AIRCRAFT PNEUMATIC SYSTEMS

It employs air instead of a liquid for transmitting power. Pneumatic systems are sometimes used for: • Brakes • Opening and closing doors • Driving hydraulic pumps, alternators, starters, water injection pumps, etc. • Operating emergency devices

Both pneumatic and hydraulic systems are similar units and use confined fluids. Liquids are practically incompressible; a quart of water still occupies about a quart of space regardless of how hard it is compressed. Gases are highly compressible Pneumatic System Components do not utilize reservoirs, hand pumps, accumulators, regulators, or engine-driven or electrically driven power pumps for building normal pressure. But similarities do exist in some components.

Air Compressors On some aircraft, permanently installed air compressors have been added to recharge air bottles whenever pressure is used for operating a unit. Relief Valves Relief valves are used in pneumatic systems to prevent damage. They act as pressure limiting units and prevent excessive pressures from bursting lines and blowing out seals. Control Valves Control valves are also a necessary part of a typical pneumatic system. Check Valves Check valves are used in both hydraulic and pneumatic systems.

Restrictors Restrictors are a type of control valve used in pneumatic systems. The small outlet port reduces the rate of airflow and the speed of operation of an actuating unit. Filters Pneumatic systems are protected against dirt by means of various types of filters. 1. Micronic filter 2. Screen-type filter

Desiccant/Moisture Separator The moisture separator in a pneumatic system is always located downstream of the compressor. Its purpose is to remove any moisture caused by the compressor. A complete moisture separator consists of a reservoir, a pressure switch, a dump valve, and a check valve. Chemical Drier Chemical driers are incorporated at various locations in a pneumatic system. Their purpose is to absorb any moisture that may collect in the lines and other parts of the system. Each drier contains a cartridge that should be blue in color.

Types of Typical air pressure system High pressure system Medium pressure system Low pressure system

High pressure system For high pressure systems air is usually stored in metal bottles Pressure ranging from 1000 to 3000psi Has two valves Charging valve and Ground operated compressor

Medium pressure system Pressure ranging from 100 to 150psi Does not include air bottles Draws air from jet engine compressor section Low pressure system Pressure ranging less than 100 psi Used in reciprocating types of aircraft Driven by electric motors or by aircraft engine