What is the wait time before checking the oil level on the APU after shutdown?

The correct wait time before checking the oil level on the Auxiliary Power Unit (APU) after shutdown is 1 hour. This period is critical because it allows the oil to settle in the engine and fully drain back into the sump. Immediately after shutdown, the APU components are still hot, and oil may be temporarily circulated or not yet fully settled, which can give an inaccurate reading. By waiting 1 hour, you ensure that the oil level reflects the true capacity of the APU's oil system, allowing for an accurate assessment of whether the oil level is adequate for proper operation. This practice helps prevent potential issues that could arise from incorrect oil levels, ensuring the APU operates efficiently and safely during flight operations.