Aircraft mechanical systems are operated by either direct mechanical linkages or electric motors or actuators powered by Pneumatic or Hydraulic pressure.Some systems like airconditioning and pressurization systems and anti-icing/de-icing systems use low-pressure pneumatics. Pneumatic pressure is either stored in bottles delivered from ground sources or created by compressors on demand or using pneumatic pumps. Pneumatic was preferred in aircraft because the operating medium air is freely available and return lines are not necessary which in turn reduce the weight. Even though there are few pros for using pneumatic there are more cons like compressed air is generally not suitable for the operation of large capacity components, leaks can be difficult to trace if there is crack or lose fitting. Further, the results of pipeline or component failure can have a very serious outcome.
Pneumatic and vacuum systems in aircraft are further subdivided as:
1.Vacuum System – A system used to supply driving power, in the form of a suction of air, to several gyro instruments, such as the ADI (artificial horizon), HSI (heading indicator) and Turn indicator. It is used as an alternative to electrical power, and is considered as a failsafe system, in that should the electrical power supply to the aircraft systems fail, then the pilot will still have some essential flight instruments available. The vacuum-driven instruments are now used only on light aircraft.
2. High-Pressure Pneumatic Systems –It is powered by engine-driven compressors and are generally fitted on the older types of piston- engine aircraft to operate services such as the landing gear, wing flaps, wheel brakes, radiator shutters and, at a reduced pressure, de-icing shoes. The use of hydraulics has now superseded this method of the power supply. Many modern aircraft also use a smaller system of high-pressure pneumatic air, using air tapped (Bleed air) from the latter stages of the turbine engine compressor. This pneumatic power is for the emergency operation of essential services.
3. Low-Pressure Pneumatic Systems – These are used on most commercial turbine-engine aircraft and are supplied with compressed air tapped from the engine compressor.
Some aircraft systems supplied by engine bleed air are:
- Air conditioning systems (Air cycle packs, Cabin compressors, Freon compressors)
- Air driven hydraulic pumps
- Engine starter motors
- Engine thrust reverser actuators
- Anti-ice or de-icing systems for wing, tail and air inlets
- Cargo heating and ventilation systems
- Galley and Lavatory ventilation systems
- Water tank pressurization systems
- Fuel tank Pressurization
- Hydraulic tank pressurization systems