At what temperature does APU's maximum start time increase to 60 seconds?

The maximum start time for the APU increases to 60 seconds when the temperature drops below 0°F. This is due to the need for the APU to expend additional effort to start in colder conditions, which can potentially lead to issues with fuel ignition and engine performance. The lower temperature affects the viscosity of the fuel, the battery performance, and the overall efficiency of the starting system, which necessitates a longer start time to ensure that the APU can successfully initiate operation without compromising reliability.

Starting an APU in colder temperatures presents challenges that require consideration of both time and operational limits. The design of the APU and its components accounts for environmental factors, and the specifications include longer start times as a standard safety measure in these conditions.