How does the APU contribute to cross-bleed starting of other engines?

How does the APU contribute to cross-bleed starting of other engines?

• A. It powers hydraulic systems
• B. It starts engines without bleed air
• C. The APU provides conditioned bleed air at sufficient pressure/temperature to facilitate starting other engines via cross-bleed lines
• D. It only reduces engine RPM

Answer: (C)

Cross-bleed starting relies on a source of high-pressure bleed air to drive the air turbine starter of another engine. The APU, when running, provides conditioned bleed air at the necessary pressure and temperature and routes it through the cross-bleed lines to the starting engine’s pneumatic starter. This air spins the engine core, enabling ignition and fuel flow to bring the engine up to idle. The APU is used specifically to supply that air when the main engines aren’t yet started or their bleed systems aren’t available. Other options don’t describe this mechanism: one is about hydraulic power, which isn’t how cross-bleed starting works; another claims starting without bleed air, which defeats the whole purpose of cross-bleed; and another notes only a reduction in RPM, which isn’t how starting initiates the engine.

Cross-bleed starting relies on a source of high-pressure bleed air to drive the air turbine starter of another engine. The APU, when running, provides conditioned bleed air at the necessary pressure and temperature and routes it through the cross-bleed lines to the starting engine’s pneumatic starter. This air spins the engine core, enabling ignition and fuel flow to bring the engine up to idle. The APU is used specifically to supply that air when the main engines aren’t yet started or their bleed systems aren’t available. Other options don’t describe this mechanism: one is about hydraulic power, which isn’t how cross-bleed starting works; another claims starting without bleed air, which defeats the whole purpose of cross-bleed; and another notes only a reduction in RPM, which isn’t how starting initiates the engine.