Starter assist MES using APU bleed air: what is the maximum altitude?

• A. 12,000 ft
• B. 18,000 ft
• C. 21,000 ft
• D. 24,000 ft

Answer: (C)

Using APU bleed air to assist the starter relies on delivering a stable, high-pressure air stream to spin the engine’s start mechanism. As you climb, the bleed-air pressure available from the APU decreases because the compressor and system have to operate against thinner air and are limited by their design. The engine start cycle needs a minimum pressure and flow to turn the compressor and reach a self-sustaining speed. Once you reach about 21,000 ft, the bleed-air pressure is no longer enough to meet those requirements, so start assist with APU bleed air is no longer reliable. That makes 21,000 ft the practical maximum altitude for this method.