Up to what altitude can the APU be started using normal electrics?

The APU (Auxiliary Power Unit) of the A320 can be started using normal electrics up to an altitude of 39,000 feet. This capability is crucial for providing electrical power and bleed air for air conditioning and engine start when the aircraft is at high altitudes. The design allows continued use of the APU during flight, which is particularly valuable for maintaining operations when more significant power or air supply is needed on the aircraft, especially in case of main engine failure or in an emergency.

Starting the APU at higher altitudes ensures that the crew has the necessary resources available to manage aircraft systems effectively, enhancing safety and operational flexibility. Hence, the limit is set at 39,000 feet, allowing operation within a wide flight envelope where most typical operations occur.