What is the APU Battery Restart Limit / maximum altitude?

What is the APU Battery Restart Limit / maximum altitude?

• A. 25,000' MSL
• B. 15,000' MSL
• C. 28,000' MSL
• D. 30,000' MSL

Answer: (A)

The key idea is that there’s a ceiling for restarting the APU using the battery because the start sequence needs sufficient air and electrical conditions to reach a self-sustaining state. As you climb, air density drops and the APU’s starter torque from the battery must spin the compressor fast enough for ignition and stable operation. Beyond a certain altitude, even with the battery providing starting current, the APU can’t reliably reach that self-sustaining speed, so a restart isn’t permitted. That altitude limit is 25,000 feet MSL. Keeping within this limit helps ensure the APU can start reliably when needed and deliver power or bleed air as required.

The key idea is that there’s a ceiling for restarting the APU using the battery because the start sequence needs sufficient air and electrical conditions to reach a self-sustaining state. As you climb, air density drops and the APU’s starter torque from the battery must spin the compressor fast enough for ignition and stable operation. Beyond a certain altitude, even with the battery providing starting current, the APU can’t reliably reach that self-sustaining speed, so a restart isn’t permitted.

That altitude limit is 25,000 feet MSL. Keeping within this limit helps ensure the APU can start reliably when needed and deliver power or bleed air as required.