What happens to APU operation above 20,000 feet?

When the APU (Auxiliary Power Unit) operates above 20,000 feet, it becomes unavailable for use. This limitation is due to the APU's design and the need for specific environmental conditions to function effectively. At higher altitudes, the APU may not be able to achieve the required performance levels, such as producing sufficient bleed air or electrical output.

This restriction ensures that the aircraft relies on its main engines and other systems for power and air supply when cruising at elevated altitudes. Understanding this operational limit is crucial for pilots and crew members, as it impacts the management of onboard systems, especially during the transition from ground operations to flight.