Which indicator must be monitored at all times during APU operation?

Monitoring the bleed air pressure indicator is crucial during APU (Auxiliary Power Unit) operation because it directly reflects the performance and health of the APU. The APU is responsible for providing bleed air for starting the main engines and for environmental control systems like cabin heating and cooling.

If the bleed air pressure is too low, it could indicate that the APU is not functioning properly, which may lead to insufficient air supply for engine starting or environmental controls. Ensuring that this indicator stays within the appropriate range helps maintain system reliability and operational readiness.

While the other indicators provide important information about fuel pressure, engine temperature, and battery voltage, their monitoring is typically secondary to ensuring that bleed air is being efficiently generated and supplied, as this is essential for the APU’s core functions.