Pneumatic / Vacuum

Bleed-air sources and distribution, its uses, leak detection, and vacuum systems.

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Main ideas

The pneumatic (bleed-air) system taps hot, high-pressure air from the engine COMPRESSOR (at more than one stage, selected by valves), supplemented by the APU load compressor and a ground air cart.
Bleed air is pressure- and temperature-regulated and fed to a distribution manifold with ISOLATION valves so systems can be separated or one source can feed the whole aircraft.
USES include air conditioning and pressurisation, wing/engine ANTI-ICE, engine STARTING (air-turbine starter), and pressurisation of hydraulic reservoirs and potable water.
A bleed-air LEAK (hot, high-pressure air near structure/wiring) is detected by OVERHEAT sensing loops that warn the crew to shut off the affected duct.
Older/light aircraft use engine-driven VACUUM pumps to run air-driven gyros; suction is regulated and filtered.
⚠ Exam trap: pneumatic air is tapped from the engine COMPRESSOR (bleed) and feeds air conditioning, anti-ice, engine start and pressurisation; a bleed-air LEAK is detected by OVERHEAT loops, not smoke detectors.

Solved examples

How is a hot bleed-air duct leak detected, and why not use a smoke detector?
A bleed-air leak releases hot air, not smoke, so it is detected by overheat sensing — a continuous element or spot sensors routed along the ducts that trigger a warning when the local temperature rises above a threshold. The crew then closes the relevant isolation/bleed valve to stop the leak. A smoke detector would not respond to hot clean air, which is why thermal (overheat) detection is used for ducting.

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