Which systems are commonly fed by APU bleed air in a typical commercial transport aircraft?

• A. Electrical generation and hydraulic pumps
• B. Cabin lighting and navigation systems
• C. Fuel system and lubrication system
• D. Environmental control system (air conditioning and cabin pressurization), aircraft anti-ice systems, and starting air for engines (via cross-bleed/start sequences)

Answer: (D)

APU bleed air powers the aircraft's pneumatic systems when the main engines aren’t providing bleed air. This high‑pressure air is used to run the environmental control system, which includes the packs that condition cabin air and support cabin pressurization. It also feeds the anti-ice system to protect surfaces like the wings and nacelles from ice. In addition, bleed air is used to start engines through cross-bleed or start sequences, providing the motive air to the engine’s starter until the engine can run on its own. The other options involve electrical or hydraulic systems rather than pneumatics; electrical generation and hydraulic pumps are driven by electrical power or hydraulic pumps, while cabin lighting and navigation are electrical, and fuel or lubrication systems are not pneumatic.