For the APU, what is the significance of the 'ISA + 39 degrees C'?

The significance of 'ISA + 39 degrees C' in relation to the Auxiliary Power Unit (APU) refers to the maximum operational temperature. This designation is based on the International Standard Atmosphere (ISA), which serves as a baseline for defining ambient temperature conditions at various altitudes. By adding 39 degrees Celsius to the standard atmosphere temperature, it helps to establish the upper limit for safe operation of the APU.

In practical terms, this means that the APU can operate effectively within this temperature range without exceeding its design specifications or risking component damage. Monitoring the temperature in relation to this standard ensures that the APU functions optimally and reliably, especially in warmer conditions where performance may be compromised.

Understanding this specification is crucial for maintaining the APU's operational integrity and for ensuring that it remains within safe operating parameters during use.