Originally Posted by 411A
...The PT6-20/27/28 types...[in case of] loss of engine oil quantity, the propeller will feather. I know, because, I've had it happen twice on BE99 aircraft.
I can attest that what 411A has written is correct. In any case of a catastrophic loss of oil quantity (or a loss of oil pressure that interrupts the supply of oil to the propeller governor) in a small PT6A engine, the propeller will feather.
The reason for this is that propeller blade angle on the small PT6A engines is controlled by a propeller governor that uses oil under pressure to move the propeller blades out of the feather position, and the force created by that oil under pressure is opposed by springs and counterweights within/on the propeller hub that act to move the propeller towards the feather position. In any case of loss of supply of oil to the propeller governor, the propeller will feather. By way of example, autofeather systems on PT6A engines operate by dumping oil from the propeller hub.
I have only experienced one complete loss of oil quantity while operating a PT6A powered aircraft. The loss of oil quantity was almost instantaneous, and so was the feathering of the propeller. By the time I turned my head to look out the window to try and determine what had happened to the engine, the propeller had feathered.
Michael