Maximum altitude for starting the APU?

Maximum altitude for starting the APU?

• A. FL320
• B. FL310
• C. FL350
• D. FL330

Answer: (B)

The key idea is how high the APU can reliably start given the need for sufficient bleed air and compressor performance. To start the APU and supply bleed air for engine starts and essential systems, there must be enough intake air for the APU’s compressor to reach the speeds and pressures required. As you climb, air density falls, making it harder for the APU to develop that pressure. For the Citation Longitude, this reliability threshold is set at 31,000 feet. Above this altitude, the APU is not guaranteed to start or to produce the needed bleed air, so the permitted start altitude is limited to 31,000 feet.

The key idea is how high the APU can reliably start given the need for sufficient bleed air and compressor performance. To start the APU and supply bleed air for engine starts and essential systems, there must be enough intake air for the APU’s compressor to reach the speeds and pressures required. As you climb, air density falls, making it harder for the APU to develop that pressure. For the Citation Longitude, this reliability threshold is set at 31,000 feet. Above this altitude, the APU is not guaranteed to start or to produce the needed bleed air, so the permitted start altitude is limited to 31,000 feet.