Is it permissible to have frost on the APU inlet screen?

Frost on the APU (Auxiliary Power Unit) inlet screen is generally permissible under specific circumstances. When the APU is operating, it pulls in ambient air for combustion and cooling, which can sometimes lead to the formation of frost, particularly in cold weather conditions. The APU is designed to function properly even with a certain amount of frost present, as it typically can still draw sufficient air flow for optimal performance.

However, it is crucial to monitor the amount of frost because excessive accumulation could impede airflow, which in turn can affect the efficiency and functionality of the APU. Proper maintenance protocols should be followed to ensure that the screen does not become blocked, but a small amount of frost does not necessarily indicate a problem, making it permissible if it is within limits that still allow for safe operation of the APU.