What is the maximum operational temperature for the APU?

The correct response regarding the maximum operational temperature for the Auxiliary Power Unit (APU) is indeed the option that states ISA + 39 Degrees C. This temperature indicates the upper limit at which the APU can function efficiently while maintaining its performance and reliability standards. Exceeding this temperature can lead to operational inefficiencies or potential system failures, which could compromise safety and reliability.

The International Standard Atmosphere (ISA) provides a baseline for understanding how environmental conditions affect aircraft performance, including APUs. The selection of 39 Degrees C above ISA not only reflects operational safety directives but also embraces manufacturer specifications that ensure the APU operates within a safe thermal envelope. Operating beyond this specified temperature can lead to thermal stress and component degradation, ultimately affecting the longevity and functionality of the APU.

In contrast, the other temperature options imply higher operational limits, which may not align with established safety practices or technical specifications.