What is the maximum temperature for fuel entering APU or engines?

• A. 59 °C
• B. 62 °C
• C. 57 °C
• D. 60 °C

Answer: (A)

Fuel entering the APU or engines is limited to a temperature well below the point where fuel properties and the hardware can be affected. If the fuel gets too hot, its viscosity drops and the pumps, lines, and fuel-control hardware can operate less reliably, risking improper atomization, cavitation, or stress on components. Keeping the entering fuel temperature within a strict limit helps maintain consistent engine performance and protects the APU/fuel system from thermal stress.

The limit used is 59 °C. This provides a safety margin below the primary design limit (60 °C) to account for measurement tolerances and sensor lag. In practical terms, if you see temperatures approaching 60 °C, corrective action is needed to prevent exceeding the limit.