Propeller Overspeed and Hunt After Cable Replacement
A single-engine airplane with a three-blade constant-speed propeller and an engine-driven governor had the governor control cable replaced at the last annual, 20 hours ago. On takeoff, if the throttle is advanced briskly, RPM overshoots to about 2,780 before settling at 2,700, which is redline. In cruise the RPM hunts about ±70. A static RPM check on the ground falls at the low end of the allowable range. Oil pressure and temperature are normal, there are no leaks at the hub, blade play is normal, and the propeller moves freely when the control is cycled with the engine off. The pilot says the condition began after the annual.
1. How do you decide whether the fault lies in the governor or in the propeller?
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Assign the functions correctly first. The propeller is passive - it goes where oil pressure pushes it. The governor decides how much oil pressure to send, and it does that by comparing actual RPM to the speed set by the cockpit control.
- Overshoot on rapid acceleration means the governor is slow to increase oil pressure, or the control is not commanding the speed the pilot thinks it is. A propeller cannot overspeed on its own at a commanded speed.
- Hunting at steady state is the classic governor signature: a worn pilot valve, air in the oil, or sloppy linkage oscillating around the set point. A propeller fault tends to show up as a fixed-pitch behavior, inability to change pitch, sluggish response, or vibration.
- Static RPM at the low end points toward rigging: the blades are not reaching the expected low-pitch angle at full power.
Perform one decisive test: at a steady condition, sweep the propeller control slowly through its full range and watch RPM. Smooth, proportional response with correct RPM at each end of travel indicates the propeller and the pitch-change mechanism are working, and the fault is in the governor or its rigging. Couple that with the timing - the condition began with the cable work.
2. Given the cable was just replaced, what specific rigging and installation checks do you make?
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Treat rigging as the primary suspect and verify it mechanically, not by feel.
- Full travel and stop contact. Confirm the governor lever reaches the correct position at both extremes of cockpit control travel, per the rigging dimension in the manual. A lever that falls short of the high-RPM stop gives low static RPM and an inability to reach the commanded speed.
- Preload. A cable adjusted too tight pulls on the governor lever and effectively changes the governed speed, and it can produce hunting because the lever is not free to float with the pilot valve. Check that the cable can be disconnected and the lever moves freely with no cable tension on it.
- Lost motion and binding. Confirm the cable is the correct part number, routed without tight radii, and that the swaged end and the clamp are secure. A cable with backlash gives imprecise speed setting.
- Governor condition. Pull and inspect the governor screen for debris, confirm the correct governor part number, and check the oil supply line for a restriction or trapped air after the system was opened.
Do not adjust the governor's internal setting to compensate for rigging. Fix the rigging and confirm the governed speed by tachometer check.
3. What is your return-to-service decision, and what does the 2,780 RPM overshoot require beyond fixing the rigging?
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The airplane does not leave until the rigging is corrected and the overshoot is gone.
Beyond the repair:
- The overshoot exceeded redline, so it is an exceedance, not an annoyance. Consult the propeller and engine manufacturers' instructions for overspeed limits and required inspections; a short overshoot may require nothing more than a record entry, while a larger one can require blade inspection or engine inspection. Do not make that call from memory - use the published limits.
- Make the maintenance record entry for the exceedance and the corrective action, including the highest RPM observed and its duration.
Return-to-service demonstration:
- Static run-up: static RPM within the allowable range, propeller control sweeps smoothly through its range, RPM stable at each setting.
- Flight test: brisk throttle application from a low-power approach attitude with no overshoot beyond redline; steady RPM in cruise with no hunting.
- Confirm the governor screen is clean after the run and there are no leaks.
A propeller that hunts and overshoots is also a fatigue issue for the hub and blades; the longer it runs that way, the more the inspection bill grows. Fix the cause now.