End-of-Life Disposal

In general, the operational life of a civil aircraft ranges from twenty to thirty years depending on operational demand and profitability. A few World War II C47s

Figure 15.37. Anti-icing subsystem using boots

(b) RJ family rain-repellent system
(Courtesy of BAE Systems)



Figure 15.41. Civil aircraft turnaround servicing locations

(Dakota) are still flying. Thousands of aircraft already have been grounded and thousands more will be grounded forever in the immediate future. Their storage occupies much land and aircraft disposal is not the same as for automobiles. The disposal of older aircraft is a serious problem.

Metal sold as scrap can be recycled but increasing amounts of composite mate­rial are accumulating. Disposal of composite materials is difficult because they serve no useful purpose as scrap – attempts are being made to make them recyclable. Avionics black boxes and microprocessors contain toxic materials; the fluids in dis­play units also are toxic. It is expensive to rid the environment of toxic materials. Incineration plants are specifically designed to keep the efflux clean.

More research is continuing to find suitable materials that are less toxic and also can cost effectively be disposed of. This is a concern of material scientists; however, aircraft designers must stay current with materials technology and make proper selections.

Utility Subsystem

Utility systems are composed of water and waste systems, as shown in Figure 15.39. Passengers need water and restroom facilities. As the number of passengers and the duration of flights increase, the demand for drinking water and waste-disposal management also increases. The entire system is self-contained.

Typically, a third of a U. S. gallon of water per passenger (i. e., 100 U. S. gallons for 300 passengers) is the quantity carried onboard. Both hot and cold water is sup­plied. Typically, 1 lavatory per 10 to 15 passengers is provided. Chemicals are used

with water to flush the commodes. Waste must be contained inside until the air­craft lands, whereupon the systems are cleaned and refilled with fresh supplies for the next sortie. Figure 15.40 shows the RJ family wastewater-management system in detail.

Aircraft must be prepared for passenger services and utility use. Specific loca­tions are designated for preparing aircraft such that they do not interfere with and delay one another. Figure 15.41 shows the typical utility-service points. Access to servicing should not interfere with other activities in and around aircraft. Freezing of water is prevented by heating the critical areas.

Hydraulic Subsystem

All larger aircraft have a hydraulic system, which includes a fluid reservoir, electricity-driven pumps, hydraulic lines, valves, and pilot interface at the flight deck. Hydraulic-driven actuators are incorporated at a higher force level to activate the following uses:

• aircraft control system (e. g., elevator, rudder, aileron, and high-lift devices)

• engine thrust reversers

• undercarriage deployment and retraction

• brake application

For modern civil aircraft, hydraulic pressure is from 2,000 psia (older designs) to

3.0 psia (current designs); military aircraft hydraulic oil pressure has reached

8.0 psia. A higher pressure lowers the system weight but requires stringent design considerations.

Figure 15.28 shows the hydraulics system scheme of a four-engine aircraft. To ensure safety and reliability, at least two independent, continuously operating hydraulic systems are positioned in separate locations. The port side is identified as

Table 15.12. Hydraulics-driven subsystems (BAe RJ family)

Separately in both systems Yellow system Green system

the yellow system and the starboard side is the green system. (Airbus introduced a third independent line – the blue line.) Each line has its own reservoir and function­ality. Table 15.12 lists the subsystems activated by the hydraulics system. All systems include gauges, switches, valves, tubing, and connectors.




15.9.3 Pneumatic System

The pneumatic system consists of the use of a high-pressure air-bleed from the engine (gas turbines) serving the (1) ECS, which consists of cabin pressurization and air-conditioning; (2) anti-icing; (3) defogging system; and (4) engine starting. The APU is linked to the pneumatic system. An aircraft’s oxygen needs are sup­plied by a separate pneumatic system that is fitted with a pressure-reducing shutoff valve (PRSOV) and a cross-flow shutoff valve (SOV) to control and isolate airflow according to the scheduled demand.

Other uses for pneumatics include pressurizing the hydraulic reservoir, fuel sys­tem, and water tank; driving the accessories; and as a medium for rain repellent. Some thrust reversers are actuated by the pneumatic system.

ECS: Cabin Pressurization and Air-Conditioning

At cruise altitude, the atmospheric temperature drops to -50°C and below, and the pressure and density reduce to less than one fifth and one fourth of sea-level val­ues, respectively. Above a 14,000-ft altitude, the aircraft interior environment must be controlled for crew and passenger comfort as well as equipment protection. The cabin ECS consists of cabin pressurization and air-conditioning. Smaller, unpressur­ized aircraft flying below a 14,000-ft altitude suffice with air-conditioning only; the simplest form uses engine heat mixing with ambient cold air supplied under con­trolled conditions.

The cabin-interior pressure maintained at sea-level conditions is ideal but expensive. Cabin pressurization is like inflating a balloon – the fuselage skin bulges. The major differential between the outside and the inside pressure requires struc­tural reinforcement, which makes an aircraft heavier and more expensive. For this reason, the aircraft cabin pressure is maintained no higher than 8,000 ft, and a max­imum differential pressure is maintained at 8.9 psi. During ascent, the cabin is pres­surized gradually; during descent, cabin depressurization is also gradual in a pre­scribed schedule acceptable to the average passenger. Passengers feel it in their ears as they adjust to the change in pressure.

Cabin air-conditioning is an integral part of the ECS along with cabin pres­surization. Supplying a large passenger load at a uniform pressure and tempera­ture is a specialized design obligation. The engine compressor, which is bled at an intermediate stage with sufficient pressure and temperature, becomes contaminated and must be cleaned with moisture removed to an acceptable level. Maintaining a proper humidity level is also part of the ECS. The bled-air is then mixed with cool ambient air. In addition, there is a facility for refrigeration. The internal system tur­bine and compressor are driven by the system pressure. The heat exchanger, water extractor, condenser, valves, and sensors comprise a complex subsystem, as shown in Figure 15.29.

Figure 15.30 depicts the BAe RJ family anti-icing system. A generic pattern for the supply of air-conditioning in the passenger cabin is shown in Figure 15.31.

defogging aij:—



turbo-compressor unit














The avionics black boxes heat up and must be maintained at a level that keeps equipment functioning. The equipment bay is below the floorboards, as shown in Figure 15.27. Typically, a separate cooling system is employed to keep the equip­ment cool. Ram-air cooling is a convenient and less expensive way to achieve the cooling. Scooping ram air increases the aircraft drag. The cargo compartment also requires some heating.

An advanced military aircraft ECS differs significantly (Figure 15.32), using a boot-strap refrigeration system, which has recently also been deployed in civil aircraft applications.

Oxygen Supply

If there is a drop in cabin pressure while an aircraft is still at altitude, the oxygen supply for breathing becomes a critical issue. The aircraft system supplies oxygen to each passenger by dropping masks from the overhead panel. Military aircraft have fewer crew members and the oxygen supply is directly integrated in a pilot’s mask, as shown in Figure 15.33.

Anti-icing, De-icing, Defogging, and Rain-Removal Systems

Icing is a natural phenomenon that occurs anywhere in the world depending on weather conditions, operating altitude, and atmospheric humidity. Ice accumulation

4 exhaust dump

from engine bleed turbo-com pressor unit

system cooling air



on the wing, empennage, and/or engine intake can have disastrous consequences. Icing increases the drag and weight, decreases the lift and thrust, and even degrades control effectiveness. On the wing and empennage, icing alters the profile geometry, leading to loss of lift. Ice accumulation at the intake degrades engine performance and can damage the engine if large chunks are ingested. It is a mandatory require­ment to keep an aircraft free from icing degradation. This can be achieved by either anti-icing, which never allows ice to form on critical areas, or by de-icing, which allows ice buildup to a point and then sheds it before it becomes harmful. De-icing results in blowing away chunks of ice, which could hit or be ingested into an engine. Figure 15.34 shows the typical anti-icing envelopes.

There are several methods for anti-icing and de-icing. Not all anti-icing, de­icing, defogging, and rain-removal systems use pneumatics; some have an electrical system. Following are the methods currently in practice:

1. Hot Air Blown Through Ducts. This pneumatic system is the dominant one used for larger civil aircraft. Both anti-icing and de-icing can use a pneumatic system, which is achieved by routing high-pressure hot air bled from the midcompres­sor stage of a gas turbine and blown around the critical areas through perfo­rated ducts (i. e., Piccolo tubes). Typical pressure and temperature in the duct is about 25 psi (regulated between 25 psi and 40 psi) and 200° C (military air­craft reaching 500°C). Designers must ensure that damage does not occur due

Figure 15.34. Typical anti-icing enve­lopes




0 5 10 20 30 40 50 60 70 8C

Altitude (1000 X ft)

Figure 15.35. Generic civil aircraft anti­icing subsystem (Piccolo tubes)

to overheating. Figure 15.35 shows a typical system. Figure 15.36 depicts the BAe RJ family anti-icing system.

2. Boots. Both anti-icing and de-icing can use a boot specially designed with an integrated electrical heater or passages for hot airflow. Rubber boots are wrapped (i. e., capped) around the critical areas (e. g., LEs of lifting surfaces, propeller LEs, and intake lips) and are heated by either electrical elements or passing hot air, as in the pneumatic system. Electrically heated boots are lighter but can be relatively more expensive. The boot-type method is used in smaller aircraft. Figure 15.37 shows a typical boot system.

3. Electric Impulse. This is a not common but quite effective de-icing system. Ice is allowed to accumulate to a point when vibrations generated by electrical impulses break the ice layer, which is then blown away. This method has low power consumption but can be a heavy and expensive system.

4. Chemicals. This also is not a common system and is used primarily for anti-icing. Glycol-based antifreeze is allowed to “sweat” through small holes in critical areas where the chemical is stored. This process is limited to the amount of antifreeze carried onboard.

The piston engine carburator and critical instruments must be heated to keep them functioning.

Defogging and Rain-Removal Systems

The defogging and rain-removal systems are like an automobile using windshield wipers with embedded electrical wire in the windscreen. Rain-repellent chemicals assist in rain removal. Figure 15.38 shows a defogging and rain-removal system. Figure 15.38a is a generic layout with wipers, and Figure 15.38b shows the RJ family rain-repellent system in better detail.

Electrical Subsystem

All aircraft must have some form of electrical supply to power the aircraft subsys­tems. The supply of electricity is executed by a combination of generators and bat­teries. Most modern aircraft require both AC and DC supplies. The typical AC voltage is 115 volts at 400 Hz, but there are higher-voltage AC supplies. Typically, the DC voltage supply is 28 volts. The electrical-supply control must ensure safety and comply with mandatory requirements.

The following systems are associated with electrical power:

• engine starting and operation; management of the fuel system

• lighting – both internal and external (Figure 15.27 shows external requirements)

• flight deck instrumentation

• communication and navigation

Figure 15.27. Aircraft lighting require­ments

:r side lights

various external

light points

• avionics system

• flight-control system using the PCU

• passenger services for civil aircraft

• APU: emergency electrical power generation and supply

• armament management, electronic defensive and countermeasures for military aircraft

Typically, the electrical supply is generated at the primary and secondary levels. Engine-driven generators supply the primary power. The secondary supply serves before an engine starts and is a standby in an emergency situation. The secondary supply is generated from batteries, the APU, or an auxiliary system such as RAT.

The below-floorboard equipment bay houses items such as batteries, chargers, power controllers, transformers, and inverters (see Figure 15.27).

The weight of an electrical system depends on the load requirements. The cable weight is significant. An avionics system can be 0.4 to 4% for civil aircraft and 0.5 to 5% for military aircraft.

Military Aircraft Application

MIL-STD-1553B: U. S. military aircraft were the first to use the data-bus architec­ture, especially to handle the large amount of data for FBW and combat operations. In the United Kingdom, it is covered by DEF STAN 00-18. MIL-STD-1773 is the fiber-optics version of MIL-STD-1553B and STANAG 3838 is the NATO standard for bus architecture.



when retracted

Table 15.11. Aircraft avionics items





System display

Air data



Analog gauges










Civil Aircraft Application

ARINC 429 (originated in the 1970s): The success of the military standard was fol­lowed by civil standards, which began in a simplified manner. The Airbus 320 was the first large transport aircraft to use a full FBW system. ARINC 629 is the updated version that replaces ARINC 429.

Line replacement units (LRUs) are a convenient hardware design to facilitate installation and maintenance of electrical and avionics transmissions and connec­tions following the bus standards. LRUs are constructed in the modular concept as a subassembly and then installed on an aircraft. LRUs are also standardized to comply with the bus architecture.

Aircraft communication and navigation equipment is part of an avionics pack­age. The components of a typical civil aircraft avionics package are listed in Table 15.11.

Typical antenna locations for aircraft communication and navigation are shown in Figure 15.26. Antennas are installed in the symmetrical plane of an aircraft. Surveillance aircraft have a specific large housing for special-purpose avionics.

Emergency Power Supply

Most midsize and larger aircraft install an APU, which performs many functions. An APU is a small power plant, invariably a turboshaft engine that uses the same fuel (i. e., AVTUR). When ground facilities are not available, the APU can provide an emergency electrical supply and air-conditioning, and it can start the main air­craft gas turbines. It is interesting that an APU exhaust can reduce aircraft drag, regardless of how small. A typical example of an aft-mounted APU is shown in Figure 15.24 (i. e., a schematic layout). The APU and its installation weight range from 100 to 300 kg depending on the size. The size of an APU in a military aircraft depends on user requirements. An APU can be started using onboard batteries.

A ram air turbine (RAT) is another way to supply emergency power. This is a propeller-driven device mounted on an aircraft surface (at the fuselage underbelly) that operates when an aircraft is in motion. A RAT is retractable. Figure 15.25 shows the schematic layout.

Figure 15.24. Auxiliary power unit

15.9.2 Avionics Subsystems

A host of avionics “black boxes” support the flight deck and beyond. The black boxes serve navigation, communication, aircraft-control, and environment-control systems; and record and process important data to analyze and monitor malfunc­tions and so forth.

With increasing features, the electrical cable length is long and relatively heavy. Multiplexing of data transmission significantly reduces cable weight. Recently, fiber optics have been used for data transmission; when used with a FBW system, it is appropriately termed FBL (see Chapter 12). This section introduces readers to design features (i. e., hardware) that assist in a more accurate prediction of weight and cost.

Most avionics black boxes have microprocessors, which help to standardize con­nections for data flow. The connection of wires is called a bus. Following are the prevailing standards for a bus architecture.

Aircraft Control Subsystem

Chapter 12 discusses the analytical consideration of aircraft motion having six degrees of freedom and its control. Figure 3.8 is a Cartesian representation of the six degrees of freedom, consisting of three linear and three rotational motions. This sec­tion describes the associated control hardware and design considerations (see also [9] and [10]). An aircraft’s control-system weight is about 1 to 2% of the MTOW.

The three axes (i. e., pitch, yaw, and roll) of aircraft control have evolved con­siderably. The use of trim tabs and aerodynamic and mass balances alleviates hinge

moments of the deflecting control surfaces, which reduces a pilot’s workload. Some operational types are as follows:

1. Wire-Pulley Type. This is the basic type. Two wires per axis act as tension cables, moving over low-friction pulleys to pull the control surfaces in each direction. Although there are many well-designed aircraft using this type of mechanism, it requires frequent maintenance to check the tension level and the possible fraying of wire strands. If the pulley has improper tension, the wires can jump out, making the system inoperable. Other associated problems include dirt in the mechanism, the rare occasion of jamming, and the elastic deformation of support structures leading to a loss of tension. Figure 15.19 shows the wire – pulley (i. e., rudder and aileron) and push-pull rod (i. e., H-tail) types of control linkages.

2. Push-Pull-Rod Type. The problems of the wire-pulley type are largely over­come by the use of push-pull rods to move the control surfaces. Designers must ensure that the rods do not buckle under a compressive load. In general, this mechanism is slightly heavier and somewhat more expensive, but it is worth installing for the ease of maintenance. Many aircraft use a combination of push – pull-rod and wire-pulley arrangement (see Figure 15.19).

3. Mechanical Control Linkage Boosted by a Power Control Unit (PCU). As an aircraft size increases, the forces required to move the control surfaces increase to a point where a pilot’s workload exceeds the specified limit. Power assistance by a PCU resolves this problem. However, a problem of using a PCU is that the natural feedback “feel” of control forces is obscured. Therefore, an artificial feel is incorporated for finer adjustment, leading to smoother flights. PCUs are either hydraulic or electric motors driven by linear or rotary actuators (there are several types). Figure 12.16 is supported by a PCU.

4. Electromechanical Control System. In larger aircraft, considerable weight can be saved by replacing mechanical linkages with electrical signals to drive the actuators. Aircraft with FBW use this type of control system (see Figure 12.16). Currently, many aircraft routinely use secondary controls (e. g., high-lift devices, spoilers, and trim tabs) driven by electrically signalled actuators.

5. Optically Signaled Control System. This latest innovation uses an optically sig­naled actuator. Advanced aircraft already have fiber-optic lines to communicate with the control system.

Modern aircraft, especially the combat aircraft control system, have become very sophisticated. A FBW architecture is essential to these complex systems so that aircraft can fly under relaxed stability margins. Enhanced performance require­ments and safety issues have increased the design complexities by incorporating various types of additional control surfaces. Figure 15.20 shows the typical subsonic- transport aircraft-control surfaces.

Figure 15.21 shows the various control surfaces and areas as well as the system retractions required for a three-surface configuration. As shown in the figure, there is more control than what most modern civil aircraft have. Military aircraft control requirements are at a higher level due to the demand for difficult maneuvers and a possible negative stability margin. The F117 is incapable of flying without FBW. Additional controls are the canard, intake-scheduling, and thrust-vectoring devices.

Figure 15.20. Civil aircraft control sur­faces

Fighter aircraft may use stabilators (e. g., the F15) in which the elevators can move differentially to improve roll capability. Stabilators are used collectively for pitch and differentially for roll control. Also, the aileron and rudder can be interconnect­ing. There also can be automatic control that parallels the basic system.

15.9.1 Engine and Fuel Control Subsystemsч

In this section, the engine and fuel control subsystems are addressed together. The engine and fuel/oil control subsystems must have a fire-extinguishing capabil­ity. A better understanding of the engine and fuel/oil control subsystems improves weight – and cost-prediction accuracy. The dry-engine weight supplied by an engine manufacturer is accounted for separately. The earliest aircraft were piston-engine – powered. Piston engines use petrol (i. e., AVGAS). Diesel-powered engines were introduced recently. Figure 15.22 shows a basic fuel system for a small piston – engine-powered aircraft.

Piston Engine Fuel Control System (The total system weight is approximately 1 to 1.5% of the MTOW)

• ignition and starting system

• throttle to control fuel flow

• fuel storage (tank) and flow management: This must incorporate fuel refueling and defueling and venting arrangements. High-wing, smaller aircraft may have

Figure 15.22. Piston engine fuel system

a gravity-fed fuel supply to the engine, but most aircraft use fuel pumps. Aero­batic aircraft should be capable of flying in an inverted position for at least a minute

• mixture control to adjust air-density changes when the altitude changes

• propeller-pitch control (see Chapter 12); smaller aircraft may have a fixed pitch

• engine-cooling system

• engine anti-icing system

• oil system

• fire protection system

• instrumentation and sensor devices

Smaller aircraft can store fuel in the wing. Although a few aircraft store fuel in the fuselage, it is not recommended. Fuel in the fuselage can affect a larger CG shift and, in the case of a crash, the occupants may get doused by leftover fuel. Fuselage fuel tanks are an optional installation in order to increase range.

Gas turbine engine control at the pilot interface is simpler in that it does not require mixture control by a pilot. For turbofans, there are no propellers; hence, there is no pitch control by a pilot. The turbofan engine/fuel control system is described as follows and shown in Figure 15.23.

Turbofan Engine Fuel Control System (The total system weight is approximately 1.5 to 2% of the MTOW)

• ignition and starting system

• throttle to control fuel flow (thrust adjustment); larger jets have thrust reversers

• fuel storage (tank) and flow management: This must incorporate fuel refueling and defueling and venting arrangements. Some combat aircraft need mid-air refueling. In an emergency, aircraft should be able to dump (i. e., jettison) fuel (this is an environmental hazard and is discouraged)

• engine-cooling system

• engine anti-icing system

• oil system

• fire-sensing and protection system

• built-in tests for the fault-detection system; there should be flight and ground crew interface

• instrumentation and sensor devices

Figure 15.23. Turbofan engine fuel control system

Modern military aircraft and commercial aircraft engine control is microprocessor – based and known as FADEC. It is linked with FBW using air data to respond cor­rectly as demanded by a pilot. A typical turbofan and fuel/oil control system (see Figure 15.23) is discussed in more detail in the following paragraphs.

Fuel Storage and Flow Management

The fuel supply to an engine must be made smoothly and accurately. There must be adequate fuel-storage capacity to meet the mission profile with mandatory excess as a reserve. These requirements are an important part of the study during the concep­tual design phase. Fuel management is complex: Fuel weight is a significant percent­age of an aircraft MTOW and consumption from full to empty has the potential for major movement of the CG, affecting the aircraft’s stability. It is important for fuel consumption to be managed for the least shift in the CG. In a demanding situation, this is achieved by an in-flight fuel transfer.

A typical commercial aircraft tank arrangement is shown in Figure 15.23. Stor­age of fuel is located primarily in the cavity of the wing box that extends from the

wing root to close to the wing tip (a tapered wing tip has a lower volumetric capac­ity). Fuel storage in the wing is advantageous because it is close to the aircraft’s CG, which lowers the range of the CG shift. If the volume available in the wing is not adequate, especially for thin-winged combat aircraft, then the fuselage space can be used for storage. Typically, fuel storage in the fuselage can be above or below the floorboards and forward, rearward, and/or at the center of the wing. When there are several tanks, it is convenient to collect fuel at a central location before delivering it to the engines. Fuel from tanks at various locations is pumped into a centrally located collector tank following a transfer schedule that minimizes the CG shift. A symmetrical fuel level in the wings also is important. Note the compartmentaliza – tion of the wing tank; surge tanks are provided at the wing tips and internal baffles restrict fuel-sloshing. Some long-range aircraft have volume available in the stabi­lizer to balance the CG shift through an in-flight fuel transfer.

The fuel-tank arrangement for a thin-winged aircraft (i. e, the supersonic type) is complex because there is insufficient volume available for the mission. Therefore, fuel is provisioned in the fuselage wherever space is available. The Concorde exam­ple, shown in Figure 15.23, carries a substantial amount of fuel and the CG shift is minimized by in-flight balancing through a fuel transfer from the forward and aft trim tanks. The military aircraft fuel-tank arrangement is similar to the Concorde. There can be as many as sixteen tanks, all interconnected to meet the fuel require­ments of the mission.

Fuel tanks can be either rigid, made of metal or composite material, or flexible, made of a rubber-neoprene-like material. Tanks are installed during component assembly. Flexible-tank maintenance requires a change of tanks, which can be a laborious task. Most modern aircraft have wet tanks, in which the skin at the joints is treated with a sealant. A wet-tank system is lighter and more volume-efficient; however, leakage is problematic and these aircraft require strict inspection, espe­cially older aircraft. Sealant technology has improved and wet tanks are favored.

Heat generated in stagnant regions of an aircraft flying faster than Mach 2.4 can be cooled by recirculating cold fuel around the hot zones before being fed to the engine. The preheating of fuel also helps in the combustion process.

Hands-On Throttle and Stick

Other examples of easing a pilot’s workload include the essentials of weapons man­agement and other requirements being incorporated on two controls so that combat pilots can keep their hands on the engine throttle control and the flight control stick. This arrangement of control buttons on the engine throttle and stick is known as HOTAS (see Figure 15.16). The essential control buttons are ergonomically located (Table 15.10). Most modern aircraft have buttons on the flight-control stick for com­munication, trimming, and so forth.

15.8.6 Voice-Operated Control

Voice-operated control (VOC) through voice recognition – still in the development stage – has been installed in advanced combat aircraft. All voice commands also are visually displayed and are very effective for a pilot operating under severe stress, especially if incapacitated by injury.

All of these advancements help a pilot but the systems still require the pilot’s familiarity. Pilots undergo extensive training and practice to gain familiarity with a mass of information in a rather claustrophobic presentation. A pilot’s workload is nearly an inhuman task. They are a special breed of personnel, rigorously trained for years to face the unknown in a life-or-death situation. It is the moral duty of any combat aircraft designer to enhance pilot survivability as best as possible in an integrated manner, embracing all types of technologies.

15.9 Aircraft Systems

Figure 2.1 shows the aircraft-design process in a systems approach. The definition of system is provided in Section 2.2. In that regard, an aircraft can be seen as a system composed of many subsystems. Chart 15.1 illustrates a typical top-level subsystem architecture of aircraft as a system. The subsystems can be designed in separate modules and then integrated with an aircraft.

Together, the system and subsystem mass is 10 to 12% of an aircraft’s MTOM. Typically, this amounts to nearly a quarter of the OEM. Practically all of the items in aircraft subsystems are bought-out. A better understanding of the subsystems improves weight and cost predictions. It is important for good information about subsystem items at the conceptual design stage for better weight and cost estimation. Designers are continually assessing cost versus performance of the subsystems to obtain the best value for the expense.

Aircraft as a System

Engine/Fuel Avionics Electrical Control Black Boxes Power Supply System Subsystems Subsystems

Pneumatic, Hydraulics,

ECS Undercarriage

Chart 15.1. Aircraft as a system

Mechanical systems are connected by direct linkages and pneumatic and hydraulic means. Larger undercarriages are actuated hydraulically.

Head-Up Display

The flight-deck displays shown in Figures 15.16 through 15.18 are on the instrument panel in front of the pilot, who must look down for flight information – more fre­quently in critical situations. When flying close to the ground or chasing a target, however, pilots should keep their head up, looking for external references. This inflicts severe strain on pilots who must frequently alternate the head-up and head – down positions. Engineers have solved the problem to a great extent by projecting the most important flight information (both primary and navigational data) in bright green light on transparent glass mounted in front of the windscreen. With a head – up display (HUD), pilots can see all necessary information without moving their head and, at the same time, they can see through the HUD for external references. Figures 15.17 and 15.18 show a modern HUD.

Initially, a HUD was installed in combat aircraft but the technology recently has trickled into civil aviation as well. HUDs are being installed on most new medium – and large-sized commercial transport aircraft if requested by operators.

15.8.5 Helmet-Mounted Display

Although the HUD has relieved pilots from frequently looking down, the head-up observation is restricted to forward vision only. Military aircraft pilots needed to ease the workload while making a peripheral visual search when the HUD is no longer in the line of sight. Engineers designed a novel device that projects flight information on a helmet-mounted visor. Now pilots can turn their head with all the relevant information still visible on the transparent visor, through which external references can be taken.

Figure 15.18. Typical civil aircraft flight deck

Table 15.10. HOTAS control buttons

On the throttle (left side)

On the stick (right side)




Weapon release




Sensor select


In-flight start


Flight control


Dive brakes

Combat Aircraft Flight Deck

Figure 15.16 shows a typical modern flight deck for military aircraft. Backup ana­log gauges are provided as well as the MFD-type EFIS. The left-hand side is the throttle and the right-hand side is the side-stick controller known as the hands-on throttle and stick (HOTAS) (see Section 15.8.6). The figure indicates which type of data and control a pilot requires. A single pilot’s workload is exceptionally high when computer assistance is required.

data-entry control unit


Flight data EFIS at the left and navigational data EFIS at the right

Figure 15.16. Schematic fighter-aircraft flight deck

15.8.1 Civil Aircraft Flight Deck

An old-type panel with analog dial gauges is shown in Figure 15.17. With two pilots, some of the displays are duplicated, which are deliberate redundancies.

The latest Airbus 380 flight-deck panel replaces myriad gauges by EFISs, which are MFD units. The minimum generic layout of a modern flight-deck panel is shown in Figure 15.18. Numerous redundancies are built into the display with inde­pendent circuits. PFDs, NDs, and SDs have several pages that display significant data.