1.6 MEDIUM PRESSURE PNEUMATIC SYSTEM

MEDIUM PRESSURE PNEUMATIC SYSTEM

1.6.1 General: 

As said earlier, a medium pressure pneumatic system is used on modern wide bodied commercial aircraft. Such aircraft is designed with a very sophisticated pneumatic system that ensures pneumatic supply as source of thermal energy and pressure energy to be used in/as:

v     Environment Control System (ECS): Aircraft pressurization and air-conditioning

v     Wing anti-icing

v     Cargo heat and ventilation

v     Avionics cooling and ventilation

v     Pressurization of hydraulic reservoir

v     Back up to hydraulic system for emergency operation of hydraulically driven components/systems

1.6.2 Supply sources: Medium pressure pneumatic system employs a pneumatic manifold system as illustrated in Figures 1.6 used in typical aircrafts. Such systems may be pressurized from the following sources for pneumatic:

v     Engine Bleed Air

v     APU Bleed Air

v     Ground Pneumatic Cart

Engine Bleed Air: Aircraft pneumatic system may take engine bleed air as a primary source. This is the great advantage to have pressurized air from engine without using air compressor as a separate unit. Air is bled off from convenient stage or stages and routed through suitable ducts to the aircraft pneumatic system. See examples in Figures 1.6 and 1.7.

  

Figure 1.6: Pneumatic sources on a medium pressure pneumatic manifold system (Typical)

Figure 1.7: Engine bleed sources (medium pressure pneumatic system)

APU Bleed Air: Modern aircraft's are generally provided with Auxiliary power unit, which is nothing but a small gas turbine engine. APU is used as alternate and emergency source for air supply to the pneumatic system if aircraft engine fails or not running.

Ground Cart: Ground cart can be used as a source of compressed air on the ground only; when aircraft engine or APU is not running. Air can be supplied from the ground source directly to the aircraft pneumatic ducts from where different systems, sub-systems or components can be operated on the ground.

1.6.3 Pneumatic Systems for Air Conditioning and Ice Control: Some of the large jet aircraft use compressor bleed air from the engines to provide a relatively large volume of compressed air at a low pressure to heat the leading edge of the wing and prevent the formation of ice, to provide air for starting the engine, and for pressurizing and controlling the temperature of the air in the aircraft cabin. Such systems are pressurized only when engines are operating. See illustration in Figure 1.8.

Figure 1.8: Pneumatic system of a jet transport aircraft, using compressor bleed air to operate the air con­ditioning and ice control systems