Automatic DF

8/17/2019 Automatic DF

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Autom atic Direction Finder (AD F)

Know ledge o[AT)F procedures oFfers several advantages to the instrument pilot, although rruiy seldom useAD F equipnnt because of the relatively simpler operation and interpretation of V I ]F equipment. AD Fprovides: (a) a backup navigation system in the event of VHF cquiprncnt failure; (b) a moans of monitoringposition enroule and providing data for plotting Hxes: (c) a naigatiori system for use in areas and at altitudeswhere VOR "line-of..sight" signals are unreliable; (d) radio coinmuniations (receiver only) on the groundwhere VIIF rcccption is imp ossible. Weather broadcasts and clearances can he received, for example, atpoints outside VHF signal range; and (e) auxiliary and staidliy navigation inIhrn'ition on instrumentapproaches.

Selection of Station. An older type A L)I' receiver is shown in Figure 8-21 , and has been described earlier onpages 124 and 1 21 The receiver illustrated can he tuncd to any station between 190-17 5 0 kHz Included inthis range are "H" Iàcilities on Rnroute low Altitude CfurLs,

Figure 8-21. AM -' receiver

Tuning. Tuning details vary with the type A DF equipm ent installed in the aircraft. The tminnlàcturer'sbrochure provided with the particular receiver will include the necessary operating information. The modemcrystal-controlled digitally-tuned. ADF previously discussed on page 125 eliminates Yn ainual tuning. The oldertype AD F (Fig. 8-21) is tuned to a stations as Ilillows:

1. Adjust the frequencyband selector to the desired hand .2. Rotate thekolwix knob clockwise to approxiimtely hall'the range.3. Select "RE C" po sition on the function switch. This selects the sense antenna for tuning the station foruse

of the receiver as a nondirectional radio receiver. Tuning with the tirnction switch on 'ADF" (automatic)position results in unnecessary hunting ofthe \DF needle as various station signals are received

4. Rotate the frequency selector to the desired frequency. [[a tuning mo ler is installed with the receiver, illsessential to tune for maximu m tuning m eter deflection. Without a tuning m eter, adjust the frequency selectorfor maxim um signal clarity and identify the station.

5. Select the ADF position on the Func tion switch. The A DF n eedle will rotate until it points to the station.Note the bearing indicated on the ADF dial and push the"test button. This will rotate the needle clockwiseuntil the test button is released. If the station is tuned properly and the signal is reliable, the needle will returnto the bearing p 'reiously no ted.

6. Adjust the voluar to the desired level, Volum e adfustmon t has no eflct on operation of the ADF needle.

ADF Orientation. Unlike the \'OR receiver, which indicates magnetic hearing10 or 1R0 M the station


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without reference to aircraft heading, the A DF n eedle pointsTO the station, regardless of airctaf heading orposition. The relative bearing indicated is thus the angular relationship betw een the aircraft heading and thestation, m easured clockw ise from the no se of the a'ircraflA beann g 15 simply the direction oFastraight line between the aircraftand station, or vice-versa. The bearing

line niasurcd clockw ise froin the nose of the aircraft is a relative bearing; m easured clockw ise from truenorth, it is a true bearing; measured clockwise from magnetic north, it is a magnetic hearing (Fig. -22 and

Fig, 8-23). As the illustrations show , a true, mag netic,or compass heading i5 Tneasured clockwise W orn theappropriate north, and a relative bearing is measured clockwisefrom the nose of the aircraft. Thus, the true,magnetic,or compass b earing to the stationis the sum of true, magnetic,or compass heading,respectively,and the relative bearing-

You will probably orient yourself more readily ifyou think in ternis of no se/tail arxl left/right nee dleindications, visualizingthe ADF dial in terms of the longitudinal axis ofthe aircraft.W hen the needle points toOI,'/ the nose of the aircraft points directly to the station; with the pointer on 2 1OT, /2, the station is 30' 2 L othe lelt oI'the tail; with thepointer on 09 OI,Y 2. the station if'olT' the right wing tip (Fig. 8-24). Th us, toturndirectly toward station A, turn left 15 Q ,/2 since the needle pointsto the left of the nose/taft line 3Oi,Y2fromthe tail position. Station B is9O1/2 to the right; therefore turn right 9(i / to head directly toward th e station.

Figure 8-22. ADF bearings.Tve N. rh

M ~ne If Nwth

nh

afr

H

True bearing to station H rei, bearing.

Mactic bearing to station = M[ I reL bearing.Stationto aircraft bearingsare true, magneticor compass bcarins I /2 I Oi4.

Figure 8-23. ADF bearing computations.


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MagnegicNorth

Magnetic bearing to sialion = nignetic heading (320i,Y2) + rel. bearing (32Oi,1 2) = 64OIV2 or 280I ,1/2

(wiicncvcr the total is greater than 36OI½, subtract 360 from the bearing).

Figure 8-24. Al)F relative bearings.

C

0

Figure 8 2 5. Determining magnetic bearing to station with ADF.


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- •- -i-MH=230 °\

_3G

MB= I 5ii-I-MHz4O

A N 20

. O -A I —: 2O

TA Nq

F.- =•2cI-t • M 1 • l

Ma= 035 1

Note that (a) the relative bearm g show n on the A DF d ial does not, by iLseli indicate aireralt position, and (b)the relative bearing m ust be related to aircraft heading to dcterrninc direction to or from the station (Fig. 8-25).

Figure 8-25 show s only one oF several methods o I'deterrnining bearings - or lines oI'position - betweenaircraft and station. Visualizing the 80I,¼ left-of-nose indication at all three po sitions sho wn , the nigncticbearing to station can h e determined by subtracting the left deflection from the magn etic heading:

ARC

Magnetic heading.........23Q, /2 11 5 40I, /2

M imic left deflection.... 8O IL'/2 8OI 80I , /

M agnetic be aring ......... 1 50 I,'/2 03 5 I /2 26OI, /

I lonhing. ADF homing is flying the aircraft on any heading required to keep the azimuth needle on0 I 1 / 2untilthe selected station has been reached (Fig -26) To head the aircraft toward the station, turn in the heading

that will zero ihe ADF needle. The heading indicator,rather than the AD F, shouki be used to imtke the turn.At the com pletion of the initial turn toward the station, check w ith the AD F needleand, if necessary, zero itwith small corrections

For example, Figure 8-26 shos an initial magn etic heading oiO5O I/2 and a relative bearingoi3O2I1/2. Aleft turn of approximately60i,1/2sliuld zero the needle, heading 3 5OIj,V2. After the needle is zeroed, it willremain so unless the heading is chang ed or crossw ind affects the aircraft track. If there is no wind, the aircraftwill Rillow a straight track to the station, assuming constant heading. TFa crosswind drills the aircra1t thehoming track will be a curve as you keep the ADF needle zeroed.

Figure 8-26. ADF horning.


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Approach to the station is indicated by increasingly frequent heading corrections to zero the needle,especially when a strong crossw ind exists, and by sidc-to-side needle dcllcctions vcry close to the station.Passage directly over the station is shown by a 1g1/2 reversal ofthe AD F need le to the tail position;passage on either side arid close to the station is shown by a rapid sw ing of the needle a s it continues10 pointto the station Horning is easy, though seldom used during instnuncnt flying Competent pilots control track bymore precise procedures.


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Tracking. A straight geographic flight path can be followed to orfrom a lows' (or trdium) frequency facityby establishing a heading that w ill maintain the desired track regardlessof wind effect. iDI' trackingprocedu res involve interpretation olthe heading indicatorand a,ftnutli needle to intercept and ho ld a desiredmagnetic beating.

Inbound Tracking (Fig. 8-27). To track inbound, turn to the heading that will zero the A tM" needle. A s youhold this heading, deflection ofthe A[)F ne edle to left or right shows a cross wind (nee dle lelIwind horn left;

needle right wind ftorn right). When a definite change in azimuth21/2 - 5j, /2) shows that the aircraft hasdrifted of course, turn in the direction of needle deflection (into the w ind) to rcintercept the initial inboundbearing. ITie angle of interception must always he greater than the num ber of degrees of drifL. The magnitudeof'any intercepting turn depends upon the observed RATP oF'hearthg change, true airspeed, and bow ciiicklyyou w ant to return to course.

A rapid rate of bearing change, while heading is constant, indicates either a strong crosswind or proximity tothe station, or both. For exam ple, iiyou are 60 miles CroT n the station with a degree left deflection, yourairerali is 3 miles to the right of the desired course. In a slow aircraft, use a large interception ang le for quickreturn to the course. in a very fast aircraft, the same interception angle could result in overshooting the desiredcourse- Likewise, the sam e 3I'/ needle deflection closer to the station m eans less deviation from the desiredcourse, and srnaUer angles olinterception result in rapid return to course. Again, when aircraft is 60 milesfrom the station, a rapid rate ofbearing change indicates a strong crosswind; at l'iaWthat distance, the samerate of bearing change m eans twice the crosswind cflèct, or conversely, at half the distance, the same w inde fléct results in double the rate o l'beanng change.

At a given angle of interception and w ith a given wind, rate of closure w ith the desired track varies directlywith true airspeed. At150 knots i'AS as com pared w ith 100 knots i'AS , the efkctivcness of agiDI1inLerception angle is propo rtionately greaterfor the same windat the same distance lom the station. Hav ing

detemiined the ang le of interception tr return to the d esired track, turn toward the track, by that amou nt. Asyou make the turn with the heading indicator, the azimuth needle rotates opposite the direction of turn. and asthe interception angle is established, the needle points to the side orthc ,rro position opposite the direction ofturn. As you app roach the course on a constant interception heading, the AD F needle continues to rotate asthe relative bearing changes. When the needle deflection from zero eqiials the angle ofiriterception. theaircraft is on the desired track If you begin the turn to the m agnetic hearing ofthe desired track when theseangles are equal, you will] overshoot the track, in such a case, you can either drift back to track and thenestablish an estimated drift correction angle, or bracket the track with successively smaller interception angles

Figure 8-27. AD F tracking-inbound.

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1. Turn the aircraft to zero the azimu th needle. M aintain this heading until off-course drift is indicated by left orright needle dcflcction.2. W hen a 5Ij, '/2 change in needle deflection is observed turn 2O Y 2 in the direction oluieedle deflection.

3. W hen the needle is deflected 2 Oi/2 (deflection= interception angle), track has been intercepted. Lead theinterception as noted in d iscussion of tracking. Turn IO I 1 /2toward the inbound course. You are now inboundwith a 1 O I/, left drill correction ang le.4, l]'you observe uIlcourse deflection in the original direction, turn ag ain to the o riginal interception heading.5. When the desired course has been rcirrtcrceptcd, turn 5I,1/2 toward the inbound course, proceeding

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inbound with a 15i,/2 drift correction.

6. If the initial IO I 1 /2drift correction is excessive, as shown by needle deflection away from the w ind, turn toparallel the desired course and let the wind drift you hack on course- When ihe needle is again crocd, turn

iao the wniii with a reduced driti correction angle.

A quicker technique is to lead the turn to the inbound heading before the track is intercepted. The amount of

lead depends upon the distance from station, rate of closure observed as you approach the desired track,

num ber of'degrees lo be turned, an d rate ol'lurri. Since these lhctors are variable, you w ill develop elTëcLivelead estimates as vou bcconx familiar with particular aircraft and practice ADF tracking

After you are back on track, holding an estimated correction for wind d rift, you rem ain on the desired trackas long as the aiirnulh needle is dellecled ftom zero op posite the direction o idniT. correction, an aniount equalto tl r drift correction angle. Ifthe needle moves further fi-om the nose p osition, the drift correction isexcessive. Reduce the correction angle allowing the aircraft to drift hack on course. This is indicted for anydrill correction (or interception) angle when A DF n eedledelleclionand drilt correction angle are e q u a llitheestimated dnit correction is insuftcient, the azimuth needle willffPDVC toward the no se, requiring a furthercorrection to regain track. W ith careful attention to h eadings, effective drift correction angles can beestablished with v ery little bracketing.

Station Approach and Station Passage. The sank suggestions apply to ADF tracking as have beenmentioned in connection w ith ADF homing and V OR lLocalizer procedures. 'the closer you are to the stationthe more aggraN atcd are your errors in drill correction and basic instrument flying technique, unless yourecognize sI.alori approach and prevent yourselF' Iforn overconirolling the observed track deviations.

W hen yo u are close to the station, slight deviations from the desired track resultin large deflections of theazimuth n eedle. It is imp ortant, therefore, that the correct drift correction angle he established as so on as

possible alter interception oI'an inbound course. With the course pinned down and heading corrections keptat a miniiriun you will be more alert to signs of station appro ach than you w ould be if you were bu sychasing headings and ADF deflections. Make small heading corrections (not over 5I'/2) as soon as the

needle show s a deviation from co urse, until it begins to rotate steadily toward a w ing-tip position or sho wserratic left/right oscillations. A t this point, hold y our last corrected heading ccnstant, and time station p assagewhen the needle show s either wing-tip position or settles at or near the I 8 Oi.'/2 position. The d ine intervalfrom the first indications oistation proxim ity to positive station p assage varies with altitude - a few seconds atlow levels to 3 minutes at high altitude.

Ou tbound 'I'racking. Procedu res for tracking outbou nd are identical to those used for inbound tracking.Ho wever, the direction ofthe azimuth needle deflections are differciit from those no ted during inbound trackinterceptions, as show nin Figure 8-28. When tracking inbound, a change ol'beading toward the desired trackre suits in mov ement of the azimuth needle toward zero. W hen tracking outbound, a change of heading tow ardthe desired track rcsuhs in needle m ovemen t further away from the 180Y ,'/-. position.

Tie/Distance Checks (ADF), Time and (listance to a station mriy he calculated with radio-compass

procedures similar to the VOR procedures already discussed. A variety of methods commonly used are

variations of the basic procedures that follow .

W ing-Tip Bearing C hange. To d etermine the time distance to the station, use the following steps:

1. After tuning in the station, determine the relative bearing from the p osition of the AD F needle.2. Turn the num ber of degrees necessary to place the needle on 09O IY2 or 27O I,1/2.I Note the time and fly a constant tmgi'ietic heading for a specific number oNegrees ofhearing change.


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The am ount of change flown varies with the observed rate of bearing change. For example, a 1 OV2 chang eat a considerable distance iorn the station may take unnecessarily long; the time/distance check can be

accomplishedin this case by timing a5 ? 1 / 2change4. Apply the observed tmie interval to thei)rnlub, or cakulate the urne to stMion by rule of thum b if a

1 0i, 2 bearing change is used (SELi lime/Distance Checks by V O K ). For example, you are flying a magneticheading oil 0Ij, 1/. TAS 130 knots, AI)I-' relative hearing 090I,'/ Maintaining the magnetic heading for 4nilnutes, you observe are lative beanngol' 1 0OI,Y2. Approximate time to station is as ftjllows;

Figure 8-23. ADF' tracking-outbound.

By lbnnnla:

60 x minutes between bearing changeMinutes to station -

Degrees of bearing change

60 x 4

10


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24 minutes

By m ic of thumb (for 1 0I'/2 change):

Minutes to station - x time in iiiirniies between bearing change.6x4

- 24 minutes

time in secondsor Minutes to station = --

10

240

10

24 minutes

To determine the distance 11Dmthe station, use the lbrmula (see Time/Distance Checks by VOR) or thefollowing computer method:

I Place the speed index opposite True Air Speed

2. Read distance Ii -urn station on miles scale opposite time 11Dm station on minutes scale.Other tim e distance checks are applications of the isosceles triangle principle:Bow -to-beam bearing gives time to station by the following steps:1. Turn the number ol'degrees necessary to place the ADF needle on 045I/ (or 31 5 I 17 .

2. Maintain heading until the needle is on 09Oi:1/2 (or 270I,1/2).

3. Time distance flown equals time/distance to station,The D ouble-the-angle-on-how m ethod. 'involves the Co llowing stcps:1. Tune m a staLion between 1O Ij,1/2 and 45 1A of the nose position, and mo te the relative bearing.2. Fly a constant magn etic heading until the angle on the nose doub les.3 The time/distance required to double the angle on the nose equals the time distance to the station

The accuracy o]'time distance checks inv olves a num ber olvariables, including existing wind, accuracy o l'timing, and heading con trol. Tim e checks, especially those involving a rapid rate ofbcaring change,dcnimdvery precise techniques in basic instrument flying w hile you m aintain heading and check elapsed time.

Automatic DF

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