For cooling purposes, how long should the APU run unloaded before shut down?

Running the APU unloaded for 2 minutes before shutdown is important for several reasons related to cooling and maintenance of the unit. When the APU operates, various components, particularly the engine and associated systems, generate heat during functioning. Allowing the APU to run unloaded for this specified duration helps to ensure that the internal components reach a safe temperature for shutdown.

During the 2-minute cooling period, the APU can bring down the temperature of critical parts, reducing the risk of heat-related damage or excessive wear on the components. This practice helps in preventing conditions such as thermal shock, which can occur if the unit is turned off too quickly while still hot.

Additionally, this cool-down period ensures proper oil circulation, as the oil needs time to cool and return to a proper viscosity for safe operation. The benefits of this practice extend the lifespan of the APU and maintain its reliability for future operations, making it an essential procedure in APU maintenance protocols.