What is the minimum warm-up time required for the APU before applying a bleed air load?

The minimum warm-up time required for the Auxiliary Power Unit (APU) before applying a bleed air load is typically one minute, plus an additional three minutes if the APU is cold. This procedure ensures that the APU reaches optimal operating conditions and pressures effectively, which is crucial for maintaining system integrity and preventing potential damage.

When the APU is cold, it requires extra time to stabilize and achieve the necessary operating temperatures and pressures before it can handle the demands of supplying bleed air. The initial minute allows for some warming, while the following three minutes provide additional time to ensure that the APU performs efficiently and reliably under the increased load. Therefore, this structured warm-up period is essential for safe and effective operation of the APU in various scenarios.