In soft alternate mode, both indicators on the engine panel light up to show two linked pieces of information: the engine is on (ON) and the alternate path is active (ALTN). This dual indication helps the crew confirm that the engine is running while the system is using its backup/alternate configuration. If only the ON light were visible, you wouldn’t know that the alternate path is engaged; if only the ALTN light were visible, you’d know the alternate mode is selected but not that the engine is currently on. If neither light is on, there’s no clear status of either condition. So seeing both ON and ALTN together is the correct, informative state in soft alternate mode.

Permissives are the prestart checks that must be true before the APU start command is allowed, to ensure a safe and reliable start. The essential conditions are that electrical power is available to drive the start motor and control circuitry, the APU bay doors are closed so there’s no risk of debris ingress and proper enclosure/airflow, and there are no faults detected in the system that would indicate a problem. When these three are satisfied, the system can initiate the start sequence without risking damage or unsafe operation. Weather information or fuel level for the APU isn’t part of this specific gating logic, and the pilot’s mood has no bearing on the start process.

Exhaust gas temperature (EGT) is a direct indicator of how hot the combustion process is and how well the turbine is handling that heat. When the APU is operating normally, the exhaust temp stays within a predictable range for the given power setting. If the readings rise beyond the expected range, it points to issues in the fuel control, ignition, or turbine condition—things like an improper fuel-to-air ratio, mis-ignition, or worn turbine components that cause more fuel to burn hotter or less efficient expansion. Persistent abnormal EGT can signal potential damage or wear in the turbine, which disrupts efficiency and raises exhaust heat. In practice, technicians monitor EGT alongside RPM and fuel flow to diagnose APU health because EGT directly reflects combustion efficiency and turbine condition. Choices that reference ambient or cabin temperature don’t relate to the APU’s exhaust heat and aren’t diagnostic of APU health.

When the EEC detects a flameout, it automatically attempts a relight by energizing both igniters. Having two igniters provides redundancy and increases the chance of a successful re-ignition under varying conditions, such as lean mixtures or unstable flames. Using only one igniter could fail if that unit has a fault or isn’t producing enough energy, so energizing both improves reliability and speeds restoration of the flame. The system continues to monitor engine parameters and will retry as needed until the engine stabilizes or safety limits dictate otherwise.