What happens if the APU is operated under certain conditions with the aircraft on jacks?

When the Auxiliary Power Unit (APU) is operated under specific conditions with the aircraft on jacks, the behavior of the oil pressure switch is critical to understand. In this context, the oil pressure switch is designed to provide protection against low oil pressure situations. However, when the aircraft is on jacks, certain parameters may prevent this protection mechanism from activating.

Operating the APU while the aircraft is on jacks may lead to an artificial indication of oil pressure, meaning that the low oil pressure protection feature may be overridden. This situation arises because the oil system may not be able to adequately circulate oil as it would during normal flight conditions. Therefore, the oil pressure switch does not function as intended, allowing the APU to operate under potentially unsafe conditions without the normal safeguards that would usually trigger a shutdown or warning.

Understanding this behavior is crucial for maintenance and operational safety, as it highlights the importance of ensuring that safety systems are fully engaged and functioning correctly before operating the APU under any non-standard conditions, such as when the aircraft is on jacks. This emphasizes the critical nature of aircraft systems and their interdependencies during different operational scenarios.