Pneumatic / Vacuum
Bleed-air sources and distribution, its uses, leak detection, and vacuum systems.
Pneumatic / vacuum, summary notes
- 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.
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.
Pneumatic / vacuum concept map
Pneumatic / Vacuum
Pneumatic / vacuum quiz
Pneumatic / Vacuum, quiz
1. Pneumatic (bleed) air is tapped from the:
Engine compressorExhaustFuel tank2. A use of bleed air is:
Air conditioning, anti-ice and engine startCharging the batteryMeasuring airspeed3. A bleed-air leak is detected by:
Overheat sensing loopsSmoke detectorsFuel gauges4. Isolation valves in the manifold allow:
Systems to be separated or one source to feed allFuel crossfeedGear retraction5. Bleed air is taken from more than one stage to:
Provide the pressure needed at different power settingsCool the fuel by passing bleed air across the fuel oil heat exchanger before combustionAdd thrust, since air taken from the compressor is returned to the exhaust at higher velocity6. On the ground, pneumatic air comes from:
The APU or a ground air cartThe batteryThe RAT7. Bleed air also pressurises the:
Hydraulic reservoirs and potable waterFuel dump systemTyres8. Older/light aircraft use a vacuum system to drive:
Air-driven gyroscopic instrumentsThe gearThe fuel pump