Final answer:
The PTU (Power Transfer Unit) in aircraft hydraulic systems is inhibited during certain situations to ensure safe operation. It is typically inhibited during engine start-up and when the aircraft is on the ground with the parking brake set.
Step-by-step explanation:
In aircraft hydraulic systems, the PTU (Power Transfer Unit) is a component responsible for transferring hydraulic power between the primary and standby hydraulic systems. The PTU is inhibited in certain situations to ensure safe operation of the aircraft. One common scenario where the PTU is inhibited is during engine start-up, as the engine-driven pump is capable of providing sufficient hydraulic power. Another scenario is when the aircraft is on the ground and the parking brake is set, as the reserve brake system can provide hydraulic power without the need for the PTU.