Double acting feathering system:
To prevent excessive drag in the event of a propeller on engine failure, the double acting propeller as with the single acting propeller requires a feathering system.
The system has mechanical and electro hydraulic means of lifting the PCU control valve. An electrically driven pump supplies the feathering oil pressure. Oil is supplied from a 'Feathering' Reserve in the main engine oil tank.
The control valve is lifted by moving the HP fuel cock lever to 'Feather' or by the piston lift solenoid valve opening and hydraulic pressure acts on the valve lift piston.Oil pressure from the feathering pump is directed to the coarse pitch line to feather the blades. When the feathering is complete a pressure operated switch de-energises the pump-solenoid to stop the pump.To unfeather the HP lever is opened and the feathering pump switched on for a few seconds.
HP fuel cock lever moved to 'feather'. Feathering pump started by Manual Feather Switch. Some systems have the feathering pump switch incorporated in the Engine Fire Handle.
This uses the torque indicating system to sense engine failure.
When torque drops to a certain valve a low torque pressure switch completes a circuit to operate the piston lift solenoid and start the feathering pump.