In below freezing temperatures, what is the minimum preheating time for the APU?

In below freezing temperatures, the correct answer regarding the minimum preheating time for the Auxiliary Power Unit (APU) is 35 minutes. This duration is essential to ensure that the APU components reach an optimal operating temperature, which helps in preventing engine start issues and ensures reliable performance. Preheating the APU is critical because cold temperatures can cause oil and other vital fluids to thicken, leading to inadequate lubrication and potential damage if the system starts without proper warming.

When preparing to operate the APU in freezing conditions, extending the preheating time minimizes wear and stress on the mechanical components and ensures that the electronics and ignition systems function properly. Using a timeframe shorter than 35 minutes can result in insufficient warming of the system, possibly leading to operational failures and safety hazards.