What altitude limit applies to the APU when using just electrics?

The correct altitude limit for the Auxiliary Power Unit (APU) when using just electrics is 41,000 ft. The APU is designed to provide electrical power and bleed air for starting the engines and supplying aircraft systems when the main engines are not operating or during ground handling activities.

The engineers establish an altitude limit for the APU to ensure it operates efficiently and effectively under various flight conditions. At high altitudes, the performance characteristics of the APU change due to reduced air density, and the operational limits are critical for aircraft safety and performance.

In this case, 41,000 ft is the maximum altitude at which the APU can reliably supply electrical power to the aircraft systems without compromising performance. This altitude ensures that the APU operates within its design specifications, maintaining sufficient capability to support the aircraft's electrical needs.

Other altitude limits mentioned do not apply to the APU's electrical operation capabilities at high altitudes, which is why they are not the correct answer. The performance of the APU's electrical output varies with altitude, making the maximum permissible altitude of 41,000 ft an important parameter for pilots and maintenance personnel to understand and follow.