2012-00085-MN-0 BAUTISTA, Pauline Ysabelle C.2012-01434-MN-0 DE LUNA, Carl Joseph T.2012-02742-MN-0 GANZALINO, Mark Paul S.BSECE 4-4 ELECTRO-NAVIGATIONAL AIDS

Topic: STRUCTURE and FACILITIES of AIRCRAFTS

INTRODUCTION

Brief History of Aircraft StructuresThe history of aircraft structure underlies the history of aviation in general. Advances in

materials and processes used to construct aircraft led to their evolution from simple wood truss structures to the sleek aerodynamic flying machines of today.

A B C D E F

GEORGE CAYLEY (A), the father of aeronautics, developed an efficient cambered airfoil in the early 1800s, as well as successful manned gliders later in that century. He established the principles of flight, including the existence of lift, weight, thrust, and drag. It was Cayley who first stacked wings and created a tri-wing glider that flew a man in 1853. Earlier, Cayley studied the center of gravity of flying machines, as well as the effects of wing dihedral. Furthermore, he pioneered directional control of aircraft by including the earliest form of a rudder on his gliders.

In the late 1800s, OTTO LILIENTHAL (B) built upon Cayley’s discoveries. He manufactured and flew his own gliders on over 2,000 flights. His willow and cloth aircraft had wings designed from extensive study of the wings of birds. Lilienthal also made standard use of vertical and horizontal fins behind the wings and pilot station. Above all, Lilienthal proved that man could fly. He quoted “To invent an airplane is nothing. To build one is something. But to fly is everything.”

OCTAVE CHANUTE (C) was a retired railroad and bridge engineer. His interest was so great that, among other things, he published a definitive work called “Progress in Flying Machines.” This was the culmination of his effort to gather and study all the information available on aviation. With the assistance of others, he built gliders similar to Lilienthal’s and then his own. In addition to his publication, Chanute advanced aircraft structure development by building a glider with stacked wings incorporating the use of wires as wing supports.

The work of all of these men was known to the WRIGHT BROTHERS (D) when they built their successful, powered airplane in 1903. The first of its kind to carry a man aloft, the Wright Flyer had thin, cloth-covered wings attached to what was primarily truss structures made of wood. The wings contained forward and rear spars and were supported with both struts and wires. Stacked wings (two sets) were also part of the Wright Flyer. Powered heavier-than-air aviation grew from the Wright design. Inventors and fledgling aviators began building their own aircraft. Early on, many were similar to that constructed by the Wrights using wood and fabric with wires and struts to support the wing structure.

In 1909, Frenchman LOUIS BLERIOT (E) produced an aircraft with notable design differences. He built a successful mono-wing aircraft. The wings were still supported by wires,

but a mast extending above the fuselage enabled the wings to be supported from above, as well as underneath. This made possible the extended wing length needed to lift an aircraft with a single set of wings. Bleriot used a Pratt truss-type fuselage frame.

More powerful engines were developed and airframe structures changed to take advantage of the benefits. As early as 1910, German HUGO JUNKERS (F) was able to build an aircraft with metal truss construction and metal skin due to the availability of stronger power plants to thrust the plane forward and into the sky. The use of metal instead of wood for the primary structure eliminated the need for external wing braces and wires. His J-1 also had a single set of wings (a monoplane) instead of a stacked set.

DefinitionAIRCRAFT is any machine supported for flight in the air by buoyancy or by the dynamic

action of air on its surfaces, especially powered airplanes, gliders, and helicopters.

Classification of Aircrafts1. Propeller - aircraft whose primary form of thrust is derived from a propellor as opposed to

a jet engine.2. Helicopter - one of the most convenient and timesaving ways to transfer between the

city and airport, alternatively an easy way to reach remote destinations. A helicopter is a type of rotorcraft in which lift and thrust are supplied by rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forward, backward, and laterally.

3. Biplane - is a fixed-wing aircraft with two main wings stacked one above the other. The first aircraft to fly, the Wright Flyer, used a biplane wing arrangement, as did most aircraft in the early years of aviation.

4. Seaplane - a powered fixed-wing aircraft capable of taking off and landing (alighting) on water. Seaplanes that can also take off and land on airfields

5. Jet aircraft - is an aircraft (nearly always a fixed-wing aircraft) propelled by jet engines.6. Glider - is a heavier-than-air aircraft that is supported in flight by the dynamic reaction of

the air against its lifting surfaces, and whose free flight does not depend on an engine. Most gliders do not have an engine, although motor-gliders have small engines for extending their flight when necessary and some are even powerful enough to take off.

7. Blimp - is an airship (dirigible) without an internal structural framework or a keel.8. Private Jet – or business jet, is a jet aircraft designed for transporting small groups of

people. These are usually owned by prominent people.9. Cargo aircraft - is a fixed-wing aircraft that is designed or converted for the carriage

of air cargo, rather than passengers.10. Jumbo jet - The biggest wide-body aircraft are known as jumbo jets due to their very

large size11. Human-powered aircraft - is an aircraft belonging to the class of vehicles known

as human-powered vehicles where in

AIRCRAFT STRUCTURE

Lift and Basic Aerodynamic Concept

In order to understand the operation of the major components and subcomponents of an aircraft, it is important to understand basic aerodynamic concepts. Four forces act upon an aircraft in relation to straight-and-level, unaccelerated flight. These forces are thrust, lift, weight, and drag.

Thrust is the forward force produced by the powerplant/propeller. It opposes or overcomes the force of drag. As a general rule, it is said to act parallel to the longitudinal axis. This is not always the case as explained later.

Drag is a rearward, retarding force, and is caused by disruption of airflow by the wing, fuselage, and other protruding objects. Drag opposes thrust, and acts rearward parallel to the relative wind.

Weight is the combined load of the airplane itself, the crew, the fuel, and the cargo or baggage. Weight pulls the airplane downward because of the force of gravity. It opposes lift, and acts vertically downward through the airplane’s center of gravity (CG).

Lift opposes the downward force of weight, is produced by the dynamic effect of the air acting on the wing, and acts perpendicular to the flightpath through the wing’s center of lift.

One of the most significant components of aircraft design is Center of Gravity. It is the specific point where the mass or weight of an aircraft may be said to center; that is, a point around which, if the aircraft could be suspended or balanced, the aircraft would remain relatively level. The position of the CG of an aircraft determines the stability of the aircraft in flight.

Major Components of Airplanes

Although airplanes are designed for a variety of purposes, most of them have the same

major components. The overall characteristics are largely determined by the original design

objectives. Most airplane structures include a fuselage, wings, an empennage, landing gear, and

a power plant.

Fuselage

The fuselage is the central body of an airplane and is designed to accommodate the crew,

passengers, and cargo. It also provides the structural connection for the wings and tail assembly.

Older types of aircraft design utilized an open truss structure constructed of wood, steel, or

aluminum tubing. The most popular types of fuselage structures used in today’s aircraft are the

monocoque (French for “single shell”) and semimonocoque.

Wings

The wings are airfoils attached to each side of the fuselage and are the main lifting

surfaces that support the airplane in flight. There are numerous wing designs, sizes, and shapes

used by the various manufacturers. Each fulfills a certain need with respect to the expected

performance for the particular airplane. Wings may be attached at the top, middle, or lower

portion of the fuselage. These designs are referred to as high-, mid-, and low-wing, respectively.

The number of wings can also vary. Airplanes with a single set of wings are referred to as

monoplanes, while those with two sets are called biplanes.

Many high-wing airplanes have external braces, or wing struts, which transmit the flight

and landing loads through the struts to the main fuselage structure. Since the wing struts are

usually attached approximately halfway out on the wing, this type of wing structure is called

semi-cantilever. A few high-wing and most low-wing airplanes have a full cantilever wing

designed to carry the loads without external struts.

The principal structural parts of the wing are spars, ribs, and stringers. These are

reinforced by trusses, I-beams, tubing, or other devices, including the skin. The wing ribs

determine the shape and thickness of the wing (airfoil). In most modern airplanes, the fuel tanks

either are an integral part of the wing’s structure, or consist of flexible containers mounted inside

of the wing.

Attached to the rear or trailing edges of the wings are two types of control surfaces

referred to as ailerons and flaps. Ailerons extend from about the midpoint of each wing outward

toward the tip, and move in opposite directions to create aerodynamic forces that cause the

airplane to roll. Flaps extend outward from the fuselage to near the midpoint of each wing. The

flaps are normally flush with the wing’s surface during cruising flight. When extended, the flaps

move simultaneously downward to increase the lifting force of the wing for takeoffs and landings.

Empennage

The empennage includes the entire tail group and consists of fixed surfaces such as the

vertical stabilizer and the horizontal stabilizer. The movable surfaces include the rudder, the

elevator, and one or more trim tabs.

The rudder is attached to the back of the vertical stabilizer. During flight, it is used to

move the airplane’s nose left and right. The elevator, which is attached to the back of the

horizontal stabilizer, is used to move the nose of the airplane up and down during flight. Trim

tabs are small, movable portions of the trailing edge of the control surface. These movable trim

tabs, which are controlled from the flight deck, reduce control pressures. Trim tabs may be

installed on the ailerons, the rudder, and/or the elevator.

A second type of empennage design does not require an elevator. Instead, it incorporates

a one-piece horizontal stabilizer that pivots from a central hinge point. This type of design is

called a stabilator, and is moved using the control wheel, just as the elevator is moved. This

increases the aerodynamic tail load and causes the nose of the airplane to move up. Stabilators

have an antiservo tab extending across their trailing edge.

The antiservo tab moves in the same direction as the trailing edge of the stabilator and

helps make the stabilator less sensitive. The antiservo tab also functions as a trim tab to relieve

control pressures and helps maintain the stabilator in the desired position.

Landing Gear

The landing gear is the principal support of the airplane when parked, taxiing, taking off, or landing. The most common type of landing gear consists of wheels, but airplanes can also be equipped with floats for water operations, or skis for landing on snow.

The landing gear consists of three wheels—two main wheels and a third wheel positioned either at the front or rear of the airplane. Landing gear with a rear mounted wheel is called conventional landing gear.

Airplanes with conventional landing gear are sometimes referred to as tailwheel airplanes. When the third wheel is located on the nose, it is called a nosewheel, and the design is referred to as a tricycle gear. A steerable nosewheel or tailwheel permits the airplane to be controlled throughout all operations while on the ground. Most aircraft are steered by moving the rudder pedals, whether nosewheel or tailwheel. Additionally, some aircraft are steered by differential braking.The Powerplant

The powerplant usually includes both the engine and the propeller. The primary function of the engine is to provide the power to turn the propeller. It also generates electrical power, provides a vacuum source for some flight instruments, and in most single-engine airplanes, provides a source of heat for the pilot and passengers. The engine is covered by a cowling, or a nacelle, which are both types of covered housings. The purpose of the cowling or nacelle is to streamline the flow of air around the engine and to help cool the engine by ducting air around the cylinders.

Subcomponents of AirplanesThe subcomponents of an airplane include the airframe, electrical system, flight controls,

and brakes.The airframe is the basic structure of an aircraft and is designed to withstand all

aerodynamic forces, as well as the stresses imposed by the weight of the fuel, crew, and payload.

The primary function of an aircraft electrical system is to generate, regulate, and distribute electrical power throughout the aircraft. There are several different power sources on aircraft to power the aircraft electrical systems. These power sources include: engine-driven alternating current (AC) generators, auxiliary power units (APUs), and external power. The aircraft’s electrical power system is used to operate the flight instruments, essential systems such as anti-icing, etc., and passenger services, such as cabin lighting.

The flight controls are the devices and systems which govern the attitude of an aircraft and, as a result, the flightpath followed by the aircraft. In the case of many conventional airplanes, the primary flight controls utilize hinged, trailing-edge surfaces called elevators for pitch, ailerons for roll, and the rudder for yaw. These surfaces are operated by the pilot in the flight deck or by an automatic pilot.

Airplane brakes consist of multiple pads (called caliper pads) that are hydraulically squeezed toward each other with a rotating disk (called a rotor) between them. The pads place pressure on the rotor which is turning with the wheels. As a result of the increased friction on the rotor, the wheels inherently slow down and stop turning. The disks and brake pads are made either from steel, like those in a car, or from a carbon material that weighs less and can absorb more energy. Because airplane brakes are used principally during landings and must absorb enormous amounts of energy, their life is measured in landings rather than miles.

Types of Aircraft ConstructionThe construction of aircraft fuselages evolved from the early wood truss structural

arrangements to monocoque shell structures to the current semimonocoque shell structures.Truss Structure

The main drawback of truss structure is its lack of a streamlined shape. In this construction method, lengths of tubing, called longerons, are welded in place to form a well-braced framework. Vertical and horizontal struts are welded to the longerons and give the structure a square or rectangular shape when viewed from the end. Additional struts are needed to resist stress that can come from any direction. Stringers and bulkheads, or formers, are added to shape the fuselage and support the covering.

As technology progressed, aircraft designers began to enclose the truss members to streamline the airplane and improve performance. This was originally accomplished with cloth fabric, which eventually gave way to lightweight metals such as aluminum. In some cases, the outside skin can support all or a major portion of the flight loads. Most modern aircraft use a form of this stressed skin structure known as monocoque or semimonocoque construction.Monocoque

Monocoque construction uses stressed skin to support almost all loads much like an aluminum beverage can. Although very strong, monocoque construction is not highly tolerant to deformation of the surface. For example, an aluminum beverage can supports considerable forces at the ends of the can, but if the side of the can is deformed slightly while supporting a load, it collapses easily.

Because most twisting and bending stresses are carried by the external skin rather than by an open framework, the need for internal bracing was eliminated or reduced, saving weight and maximizing space. One of the notable and innovative methods for using monocoque construction was employed by Jack Northrop. In 1918, he devised a new way to construct a monocoque fuselage used for the Lockheed S-1 Racer. The technique utilized two molded plywood half-shells that were glued together around wooden hoops or stringers. To construct the half shells, rather than gluing many strips of plywood over a form, three large sets of spruce strips were soaked with glue and laid in a semi-circular concrete mold that looked like a bathtub.

Then, under a tightly clamped lid, a rubber balloon was inflated in the cavity to press the plywood against the mold. Twenty-four hours later, the smooth half-shell was ready to be joined to another to create the fuselage. The two halves were each less than a quarter inch thick. Although employed in the early aviation period, monocoque construction would not reemerge for several decades due to the complexities involved. Every day examples of monocoque construction can be found in automobile manufacturing where the unibody is considered standard in manufacturing.Semimonocoque

Semimonocoque construction, partial or one-half, uses a substructure to which the airplane’s skin is attached. The substructure, which consists of bulkheads and/or formers of various sizes and stringers, reinforces the stressed skin by taking some of the bending stress from the fuselage. The main section of the fuselage also includes wing attachment points and a firewall. On single-engine airplanes, the engine is usually attached to the front of the fuselage. There is a fireproof partition between the rear of the engine and the flight deck or cabin to protect the pilot and passengers from accidental engine fires. This partition is called a firewall and is usually made of heat-resistant material such as stainless steel. However, a new emerging process of construction is the integration of composites or aircraft made entirely of composites.

AIRCRAFT FACILITIES

Runaways and othersTake-off and landing areas are based on either a runway or helipad. The landing/take-off

area consists not only of the runway and helipad surface, shoulders, and overruns, but also approach slope surfaces, safety clearances and other imaginary airspace surfaces. Aviation facilities normally have only one runway. Additional runways may be necessary to accommodate operational demands, minimize adverse wind conditions or overcome environmental impacts. A parallel runway may be provided based on operational requirements. Class A runways are primarily intended for small light aircraft. These runways do not have the potential or foreseeable requirement for development for use by high performance and large heavy aircraft. Ordinarily, these runways are less than 2,440 meters [8,000 feet] long and have less than 10 percent of their operations that involve aircraft in the Class B category. Class B runways are primarily intended for high performance and large heavy aircraft.

The capacity of a single runway system will vary from approximately 40 to 50 operations per hour under IFR conditions, up to 75 operations per hour under VFR conditions. Parallel runways are the most commonly used system for increased capacity. In some cases, parallel runways may be staggered with the runway ends offset from each other and with terminal or service facilities located between the runways. Where practical, parallel runway centerline separation of at least 5,000 feet (1 525 m) is recommended. Crosswind runways may be either the open-V or the intersecting type of runway. The crosswind system is adaptable to a wider variety of wind conditions than the parallel system. When winds are calm, both runways may be used simultaneously. An open-V system has a greater capacity than the intersecting system.

Runway overruns keep the probability of serious damage to an aircraft to a minimum in the event the aircraft runs off the runway during a take-off or lands short during a landing. Overruns are required for the landing and take-off area. Aircraft arresting systems consist of engaging devices and energy absorbers. Engaging devices are net barriers, disc supported pendants (hook cables), and cable support systems which allow the pendant to be raised to the battery position or retracted below the runway surface. Energy absorbing devices are ships

anchor chains, rotary friction brakes, such as the BAK-9 and BAK-12, or rotary hydraulic systems such as the BAK-13 and E-28. The systems designated "Barrier, Arresting Kit" (BAK) are numbered in the sequence of procurement of the system design. There is no connection between the Air Force designations of these systems and their function.

An alert pad, often referred to as an alert apron, is an exclusive paved area for armed aircraft to park and have immediate, unimpeded access to a runway. In the event of a declared alert, alert aircraft must be on the runway and airborne in short notice. Locating the alert pad adjacent to a runway end will allow alert aircraft to proceed directly from the apron to the runway threshold without interruptions from other traffic. Alert pads are located close to the runway threshold to allow alert aircraft to be airborne within the time constraints stipulated in their mission statements. The preferred location of alert pads is on the opposite side of the runway, away from normal traffic patterns to allow aircraft on the alert pad direct, unimpeded access to the runway.

A warm-up pad, also referred to as a holding apron, is a paved area adjacent to a taxiway at or near the end of a runway. The intent of a warm-up pad is to provide a parking location, off the taxiway, for aircraft which must hold due to indeterminate delays. It allows other departing aircraft unencumbered access to the runway. Typically the end cross over taxiway is widened to 46 m [150 ft] which provides room to accommodate aircraft warming up or waiting for other reasons. The most advantageous position for a warm-up pad is adjacent to the end turnoff taxiway, between the runway and parallel taxiway. However, other design considerations such as airspace and navigational aids may make this location undesirable. If airspace and navigational aids prevent locating the warm-up pad adjacent to the end turnoff taxiway, the warm-up pad should be located at the end of and adjacent to the parallel taxiway.The arm/disarm pad is used for arming aircraft immediately before takeoff and for disarming (safing) weapons retained or not expended upon their return.

An aircraft compass calibration pad is a paved area in a magnetically quiet zone where an aircraft's compass is calibrated.

Hazardous cargo pads are paved areas for loading and unloading explosives and other hazardous cargo from aircraft. Hazardous cargo pads are required at facilities where the existing aprons cannot be used for loading and unloading hazardous cargo.

Taxiways provide for free ground movement to and from the runways, helipads, maintenance, cargo/passenger, and other areas of the aviation facility. The objective of taxiway system planning is to create a smooth traffic flow. This system allows unobstructed ground visibility; a minimum number of changes in the aircraft's taxiing speed; and, ideally, the shortest distance between the runways or helipads and apron areas. At airfields with high levels of activity, the capacity of the taxiway system can become the limiting operational factor. Runway capacity and access efficiency can be enhanced or improved by the installation of parallel taxiways. A full length parallel taxiway may be provided for a single runway with appropriate connecting lateral taxiways to permit rapid entrance and exit of traffic between the apron and the runway.

Aircraft parking aprons are the paved areas required for aircraft parking, loading, unloading, and servicing. They include the necessary maneuvering area for access and exit to parking positions. Aprons will be designed to permit safe and controlled movement of aircraft under their own power. Aircraft apron dimensions and size are based on mission requirements.

Support Structures and FacilitiesHangars provide space for various aircraft activities: scheduled inspections; landing gear

tests; weighing of aircraft; major work and maintenance of fuel systems and airframes; and technical order compliance and modifications. These activities can be more effectively ccomplished while the aircraft is under complete cover. Pavement for hangar floors must be designed to support aircraft loads. Hangars provide covered floor space to accommodate aircraft. Clearance must be provided between the aircraft and the door opening, walls, and ceiling of the hangar.

The aircraft maintenance facility includes, but is not limited to: aircraft maintenance hangars, special purpose hangars, hangar access aprons, weapons system support shops, aircraft system testing and repair shops, aircraft parts storage, corrosion control facilities, and special purpose maintenance pads. The aircraft maintenance area includes utilities, roadways, fencing, and security facilities and lighting. Aircraft maintenance facilities are generally located

on one side of the runway to allow simplified access among maintenance areas, aircraft, and support areas.

Aviation operations support facilities include those facilities that directly support the flying mission. Operations support includes air traffic control, aircraft rescue and firefighting, fueling facilities, airfield operations center (airfield management facility), squadron operations/aircraft maintenance units, and air mobility operations groups. Aviation operations support facilities are generally located along the hangar line with the central area typically being allocated to airfield operations (airfield management facility), air traffic control, aircraft rescue and firefighting, and flight simulation.

Aircraft fuel storage and dispensing facilities are provided at most aviation facilities. Operating fuel storage tanks are provided where dispensing facilities are remote from bulk storage. Bulk fuel storage areas require locations which are accessible by tanker-truck, tanker-rail car, or by waterfront. Both bulk storage and operating storage areas provide for the loading and parking of fuel vehicles to service aircraft. Where hydrant fueling systems are authorized, bulk fuel storage locations take into account systems design requirements (e.g., the distance from the fueling apron to the storage tanks). Fuel storage and operating areas have requirements for minimum clearances from buildings, aircraft parking, roadways, radar, and other structures/areas, as established in service directives. Aviation fuel storage and operating areas also require lighting, fencing, and security alarms. Liquid fuel storage facility sitings address spill containment and leak protection/detection.

Navigational AidsNavigational Aids (NAVAIDS) assist the pilot in flight and during landing. A lighting equipment

vault is provided for airfields and heliport facilities with navigational aids, and may be required at remote or stand-alone landing sites. A (NAVAID) building will be provided for airfields with navigational aids. Each type of NAVAID equipment is usually housed in a separate facility.

The microwave landing system (MLS) provides the pilot of a properly equipped aircraft with electronic guidance to control the aircraft's alignment and descent until the runway environment is in sight. MLS is also used to define a missed approach course or a departure course. MLS is not particularly susceptible to signal interference as a result of buildings, trees, power lines, metal fences, and other large objects. However, when these objects are in the coverage area, they may cause multipath (signal reflection) or shadowing (signal blockage) problems. MLS antenna systems do not use the ground to form the desired signal. Grading for MLS installations is usually limited to that needed for the antenna and monitors, a service road, and a vehicle parking area.

Azimuth Antenna (AZ) provides alignment guidance. The signal coverage area extends 40 degrees either side of the intended course (runway centerline). The AZ antenna is located on the extended runway centerline at a distance of 1,000 to 1,500 feet (300 to 450 m) beyond the stop end of the runway. AZ antennas are 8 feet (2.4 m) in height and are mounted on low impact resistant supports.

Elevation Antenna (EL) provides descent guidance. The signal area extends from the horizon to 30 degrees above the horizon. The EL antenna height depends upon the beam width but would not exceed 18.6 feet (5.7 m). The EL antenna site is at least 400 feet (120 m) from the runway centerline and 800 to 1,000 feet (240 to 300 m) from the runway threshold and should provide a threshold crossing height of 50 feet (15 m).

Distance Measuring Equipment (DME) provides range information. DME antennas are 22 feet (6.7 m) in height and normally are collocated with the AZ antenna. To preclude penetration of an approach surface, the collocated AZ/DME antennas should be placed 1,300 feet (390 m) from the runway end.

The instrument landing system (ILS) provides pilots with electronic guidance for aircraft alignment, descent gradient, and position until visual contact confirms the runway alignment and location. The ILS uses a line-of-sight signal from the localizer antenna and marker beacons and a reflected signal from the ground plane in front of the glide slope antenna. ILS antenna systems are susceptible to signal interference sources such as power lines, fences, metal buildings, etc. Since ILS uses the ground in front of the glide slope antenna to develop the signal, this area should be graded to remove surface irregularities.

The Localizer Antenna (LOC) signal is used to establish and maintain the aircraft's horizontal position until visual contact confirms the runway alignment and location. The LOC antenna is sited on the extended runway centerline 1,000 to 2,000 feet (300 to 600 m) beyond the stop end of the runway. The LOC equipment shelter is placed at least 250 feet (75 m) to either side of the antenna array and within 30 degrees of the extended longitudinal axis of the antenna array.

The Glide Slope Antenna (GS) signal is used to establish and maintain the aircraft's descent rate until visual contact confirms the runway alignment and location. A GS differentiates precision from nonprecision approaches. The GS antenna may be located on either side of the runway. The most reliable operation is obtained when the GS is located on the side of the runway offering the least possibility of signal reflections from buildings, power lines, vehicles, aircraft, etc.

Marker beacons radiate cone or fan shaped signals vertically to activate aural and visual indicators in the cockpit marking specific points in the ILS approach. Marker beacons are located on the extended runway centerline at key points in the approach. The outer marker (OM) beacon is located 4 to 7 nautical miles (7.4 to 13 km) from the ILS runway threshold to mark the point at which glide slope altitude is verified or at which descent without glide slope is initiated. A middle marker (MM) beacon is located 2,000 to 6,000 feet (600 to 1 800 m) from the ILS runway threshold. It marks (approximately) the decision point of a CAT I ILS approach. An inner marker (IM) beacon may be located to mark the decision point of a CAT II or CAT III ILS approach. A "back course" marker beacon (comparable to an outer marker beacon) may be located to the rear of a bidirectional localizer facility to permit development of a nonprecision approach. Off airport marker beacons are located in a fenced 6-foot by 6-foot (2 m by 2 m) tract situated on the extended runway centerline. A vehicle access and parking area is required at the site.

The non-directional beacon (NDB) radiates a signal which provides directional guidance to and from the transmitting antenna. An NDB is normally mounted on a 35 foot (11 m) pole. A NDB may be located on or adjacent to the airport. Metal buildings, power lines, or metal fences should be kept 100 feet (30 m) from a NDB antenna. Electronic equipment is housed in a small collocated shelter.

The standard very high frequency omnirange (VOR) located on an airport is known as a TVOR. TVORs radiate azimuth information for nonprecision instrument approach procedures. If the airport has intersecting runways, TVORs should be located adjacent to the intersection to provide approach guidance to both. TVORs should be located at least 500 feet (150 m) from the centerline of any runway and 250 feet (75 m) from the centerline of any taxiway. TVOR sites should be level within 1000 feet (300 m) of the antenna. However, a downward slope of as much as 4 percent is permitted between 200 feet (60 m) and 1,000 feet (300 m) of the antenna.

From airport traffic control towers (ATCTs), ATC personnel control flight operations within the airport's designated airspace and the operation of aircraft and vehicles on the movement area. A typical ATCT site will range from 1 to 4 acres (0.4 to 1.6 hectares). Additional land may be needed for combined flight service stations/towers.

Airport surveillance radars (ASR) are used to control air traffic. ASR antennas scan through 360 degrees to present the controller with the location of all aircraft within 60 nautical miles of the airport. The site for the ASR antenna is flexible. The ASR antenna should be located as close to the ATCT control room as practical. Antennas should be located at least 1,500 feet (450 m) from any building or object that might cause signal reflections and at least one-half mile (.8 km) from other electronic equipment. ASR antennas may be elevated to obtain line-of-sight clearance. Typical ASRs heights range from 25 to 85 feet (7.5 to 25.5 m) above ground.

Airport surface detection equipment (ASDE) compensates for the loss of line of sight to surface traffic during periods of reduced visibility. ASDE should be sited to provide line-of-sight coverage of the entire aircraft movement area. While the ideal location for the ASDE antenna is on the ATCT cab roof, the antenna may be placed on a freestanding tower up to 100 feet (30 m) tall located within 6,000 feet (1 800 m) of the ATCT cab.

AIRCRAFT MAINTENANCE

Location Numbering System

Even on small, light aircraft, a method of precisely locating each structural component is required. Various numbering systems are used to facilitate the location of specific wing frames, fuselage bulkheads, or any other structural members on an aircraft. Most manufacturers use some system of station marking. For example, the nose of the aircraft may be designated “zero station,” and all other stations are located at measured distances in inches behind the zero station. Thus, when a blueprint reads “fuselage frame station 137,” that particular frame station can be located 137 inches behind the nose of the aircraft.

To locate structures to the right or left of the center line of an aircraft, a similar method is employed. Many manufacturers consider the center line of the aircraft to be a zero station from which measurements can be taken to the right or left to locate an airframe member. This is often used on the horizontal stabilizer and wings.

The applicable manufacturer’s numbering system and abbreviated designations or symbols should always be reviewed before attempting to locate a structural member. They are not always the same. The following list includes location designations typical of those used by many manufacturers.

• Fuselage stations (Fus. Sta. or FS) are numbered in inches from a reference or zero point known as the reference datum. The reference datum is an imaginary vertical plane at or near the nose of the aircraft from which all fore and aft distances are measured. The distance to a given point is measured in inches parallel to a center line extending through the aircraft from the nose through the center of the tail cone. Some manufacturers may call the fuselage station a body station, abbreviated BS.

• Buttock line or butt line (BL) is a vertical reference plane down the center of the aircraft from which measurements left or right can be made.

• Water line (WL) is the measurement of height in inches perpendicular from a horizontal plane usually located at the ground, cabin floor, or some other easily referenced location.

• Aileron station (AS) is measured outboard from, and parallel to, the inboard edge of the aileron, perpendicular to the rear beam of the wing.

• Flap station (KS) is measured perpendicular to the rear beam of the wing and parallel to, and outboard from, the inboard edge of the flap.

• Nacelle station (NC or Nac. Sta.) is measured either forward of or behind the front spar of the wing and perpendicular to a designated water line.

In addition to the location stations listed above, other measurements are used, especially on large aircraft. Thus, there may be horizontal stabilizer stations (HSS), vertical stabilizer stations (VSS) or powerplant stations (PPS). In every case, the manufacturer’s terminology and station location system should be consulted before locating a point on a particular aircraft.Another method is used to facilitate the location of aircraft components on air transport aircraft. This involves dividing the aircraft into zones. These large areas or major zones are further

divided into sequentially numbered zones and subzones. The digits of the zone number are reserved and indexed to indicate the location and type of system of whichthe component is a part.

Access and Inspection PanelsKnowing where a particular structure or component is located on an aircraft needs to be

combined with gaining access to that area to perform the required inspections or maintenance. To facilitate this, access and inspection panels are located on most surfaces of the aircraft. Small panels that are hinged or removable allow inspection and servicing. Large panels and doors allow components to be removed and installed, as well as human entry for maintenance purposes.

The underside of a wing, for example, sometimes contains dozens of small panels through which control cablecomponents can be monitored and fittings greased. Various drains and jack points may also be on the underside of the wing. The upper surface of the wings typically have fewer access panels because a smooth surface promotes better laminar airflow, which causes lift. On large aircraft, walkways are sometimes designated on the wing upper surface to permit safe navigation by mechanics and inspectors to critical structures and components located along the wing’s leading and trailing edges. Wheel wells and special component bays are places where numerous components and accessories are grouped together for easy maintenance access.

Panels and doors on aircraft are numbered for positive identification. On large aircraft, panels are usually numbered sequentially containing zone and subzone information in the panel number. Designation for a left or right side location on the aircraft is often indicated in the panel number. This could be with an “L” or “R,” or panels on one side of the aircraft could be odd numbered and the other side even numbered. The manufacturer’s maintenance manual explains the panel numbering system and often has numerous diagrams and tables showing the location of various components and under which panel they may be found. Each manufacturer is entitledto develop its own panel numbering system.

APPLICATIONS

Military

A military aircraft is any aircraft that is operated by a legal or insurrectionary armed service of any type. Military aircraft can be either combat or non-combat:

Combat aircraft are aircraft designed to destroy enemy equipment using its own armament.[18] Combat aircraft divide broadly intofighters and bombers, with several in-between types such as fighter-bombers and ground-attack aircraft (including attack helicopters).

Non-combat aircraft are not designed for combat as their primary function, but may carry weapons for self-defense. Non-combat roles include search and rescue, reconnaissance, observation, transport, training, and aerial refueling. These aircraft are often variants of civil aircraft.

Most military aircraft are powered heavier-than-air types. Other types such as gliders and balloons have also been used as military aircraft; for example, balloons were used for observation during the American Civil War and World War I, and military gliders were used during World War II to land troops.

Civil

Civil aircraft divide into commercial and general types, however there are some overlaps.

Commercial aircraft include types designed for scheduled and charter airline flights, carrying passengers, mail and other cargo. The larger passenger-carrying types are the airliners, the largest of which are wide-body aircraft. Some of the smaller types are also used in general aviation, and some of the larger types are used as VIP aircraft.

General aviation is a catch-all covering other kinds of private (where the pilot is not paid for time or expenses) and commercial use, and involving a wide range of aircraft types such as business jets (bizjets), trainers, homebuilt, gliders, warbirds and hot air balloons to name a few. The vast majority of aircraft today are general aviation types.

Experimental

An experimental aircraft is one that has not been fully proven in flight, or that carries an FAA airworthiness certificate in the "Experimental" category. Often, this implies that the aircraft is testing new aerospace technologies, though the term also refers to amateur and kit-built aircraft—many based on proven designs.

Model

A model aircraft is a small unmanned type made to fly for fun, for static display, for aerodynamic research or for other purposes. A scale model is a replica of some larger design.