APU maximum starting altitude is?

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Multiple Choice

APU maximum starting altitude is?

Explanation:
Starting an APU relies on pulling in enough air and delivering fuel and ignition in a stable way. As you climb, air density drops, so the compressor can’t generate the necessary flow to reach and sustain ignition and acceleration. That limits the altitude at which the APU can be started. For most aircraft, this start envelope tops out around 37,000 feet, or FL370, which is why FL370 is the correct limit. Starting above this altitude would typically fail due to insufficient airflow and compressor stability.

Starting an APU relies on pulling in enough air and delivering fuel and ignition in a stable way. As you climb, air density drops, so the compressor can’t generate the necessary flow to reach and sustain ignition and acceleration. That limits the altitude at which the APU can be started. For most aircraft, this start envelope tops out around 37,000 feet, or FL370, which is why FL370 is the correct limit. Starting above this altitude would typically fail due to insufficient airflow and compressor stability.

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