What is the maximum altitude for an APU Start?

What is the maximum altitude for an APU Start?

• A. Flight Level 350
• B. Flight Level 370
• C. Flight Level 390
• D. Flight Level 320

Answer: (B)

Starting an APU relies on having enough ambient air pressure for the APU’s starter and compressor to light off and produce the needed bleed air or electrical power. As you climb, air gets thinner, making it harder for the APU to light off. The system is designed with a ceiling for light-off at about 37,000 feet. So the maximum altitude at which you can initiate an APU start is Flight Level 370 (37,000 feet). Below that, start is possible and the APU can then provide power or bleed air as needed; above that, starting is not supported. The other options are below or above the limit, but the highest permissible altitude for starting is 37,000 feet.

Starting an APU relies on having enough ambient air pressure for the APU’s starter and compressor to light off and produce the needed bleed air or electrical power. As you climb, air gets thinner, making it harder for the APU to light off. The system is designed with a ceiling for light-off at about 37,000 feet. So the maximum altitude at which you can initiate an APU start is Flight Level 370 (37,000 feet). Below that, start is possible and the APU can then provide power or bleed air as needed; above that, starting is not supported. The other options are below or above the limit, but the highest permissible altitude for starting is 37,000 feet.