14 CFR 23.1027 - Propeller feathering system.
(a) If the propeller feathering system uses engine oil and that oil supply can become depleted due to failure of any part of the oil system, a means must be incorporated to reserve enough oil to operate the feathering system.
(b) The amount of reserved oil must be enough to accomplish feathering and must be available only to the feathering pump.
(c) The ability of the system to accomplish feathering with the reserved oil must be shown.
(d) Provision must be made to prevent sludge or other foreign matter from affecting the safe operation of the propeller feathering system.