Propeller RPM instability after governor replacement
A constant-speed propeller on a six-cylinder fuel-injected engine has just had its propeller governor replaced because of an external oil leak. The replacement governor is the same part number. The control cable was disconnected and reconnected, and the propeller was not removed. On the first ground run, oil pressure is 75 psi at 1800 rpm and oil temperature is normal. When the prop control is cycled, RPM falls from 1800 to 1650, then overshoots to 2150 before settling at 1800; the pilot reports that in cruise the RPM slowly creeps up about 80 rpm over 10 minutes and can be corrected only by moving the prop control. There is no visible oil leak, the governor screen is clean, and the engine has 12 hours since oil change. The aircraft is a retractable single with a Hartzell governor and a two-blade metal propeller. The maintenance manual requires the governor control rigging to be checked with the cable connected and the propeller at the low-pitch stop.
1. Is this a governor internal fault, an installation or rigging fault, or a propeller fault? Explain your reasoning.
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The symptoms point first to installation or rigging, not automatically to a faulty governor. The governor is the same part number and was just replaced, so the control cable, rod ends and governor lever are the changed variables. If the cable does not allow the governor lever to reach its stops, or if there is backlash or binding, the governor cannot schedule pitch correctly. The overshoot during cycling and the slow RPM creep in cruise are typical of a control system that is not holding the selected speed, or of air trapped in the governor oil circuit.
The propeller was not removed, which makes a sudden blade or hub fault less likely, although blade friction and hub condition should still be checked. A governor internal fault is possible, especially if the unit was not bled or was incorrectly set up, but condemning it before checking rigging and air in the system would be premature.
2. What checks would you perform before condemning the governor?
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With the engine off, check the propeller blades for free movement and normal friction, and inspect the hub for oil leakage or mechanical damage. Check the governor control cable for correct rigging per the manual: the governor lever must reach the low-pitch and high-pitch stops, the cable must not bind, and the rod ends must have no excessive play. Confirm the cable is connected to the correct lever and that the lever moves through its full travel.
Check the governor part number and model against the approved equipment list. Inspect the governor oil line and fittings for restriction or air, and verify oil level and viscosity. Start the engine and cycle the propeller several times to bleed the system, then repeat the ground run. If the RPM still overshoots or creeps after correct rigging and bleeding, remove and bench-test the governor. Document all rigging dimensions and test results.
3. What release decision would you make, and what operational limitation, if any, would you apply?
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I would not release the aircraft with the reported overshoot and cruise creep. An uncommanded RPM increase can lead to overspeed, high power and propeller or engine damage. If the fault is corrected by rigging or bleeding and the ground run shows stable RPM through the full cycle with no creep, I would release the aircraft after a satisfactory operational check.
If the governor must be replaced or bench-tested, I would record the removal and installation, part number and rigging. I would not apply a blanket operational limitation as a substitute for repair; a temporary restriction would be acceptable only if approved by the manufacturer or regulator and clearly documented. A post-maintenance flight test should confirm stable RPM in climb and cruise before normal service.