How long should the APU be warmed up before applying an air load?

The recommendation to warm up the Auxiliary Power Unit (APU) for a duration of 2 minutes before applying an air load is based on the need to ensure that the APU reaches a stable operational temperature and pressure. During this time, the APU's components can properly lubricate and ensure efficient operation.

Warming up for too short a duration may not allow the necessary systems to stabilize, potentially leading to performance issues or undue stress on the engine. The 2-minute warm-up period strikes a balance between ensuring the APU is ready for reliable operation and maintaining operational efficiency. This timeframe is particularly critical in aviation applications where reliability and performance are paramount for safety and operational readiness.