14 CFR § 25.1027

If the propeller feathering system depends on engine oil, there must be means to trap an amount of oil in the tank if the supply becomes depleted due to failure of any part of the lubricating system other than the tank itself.

The amount of trapped oil must be enough to accomplish the feathering operation and must be available only to the feathering pump.

The ability of the system to accomplish feathering with the trapped oil must be shown. This may be done on the ground using an auxiliary source of oil for lubricating the engine during operation.

Provision must be made to prevent sludge or other foreign matter from affecting the safe operation of the propeller feathering system.