What is the maximum altitude for APU engine start?

What is the maximum altitude for APU engine start?

• A. 12,000 ft
• B. 21,000 ft
• C. 18,000 ft
• D. 24,000 ft

Answer: (B)

APU start altitude is limited by the APU’s ability to produce enough starter power and bleed air as air density falls with altitude. The ERJ 175 used by SkyWest is certified to start the APU up to 21,000 feet. That means you must perform the APU start at or below 21,000 ft; above this, the APU is not within its certified envelope to light off reliably, so a start isn’t approved or guaranteed. The other options don’t fit the certification: 24,000 ft is above the limit, while 12,000 and 18,000 ft are below the maximum but not the enforced ceiling.

APU start altitude is limited by the APU’s ability to produce enough starter power and bleed air as air density falls with altitude. The ERJ 175 used by SkyWest is certified to start the APU up to 21,000 feet. That means you must perform the APU start at or below 21,000 ft; above this, the APU is not within its certified envelope to light off reliably, so a start isn’t approved or guaranteed. The other options don’t fit the certification: 24,000 ft is above the limit, while 12,000 and 18,000 ft are below the maximum but not the enforced ceiling.