Which of the following is NOT a source of bleed air?

• A. Engine compressor section
• B. APU/GTC
• C. External air source
• D. Hydraulic pump

Answer: (D)

Bleed air comes from sources that generate pressurized air in the pneumatic system. The engine compressor section provides bleed air by tapping off air from its high- or intermediate-pressure stages, which is then conditioned for use in cabin pressurization, environmental controls, and anti-ice. The APU or geared turbine compressor (GTC) can also supply bleed air when the main engines aren’t running, useful for starting engines or providing conditioned air on the ground or in flight. An external air source on the ground can be connected to supply conditioned air to the packs without using the engines. A hydraulic pump, however, is part of the hydraulic system and moves hydraulic fluid under pressure; it does not generate or supply compressed air, so it is not a source of bleed air.