Every aircraft carries systems that run on air. On a piston aeroplane the demand is modest and a small pump covers it; on a jet, compressed air tapped from the engines heats, pressurises, de-ices and starts things, and the plumbing that distributes it is a system in its own right, with valves, coolers, sensors and a famous failure mode. This lesson follows the supply side; the next lesson spends the air on the cabin.
Air for piston aeroplanes
A piston aeroplane's pneumatic needs are served by an engine-driven compressor, or a vacuum pump (the same device working by suction rather than pressure). An air supply is required for instrumentation, heating and de-icing:
instrumentation — the classic air-driven gyroscopic instruments spin on pump suction;
heating — cabin heat is commonly taken from air warmed around the exhaust;
de-icing — the boots inflate on pump pressure.
Small demands, small machine: one pump, a regulator, and a filter cover the fleet's light end.
Bleed sources on the turbine aircraft
A gas turbine engine is, among its other jobs, a magnificent air compressor, and the jet's pneumatic system taps it. The possible bleed-air sources:
The engines — compressor air in flight;
The APU — the auxiliary power unit's dedicated compressor on the ground (and, within limits, in flight);
A ground supply — a ground cart plugged into the same ducting when nothing on board is running.
The users read like the aircraft's utility bill:
ice protection — hot bleed air fed through wing leading edges and engine intakes (the next module's subject);
the engine air starter — the pneumatic starter motor that spins a big engine to life, fed from the APU, the other engine or the ground cart;
pressurisation of hydraulic reservoirs — the positive head that keeps the hydraulic module's pumps from cavitating at altitude;
air-driven hydraulic pumps — the pneumatic members of that module's pump inventory;
pressurisation and air conditioning — the biggest customer by far, and the whole of the next lesson.
The plumbing
The bleed-air supply system comprises a short list of components, each earning its place:
Pneumatic ducts — the distribution arteries, running hot, high-pressure air through wing roots and belly fairings; their routing is why a duct leak is treated so seriously.
Engine bleed valves (HP/IP valves) — the engine offers air at two tappings: an intermediate-pressure (IP) port that suffices at high engine power, and a high-pressure (HP) port whose valve opens when the engine is throttled back and IP air alone is too weak. The system always takes the cheapest air that will do.
Pressure-regulating valve — throttles whatever the engine offers down to the duct's working pressure, protecting everything downstream from the compressor's enthusiasm.
Fan-air pre-cooler — a heat exchanger blowing cold fan air across the hot bleed, cooling it to duct-safe temperature before it leaves the pylon.
Isolation valve — the crossbleed gate in the middle of the duct: open, either engine (or the APU) can feed the whole aircraft; closed, the two sides are independent, which is how a leaking side is quarantined.
Temperature and pressure sensors — the instrumentation the control and monitoring below depend on.