What is the APU max start altitude?

What is the APU max start altitude?

• A. 20,000 ft
• B. 30,000 ft
• C. 40,000 ft
• D. 35,000 ft

Answer: (B)

APU starting capability is limited by ambient air density and the starting system’s ability to spool the turbine to ignition. To ensure a reliable start, there is a defined maximum altitude at which the APU can be started. Once started, the APU can supply power and bleed air up to higher levels, but starting above the limit isn’t permitted. Among the given options, the altitude in the middle represents that maximum start limit. The lower value is below the limit and would still permit a start, while the higher values exceed the limit and would not be allowed.

APU starting capability is limited by ambient air density and the starting system’s ability to spool the turbine to ignition. To ensure a reliable start, there is a defined maximum altitude at which the APU can be started. Once started, the APU can supply power and bleed air up to higher levels, but starting above the limit isn’t permitted. Among the given options, the altitude in the middle represents that maximum start limit. The lower value is below the limit and would still permit a start, while the higher values exceed the limit and would not be allowed.