Like most, I learned in ground school that in the event of a governor failure, a single-engine constant-speed prop would go to fine pitch (high RPM). Apparently this is not always the case.
A few days ago, as we leveled off after my departure climb, the engine RPM began oscillating wildly between low RPM and overspeed every second or two. Initially we assumed it was an engine (or fuel) issue and changed tanks, but since I'd recently had the prop governor overhauled and the prop control had no effect I guessed that it might be related to that. When I pulled the throttle back to about 12.5 inhg to avoid overspeeding the engine, the oscillations stopped and the engine stabilized at 2550 RPM. Slowly inching the throttle forward didn't retrigger oscillations, and I was able to maintain altitude until reaching a nearby airport.
After landing and taxiing off the runway, we shut down and took off the upper cowl. We found the cable to the prop control lever had become detached from the governor control arm (see photo). It appears the spring clip that attaches the cable end to the pivot bolted to the control arm may not have been fully engaged when the governor was reinstalled, and had worked itself loose. After the cable was reattached everything works normally. Engine logs show no loss of oil pressure or any other anomaly (aside from the RPM oscillations).
What everyone found surprising was that the governor didn't go to its max RPM configuration when the cable detached. This is obviously a different situation than a loss of oil pressure, in which case the prop would go to fine pitch by itself. But why the rapid oscillations in prop pitch?
Thoughts? Ideas?
A few days ago, as we leveled off after my departure climb, the engine RPM began oscillating wildly between low RPM and overspeed every second or two. Initially we assumed it was an engine (or fuel) issue and changed tanks, but since I'd recently had the prop governor overhauled and the prop control had no effect I guessed that it might be related to that. When I pulled the throttle back to about 12.5 inhg to avoid overspeeding the engine, the oscillations stopped and the engine stabilized at 2550 RPM. Slowly inching the throttle forward didn't retrigger oscillations, and I was able to maintain altitude until reaching a nearby airport.
After landing and taxiing off the runway, we shut down and took off the upper cowl. We found the cable to the prop control lever had become detached from the governor control arm (see photo). It appears the spring clip that attaches the cable end to the pivot bolted to the control arm may not have been fully engaged when the governor was reinstalled, and had worked itself loose. After the cable was reattached everything works normally. Engine logs show no loss of oil pressure or any other anomaly (aside from the RPM oscillations).
What everyone found surprising was that the governor didn't go to its max RPM configuration when the cable detached. This is obviously a different situation than a loss of oil pressure, in which case the prop would go to fine pitch by itself. But why the rapid oscillations in prop pitch?
Thoughts? Ideas?
