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.

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