If an IDG BUS EICAS message appears at cruise, below what altitude must the APU be started?

The correct answer is based on operational protocols for the aircraft regarding the Integrated Drive Generator (IDG) Bus failure. If an IDG BUS EICAS message appears while cruising, it indicates a loss of electrical power generated by one of the IDGs, which can compromise aircraft systems that depend on that power.

Starting the Auxiliary Power Unit (APU) becomes necessary to ensure continued electrical power supply and redundancy for essential systems. The requirement to start the APU is specifically related to maintaining safe operational standards and ensuring that power is available in case of emergencies.

The altitude threshold for starting the APU in this scenario is set at 30,000 feet, as this altitude marks a critical point where the aircraft must have alternative means of power available due to reduced engine efficiency and the potential for increased electrical load. Therefore, below this altitude, it is essential to initiate APU operations promptly to mitigate any risks associated with electrical failure and to maintain aircraft safety.