Which of the following is a typical indicator that the APU is in start or run mode?

• A. Altimeter reading
• B. N2 gauge, EGT readout, APU start/operating lights, and audible turbine sounds
• C. Cabin temperature
• D. Gear position indicator

Answer: (B)

APU start or run status is best read from indicators that reflect the engine’s actual operating state: rotor speed, turbine temperature, status lights, and the audible sound of the turbine. When the APU is starting or running, the N2 gauge climbs to the normal operating range, the EGT rises as combustion begins, the start/operating lights come on to confirm the APU is in that mode, and you can hear the turbine spool and associated airflow. These signals together confirm that the APU is actually spinning, generating heat, and delivering power.

An altimeter reading is related to aircraft altitude, not the APU’s state. Cabin temperature depends on the environmental control system and can change for reasons unrelated to APU status. The gear position indicator shows the landing gear state, which has no direct bearing on whether the APU is starting or running.