What is the consequence of not allowing the APU appropriate warm-up time before applying an air load?

Allowing the Auxiliary Power Unit (APU) to warm up properly before applying an air load is crucial for maintaining the integrity and longevity of its components. When an APU is started, its internal components—including the engine, fuel system, and control systems—need to reach optimal operating temperatures to function effectively. If an air load is applied too early, these components might be subjected to excessive stress, leading to potential overheating or mechanical failure.

The risk of damage arises because the APU's lubrication system may not be fully effective until the unit has warmed up, which can lead to increased friction and wear on moving parts. Additionally, operating under load before the APU is ready can create imbalances that might strain the engine and its allied systems, potentially leading to premature wear or catastrophic failure.

Thus, it is critical to follow recommended warm-up procedures to ensure that the APU functions safely and efficiently, minimizing the likelihood of component damage that could result in costly repairs or replacements.