should be spoken in accordance with the ICAOspelling alphabet. Where the prefixes K, U or S, speci-fied in 2., above, are used in voice communications,they should be pronounced as:

K = “Kopter” U = “Upper” S = “Supersonic”

as in the English language.

Where suffixes “F”, “G”, “Y” or “Z” specified in 3.,above, are used, the flight crew should not be requiredto use them in voice communications.

Example:

A11 will be spoken Alfa Eleven

UR5 will be spoken Upper Romeo Five

KB34 will be spoken Kopter Bravo Thirty Four

UW456 F will be spoken Upper Whiskey Four Fifty Six

Figure 3-36 depicts published RNAV routes in theGulf of Mexico (black Q100, Q102, and Q105) thathave been added to straighten out the flight segmentsand provide an alternative method of navigation to theLF airway (brown G26), that has since been termi-nated in this case. The “Q” designation is derived fromthe list of basic route designators previously covered,and correlates with the description for RNAV routesthat do not form part of the regional networks of ATSroutes. Notice the indirect reference to the RNAVrequirement, with the note, “Navigational EquipmentOther than LF or VHF Required.”

Notice in Figure 3-37 that this en route chartexcerpt depicts three published RNAV jet routes,J804R, J888R, and J996R. The “R” suffix is a sup-plementary route designator denoting an RNAVroute. The overlapping symbols for the AMOTTintersection and waypoint indicate that AMOTTcan be identified by conventional navigation or bylatitude and longitude coordinates. Although coordi-nates were originally included for aircraft equippedwith INS systems, they are now a good way to crosscheck between the coordinates on the chart and in theFMS or GPS databases to ensure you are tracking onyour intended en route course. The AMOTT RNAVwaypoint includes bearing and distance from theANCHORAGE VORTAC. In an effort to simplifythe conversion to RNAV, some controlling agencies

Figure 3-36. Published RNAV Routes Replacing LF Airways.

3-33

outside the U.S. have simply designated all conven-tional routes as RNAV routes at a certain flightlevel.

RNAV MINIMUM EN ROUTE ALTITUDERNAV MEAs are depicted on some NACO IFR enroute charts, allowing both RNAV and non-RNAVpilots to use the same chart for instrument navi-gation.

MINIMUM IFR ALTITUDEThe Minimum IFR altitude (MIA) for operations isprescribed in Part 91. These MIAs are published on

NACO charts and prescribed in Part 95 for airways androutes, and in Part 97 for standard instrument approachprocedures. If no applicable minimum altitude is pre-scribed in Parts 95 or 97, the following MIA applies: Indesignated mountainous areas, 2,000 feet above thehighest obstacle within a horizontal distance of 4 NMfrom the course to be flown; or other than mountainousareas, 1,000 feet above the highest obstacle within ahorizontal distance of 4 NM from the course to beflown; or as otherwise authorized by the Administratoror assigned by ATC. MIAs are not flight checked forcommunication.

Figure 3-37. Published RNAV Jet Routes.

3-34

WAYPOINTSWaypoints are specified geographical locations, orfixes, used to define an RNAV route or the flight pathof an aircraft employing RNAV. Waypoints may be anyof the following types: predefined, published way-points, floating waypoints, or user-defined waypoints.Predefined, published waypoints are defined relative toVOR-DME or VORTAC stations or, as with GPS, interms of latitude/longitude coordinates.

USER-DEFINED WAYPOINTSPilots typically create user-defined waypoints for usein their own random RNAV direct navigation. They arenewly established, unpublished airspace fixes that aredesignated geographic locations/positions that helpprovide positive course guidance for navigation and ameans of checking progress on a flight. They may ormay not be actually plotted by the pilot on en routecharts, but would normally be communicated to ATC interms of bearing and distance or latitude/longitude. Anexample of user-defined waypoints typically includesthose derived from database RNAV systems wherebylatitude/longitude coordinate-based waypoints are gen-

erated by various means including keyboard input, andeven electronic map mode functions used to establishwaypoints with a cursor on the display. Another exampleis an offset phantom waypoint, which is a point-in-spaceformed by a bearing and distance from NAVAIDs, such asVORTACs and tactical air navigation (TACAN) stations,using a variety of navigation systems. When specifyingunpublished waypoints in a flight plan, they can be com-municated using the frequency/bearing/distance format orlatitude and longitude, and they automatically becomecompulsory reporting points unless otherwise advised byATC. All airplanes with latitude and longitude navigationsystems flying above FL 390 must use latitude andlongitude to define turning points.

FLOATING WAYPOINTSFloating waypoints, or reporting points, represent air-space fixes at a point in space not directly associatedwith a conventional airway. In many cases, they may beestablished for such purposes as ATC metering fixes,holding points, RNAV-direct routing, gateway way-points, STAR origination points leaving the en routestructure, and SID terminating points joining the en

Figure 3-38. Floating Waypoints.

3-35

route structure. In Figure 3-38, in the top example, aNACO low altitude en route chart depicts three floatingwaypoints that have been highlighted, SCORR, FILUP,and CHOOT. Notice that waypoints are named withfive-letter identifiers that are unique and pronouncable.Pilots must be careful of similar waypoint names.Notice on the high altitude en route chart excerpt in thebottom example, the similar sounding and spelledfloating waypoint named SCOOR, rather thanSCORR. This emphasizes the importance of cor-rectly entering waypoints into database-drivennavigation systems. One waypoint characterincorrectly entered into your navigation systemcould adversely affect your flight. The SCOORfloating reporting point also is depicted on aSevere Weather Avoidance Plan (SWAP) en routechart. These waypoints and SWAP routes assistpilots and controllers when severe weather affectsthe East Coast.

COMPUTER NAVIGATION FIXESAn integral part of RNAV using en route chartstypically involves the use of airborne navigationdatabases. Database identifiers are depicted onNACO en route charts enclosed in parentheses, forexample AWIZO waypoint, shown in Figure 3-39.These identifiers, sometimes referred to as computernavigation fixes (CNFs), have no ATC function andshould not be used in filing flight plans nor shouldthey be used when communicating with ATC.Database identifiers on en route charts are shown

only to enable you to maintain orientation as you usecharts in conjunction with database navigation sys-tems, including RNAV.

Many of the RNAV systems available today make itall too easy to forget that en route charts are stillrequired and necessary for flight. As important asdatabases are, they really are onboard the airplane toprovide navigation guidance and situational aware-ness; they are not intended as a substitute for papercharts. When flying with GPS, FMS, or planning aflight with a computer, it is important to understandthe limitations of the system you are using, for exam-ple, incomplete information, uncodeable procedures,complex procedures, and database storage limitations.For more information on databases, refer to AppendixA, Airborne Navigation Database.

HIGH ALTITUDE AIRSPACE REDESIGNHistorically in the U.S., IFR flights have navigatedalong a system of Federal Airways that require pilots tofly directly toward or away from ground-based naviga-tion aids. RNAV gives users the capability to fly directroutes between any two points, offering far more flexi-ble and efficient en route operations in the high-altitudeairspace environment. As part of the ongoing NationalAirspace Redesign (NAR), the FAA has implementedthe High Altitude Redesign (HAR) program with thegoal of obtaining maximum system efficiency by intro-ducing advanced RNAV routes for suitably equippedaircraft to use.

Figure 3-39. Computer Navigation Fix.

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Q-ROUTESNaturally, the routes between some points are verypopular, so these paths are given route designators andpublished on charts. The U.S. and Canada use "Q" asa designator for RNAV routes. Q-Routes 1 through499 are allocated to the U.S., while Canada is allo-cated Q-Routes numbered from 500 through 999. Thefirst Q-Routes were published in 2003. One benefit ofthis system is that aircraft with RNAV or RNP capa-bility can fly safely along closely spaced parallelflight paths on high-density routes, which eases air-space congestion. While the initial overall HARimplementation will be at FL390 and above, some ofthe features may be used at lower altitudes, and someQ-Routes may be used as low as FL180. A Q-Route isshown in figure 3-40.

NON-RESTRICTIVE ROUTINGHAR also includes provisions for pilots to choose theirown routes, unconstrained by either conventional air-ways or Q-Routes. This non-restrictive routing (NRR)allows pilots of RNAV-equipped aircraft to plan themost advantageous route for the flight. There are twoways to designate an NRR route on your flight plan.One method, point-to-point (PTP), uses the traditionalfixes in the aircraft equipment database and is shownby placing “PTP” in the first part of the “Remarks”block of the flight plan. Foraircraft that have the addi-tional waypoints of theNavigation Reference System(NRS) in their databases,“HAR” is placed in the firstpart of the “Remarks” block.

NAVIGATION REFERENCESYSTEMThe NRS is a grid of way-points overlying the U.S. thatwill be the basis for flight planfiling and operations in theredesigned high altitude envi-ronment. It will provideincreased flexibility to aircraftoperators and controllers. TheNRS supports flight planningin a NRR environment andprovides ATC with the abilityto more efficiently managetactical route changes for air-craft separation, traffic flowmanagement, and weatheravoidance. It provides naviga-tion reference waypoints thatpilots can use in requestingroute deviations aroundweather areas, which willimprove common understand-

ing between pilots and ATC of the desired flight path.The NRS will initially include waypoints every 30 min-utes of latitude and every two degrees of longitude. In itsfinal version, the NRS waypoints will have a grid resolu-tion of 1-degree longitude by 10 minutes of latitude. Asdatabase capabilities for the preponderance of aircraftoperating in the high altitude airspace environmentbecomes adequate to support more dense NRS resolu-tion, additional NRS waypoints will be established.

T-ROUTEST-Routes are being created for those who operate atlower altitudes. T-Routes have characteristics that aresimilar to Q-Routes, but they are depicted on low alti-tude en route charts and are intended for flights belowFL180. The first T-Routes are being pioneered inAlaska.

IFR TRANSITION ROUTESIn order to expedite the handling of IFR overflighttraffic through Charlotte Approach Control Airspace,several RNAV routes are published in theAirport/Facility Directory and available for you whenfiling your flight plan. Any RNAV capable aircraft fil-ing flight plan equipment codes of /E, /F, or /G mayfile for these routes. Other aircraft may request vectorsalong these routes but should only expect vector routes

Figure 3-40. Q-Route

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as workload permits. Altitudes are assigned by ATCbased upon traffic. [Figure 3-41]

IFR transition routes through Class B airspace for gen-eral aviation aircraft en route to distant destinations arehighly desirable. Since general aviation aircraft cruiseat altitudes below the ceiling of most Class B airspaceareas, access to that airspace for en route transitionreduces cost and time, and is helpful to pilots in theirflight planning. Establishing RNAV fixes could facili-tate the implementation of IFR transition routes,although every effort should be made to design routesthat can be flown with RNAV or VOR equipment. IFRtransition routes are beneficial even if access is notavailable at certain times because of arriving ordeparting traffic saturation atthe primary airport. For theselocations, information can bepublished to advise pilots whenIFR transition access is notavailable.

REQUIRED NAVIGATIONPERFORMANCE As RNAV systems grow insophistication, high technologyFMS and GPS avionics aregaining popularity as NDBs,VORs, and LORAN are beingphased out. As a result, new pro-cedures are being introduced,including RNP, RVSM, andminimum navigation perform-ance specifications (MNPS).ICAO defines an RNP “X” spec-ification as requiring on-boardperformance monitoring andalerting. Even such terms asgross navigation errors(GNEs) are being introducedinto the navigation equation. Ifyou commit a GNE in theNorth Atlantic oceanic regionof more than 25 NM laterallyor 300 feet vertically, it has adetrimental effect on the over-all targeted level of safety ofthe ATC airspace system inthis region. This applies tocommercial operators, as wellas Part 91 operators, all ofwhom must be knowledgeableon procedures for operationsin North Atlantic airspace,contained in the North AtlanticMNPS Operations Manual.

RNP types are identified by a single accuracy value.For example, RNP 1 refers to a required navigationperformance accuracy within 1 NM of the desiredflight path at least 95 percent of the flying time.Countries around the world are establishing requirednavigation performance values. For Federal Airwaysin the U.S. that extend 4 NM from either side of theairway centerline, the airway has an equivalent RNPof 2. Figure 3-42 on page 3-38 shows ICAO RNP con-tainment parameters, including reference to lateraland longitudinal total system errors (TSEs).

RNP requires you to learn new procedures, communi-cations, and limitations; and to learn new terminologythat defines and describes navigation concepts. One of

Figure 3-41. IFR Transition Routes in the Airport/Facility Directory.

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these terms is RNP Airspace, a generic term designat-ing airspace, routes, legs, operations, or procedureswhere minimum RNP has been established. P-RNAVrepresents a 95 percent containment value of ±1 NM.B-RNAV provides a 95 percent containment value of±5 NM. RNP is a function of RNAV equipment thatcalculates, displays, and provides lateral guidanceto a profile or path. Estimated position error (EPE)is a measure of your current estimated navigationalperformance, also referred to as actual navigationperformance (ANP).

RNP RNAV is an industry-expanded specificationbeyond ICAO-defined RNP. Some of the benefits ofRNP RNAV includes being an aid in both separationand collision risk assessment. RNP RNAV can furtherreduce route separation. Figure 3-43 depicts route sep-aration, that can now be reduced to four times the RNPvalue, which further increases route capacity within thesame airspace. The containment limit quantifies thenavigation performance where the probability of anunannunciated deviation greater than 2 x RNP is lessthan 1 x 10-5. This means that the pilot will be alertedwhen the TSE can be greater than the containmentlimit. Figure 3-44 shows the U.S. RNP RNAV levels byairspace control regions, including RNP 2 for the en

route phase of flight, and Figure 3-45 on page 3-40illustrates the U.S. standard RNP (95%) levels.

REDUCED VERTICAL SEPARATION MINIMUMS In 1960, the minimum vertical separation between airplanesabove FL 290 was officially increased to 2,000 feet. Thiswas necessary because of the relatively large errors in baro-metric altimeters at high altitudes. Since that time, increasedair traffic worldwide has begun to approach (and sometimesexceed) the capacity of the most popular high-altituderoutes. Likewise, very accurate altitude determination bysatellite positioning systems makes it possible to change theminimum vertical separation for properly equipped air-planes back to the pre-1960 standard of 1,000 feet. [Figure3-46 on page 3-41] RVSM airspace is any airspace betweenFL 290 and FL 410 inclusive, where airplanes are separatedby 1,000 feet vertically. In the early 1980’s, programswere established to study the concept of reduced verti-cal separation minimums (RVSM). RVSM was foundto be technically feasible without imposing unreason-able requirements on equipment. RVSM is the mosteffective way to increase airspace capacity to cope withtraffic growth. Most of the preferred international anddomestic flight routes are under both RVSM and RNPRNAV rules.

{{ { {

Distance to Waypoint

TrueAircraftPosition

Lateral TSE= RNP Type

Lateral TSE= RNP Type

Longitudinal TSE= RNP Type

Longitudinal TSE= RNP Type

Nav SystemIndicatedPosition

DesiredFlight Path

Inside the box95% of TotalFlight Time

Figure 3-42. ICAO RNP Containment Parameters.

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2 X RNPRNP: 95%

RNP

Containment Limit

Containment Limit: 99.999%

Defined Path

Desired Path

RNP RNAV is referenced to the airplane defined path.ICAO RNP is referenced to the airspace desired path.

Figure 3-43. RNP RNAV Containment.

U.S. RNP RNAVLEVELS

ADS-B RNP 1

RNP 2RNP 0.3

RNP 0.3

Airport Surface

Final Approach/Initial Departure

Approach/Departure Transition

Arrival/Departure

En Route

Figure 3-44. Airspace Control Regions.

3-40

In 1997, the first RVSM 1,000-foot separation wasimplemented between FL 330 and FL 370 over theNorth Atlantic. In 1998, RVSM was expanded toinclude altitudes from FL 310 to FL 390. Today States(governments) around the globe are implementingRVSM from FL 290 to FL 410. There are manyrequirements for operator approval of RVSM. Eachaircraft must be in compliance with specific RVSMcriteria. A program must be in place to assure contin-ued airworthiness of all RVSM critical systems. Flightcrews, dispatchers, and flight operations must be

properly trained, and operational procedures, check-lists, etc. must be established and published in the OpsManual and AFM, plus operators must participate in aheight monitoring program.

Using the appropriate suffix in Block 3 on the IFRflight plan lets ATC know that your flight conforms tothe necessary standards and is capable of using RNProutes or flying in RVSM airspace. The equipmentcodes changed significantly in 2005 and are shown inFigure 3-47.

Figure 3-45. U.S. Standard RNP Levels.

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FL 330

FL 310

FL 290

FL 280

FL 270

2000 ft

1000 ft

Figure 3-46. Prior to implementation of RVSM, all traffic above FL290 required vertical seraration of 2,000 feet.

Figure 3-47. When filed in your IFR flight plan, these codes inform ATC about your aircraft navigation capability.

No DME DME TACAN only Area Navigation (RNAV) LORAN, VOR/DME, or INS

/D

/B

/A

/M

/N

/P

/Y

/C

/ I

/X

/T

/U

/E

/F

/G

/R

RVSM

/J

/K

/L

/Q

/W

No transponder

Transponder without Mode C

Transponder with Mode C

Advanced RNAV with transponder and Mode C(If an aircraft is unable to operate with a transponder

and/or Mode C, it will revert to the appropriate code

listed above under Area Navigation.) With RVSM

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