Which condition is a listed APU starting limitation related to altitude?

Which condition is a listed APU starting limitation related to altitude?

• A. Below 20,000 ft
• B. Above 20,000 ft
• C. Below 30,000 ft
• D. Below 15,000 ft

Answer: (A)

Starting the APU is restricted by altitude because the APU needs sufficient air density and a reliable power source to spool up and begin delivering bleed air and electrical power. Below 20,000 feet the air is dense enough for a confident start, so the procedure allows APU start in that range. Once you’re above 20,000 feet, starting the APU becomes unreliable, which is why the listed limit specifies you may start only below that altitude. The other thresholds don’t match the documented rule: they imply different altitude boundaries that aren’t the published limitation.

Starting the APU is restricted by altitude because the APU needs sufficient air density and a reliable power source to spool up and begin delivering bleed air and electrical power. Below 20,000 feet the air is dense enough for a confident start, so the procedure allows APU start in that range. Once you’re above 20,000 feet, starting the APU becomes unreliable, which is why the listed limit specifies you may start only below that altitude. The other thresholds don’t match the documented rule: they imply different altitude boundaries that aren’t the published limitation.