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

• A. 10,000 ft
• B. 15,000 ft
• C. 21,000 ft
• D. 25,000 ft

Answer: (C)

Using APU bleed air to assist the starter relies on delivering enough high‑pressure pneumatic power to spin the engine up to ignition speed. As you climb, ambient pressure and air density fall, and the APU bleed system can’t maintain the same pressure and volume. That reduces the starter’s ability to turn the engine fast enough, so there’s a ceiling at which this method becomes unreliable. For this system, the maximum altitude is twenty-one thousand feet. Below that, the bleed air can usually drive the starter effectively; above it, the supply isn’t sufficient to guarantee a reliable start.