The weight of the hydraulics system depends on the load requirements. Together with the pneumatic system, the hydropneumatic system can be 0.4 to 1% for civil aircraft and 0.5 to 2% for military aircraft.

15.9.7 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.

Figure 15.33. Military aircraft oxygen system

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 perfo-

rated 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