What happens to a hydraulic variable pitch propeller when it loses oil pressure?

When a hydraulic variable pitch propeller loses oil pressure, the mechanism that controls the pitch of the blades defaults to a specific position due to the nature of the design. In many cases, hydraulic variable pitch propellers are constructed to revert to a fine pitch position when oil pressure is lost. This is primarily a safety feature designed to ensure that the propeller continues to operate in a manner that allows for the most efficient performance and to prevent potential damage to the engine by avoiding excessively coarse pitch, which could lead to overloading.

In this scenario, the system relies on spring tension or other mechanical means to ensure that, without the actuation of hydraulic pressure, the blades will rotate to a position that leads to less resistance and allows the engine to maintain some degree of operation. Full fine pitch is often the default position chosen because it minimizes drag on the engine and can help preserve engine power and reduce the risk of engine failure or stoppage. Therefore, when oil pressure is lost, the pitch indeed moves to a fine position.

Although the other options suggest various scenarios — increased pitch leading to a more coarse setting, propeller stoppage, or establishing a fixed pitch — they do not align with the functional characteristics of hydraulic variable pitch systems designed for safe failure modes.