We discussed in the previous article about the types of aircraft pneumatic system. The role of the pneumatic system in aircraft is indispensable and a failure will course serious flight safety consequences. Saying that here we will see more about the low-pressure pneumatic system which is more used in modern commercial jets for the various operations. First Let’s check what are the sources of pneumatic pressure in flight and ground
Pneumatic sources in flight are:
- Engine compressors
- Airborne Auxiliary Power Unit (APU), if designed for bleed air
Pneumatic sources on ground are:
- Mobile starter units or ground pneumatic networks that fixed installed in hangars or on the tarmac.
- APU (Auxiliary Power Unit)
- Engine compressor
Now we will see more about the Engine compressor bleed air system which is used in commercial aircraft. The engine bleed air system consists of the power source (the engine compressors) and the control devices to regulate temperature and pressure during system operation.
As there is a great variation of air output from a turbine engine between idle and maximum power there is a need to maintain a constant supply of air during low power operation as well as restricting excessive pressure and temperature when the engine is at maximum power. The air is ducted from two different stages of the compressor, a low-pressure stage, and a high-pressure stage. The high-pressure control valve is used to determine the source of engine compressor stage. It does when low stage air is insufficient for the pneumatic system to maintain the prescribed flow or temperature requirements, the high-pressure control valve will open and allow high stage air to enter the bleed air ducting, so increasing the engine’s output of bleed air.
As the output of the compressor air depends on engine power, the high-pressure control valve will be open at low power (during idle or descent). As the engine power increases low stage air pressure will increase and close the valve, so in normal cruise flight bleed air will come from the low stage.
The LP check valve prevents reverse flow to the engine low stage compressor from high stage compressor(Shown with an arrow mark in the figure ). The pressure regulating valve controlled electrically shuts-off or regulates the pneumatic supply pressure to a constant value. In case of electrical power loss, the valve opens and regulates a constant pressure.
The fan air valve or Pre cooler control valve regulates the amount of fan cooling airflow via the heat exchanger to control the required pneumatic temperature. The valve is normally pneumatically actuated with control pressure from a thermostat or from an electrically controlled torque motor, depending on the aircraft type.
The bleed air is normally supplied from the engine low-pressure compressor stage to minimize the engine fuel consumption. During low engine speed, if the low stage pressure and the temperature is insufficient, the air is supplied from the high-pressure compressor stage. Normally the system is controlled and monitored by a Pneumatic System Controller (PSC) or a Bleed Monitoring Computer (BMC). The controller or computer receives all the necessary input signals from the engine pneumatic system sensors, valves and bleed air control switches to monitor the system or shut off the system in case of dangerous conditions.
We will see the operation with respect to operating pressure and temperature in the next article.