Which statement describes the altitude dependence of the APU generator amperage?

Which statement describes the altitude dependence of the APU generator amperage?

• A. It remains constant at 400 amps at all altitudes.
• B. It increases above 30,000 ft.
• C. It is not affected by altitude.
• D. It decreases to 300 amps above 30,000 ft.

Answer: (D)

Altitude affects how much current the APU generator can safely deliver because cooling is less effective as air density drops. The APU and its generator have thermal limits, and at higher altitudes the thinner air makes it harder to dissipate heat. To protect the unit and keep voltage and frequency stable under load, the generator is de-rated, typically down to about 300 amps above 30,000 ft. At lower altitudes, where cooling is better, the APU can supply more current up to its higher limit. So the idea is that the APU doesn’t stay at a constant maximum current with altitude; it reduces output at higher altitudes to avoid overheating and maintain stable electrical performance.

Altitude affects how much current the APU generator can safely deliver because cooling is less effective as air density drops. The APU and its generator have thermal limits, and at higher altitudes the thinner air makes it harder to dissipate heat. To protect the unit and keep voltage and frequency stable under load, the generator is de-rated, typically down to about 300 amps above 30,000 ft. At lower altitudes, where cooling is better, the APU can supply more current up to its higher limit.

So the idea is that the APU doesn’t stay at a constant maximum current with altitude; it reduces output at higher altitudes to avoid overheating and maintain stable electrical performance.