Recently, I sent out both prop governors on my PA-39 out to be overhauled. They were reinstalled, and since then I've had a problem where the left prop sometimes goes into uncommanded feather on the roll-out (low-rpm, low power condition with the prop-lever in the full fine pitch position).

I had the pressure on the nitrogen charge in the dome checked, and they were indeed high, but fixing that hasn't fully resolved the problem.

I don't think there is any thing directly related to the governor overhaul that would cause an uncommanded feather. So that leaves possible problems with the reinstallation of the governor. But so far, my mechanic hasn't been able to isolate the cause.

Anyone experience anything like this?

Thanks.

Tomoharu

If the problem didn't exist before the O/H and it does now, very likely, as you suggest, something to do with install or O/H. Might consider calling O/H shop or Hartzell. Does it come out of uncommanded feather? If you had to go-around, is there a risk the prop/engine won't be there for you?

Did have this problem once on departure, B-nut backed off and engine 'failed' due to fuel starvation. Do not use paddles to correct this condition. Husband was solo and handled beautifully.

Thanks for your response. We did call Sesenich who did the governor overhaul, and aside from the dome pressure issue (which we reverified twice), they couldn't figure out a scenario where a governor overhaul would cause a situation where the prop would go into uncommanded feather. One possibility is that the bearing in the oil transfer system is leaking oil back into the engine, and is unable to maintain oil pressure at low RPMs, but that seems unlikely in a low time engine (<500 hours).

One thing I might try is to go to a single grade oil -- the two times it has occurred were on hot days with a hot engine (was doing landings), so maybe the oil is too thin to maintain pressure. (I've had the plane for about a year, and it spent most of last summer in maintenance, so this is really my first summer with the plane. So I just don't know if it is something that was happening previously during hot weather.)

I think the risk of the prop feathering, and not being there for a go around is relatively small. It does take a few seconds for the prop to go into feather, and you can usually tell from the vibration the engine makes from the high load, and bumping the RPM back up seems to restore the enough oil pressure to prevent it from fully feathering.

Tomoharu

Simple answer here. Governor is not putting out enough pressure to keep the prop out of feather above 1100 rpm (at which point the centrifugal stop pins extend and lock the prop from feathering). On a twin, the dome pressure and spring drive the blades to feather, and the governor keeps it out of feather by supplying oil pressure against the dome force. If it made pressure before, and now it doesn't, and nothing else was changed, the governor is the culprit...

Zach

Thanks Zach for the reply. Come annual time I will have the governor pulled again to have it looked at again.

Tomoharu

Just in case this is of use to anyone.

So after a frustrating summer of trying to track down the problem of the uncommanded feather with a hot engine in low-RPMs, we eventually figured out the cause, which turned out to be a problem with the oil transfer system between the engine and the governor.

I initially tried switching to a higher viscosity single weight oil during the summer, which seemed to help. But we finally diagnosed the problem when I had the A&P remove the governor, attach a blank plate in its place, and conduct a pressure test on the oil transfer system. The engine failed that test. The fact that my initial experience of the problem coincided with an governor overhaul seems to have been a coincidence.

This basically required an engine IRAN (ouch!). The plane is back and the engine is being broken in. The engine only had about 500 hours on it, BUT, the previous owner hadn't flown it much and it had been 12 years since the last overhaul. I guess an engine that sits a lot just isn't happy.

Tomoharu