If you are experiencing control problems with your constant speed propeller, you should consider the possibility that the source of the problem may not be the propeller or governor.
Lycoming and Continental engines incorporate provisions in the front main bearing journal area designed to transfer oil to and from the prop governor and back and forth through the crankshaft to the prop. Lycoming engines transfer this oil through an annulus in the front main bearing, while Continental engines typically utilize a transfer collar between the front main bearings.
Lycoming published Service Instruction No. 1462A Propeller Oil Control Leak Test Procedure (With Propeller Installed on Engine) It is prescribed:
- Whenever sluggish propeller action is reported.
- Whenever the engine does not hold RPM during cruise, climb or descent.
- Whenever the engine is going into feather during landing roll out with reduced throttle setting.
Service Instruction 1462A describes an air pressure check (differential pressure) designed to show the condition of governor oil passages, front main bearing clearance and positioning of the governor circuit oil plug. While Continental does not have a published procedure to test the integrity oil passages, transfer collar or governor circuit oil plug a similar differential pressure check can be a useful troubleshooting tool. A well fitted Continental transfer collar will generally exhibit a differential pressure of 78-80/80 until residual oil is displaced allowing air to escape.
Loss of engine oil pressure can also lead to a shortage of oil available to the governor. This condition will also exhibit similar symptoms. If you are experiencing loss of prop control, it is important to check engine oil screens and filters to confirm that the engine is not sustaining a failure and producing metal.