What is the significance of APU EGT in diagnosing APU health?

• A. EGT indicates combustion efficiency and turbine health; abnormal readings point to fuel control, ignition issues, or turbine wear/damage.
• B. EGT has no diagnostic value; only oil pressure matters.
• C. EGT measures ambient air temperature.
• D. EGT is used to measure cabin temperature.

Answer: (A)

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.