What does the APU indicator light signify in the cockpit?

The APU indicator light in the cockpit signifies the operational status and any malfunctions related to the Auxiliary Power Unit. This light is crucial for pilots and crew as it provides immediate feedback on the functionality of the APU, which is essential for powering electrical systems and providing pneumatic pressure for engine start and air conditioning while on the ground.

When the APU is functioning normally, the indicator light will typically be off. However, if there is a malfunction or the APU is inoperable, the indicator light will illuminate, alerting the crew to check the APU status and necessary systems. Early detection of issues can help prevent further complications during ground operations or engine start procedures.

The other options represent different systems or measurements that are not directly tied to the APU's performance. Flight altitude is indicated by altimeters, engine temperature is monitored by engine gauges, and fuel levels are displayed on fuel quantity indicators, none of which relate to the APU's operational status.