Prop Governor Issue?

[joemoriss]

Hello Mooney Brain Trust, I encountered an interesting propeller control issue last week and wanted to gather input and hopefully get a better understanding of what might be going on and what to do about it. I have a 1964 Mooney M20E, I’ve reviewed many other governor related posts and the issues there don’t seem to be quite the same.

Initial problem: on climbout, I reduced power to 25”/2500rpm, pitched for Vy as normal. I noticed that my RPM started to increase back to 2700rpm, at which point I screwed out the prop control farther than I normally do to hold the RPM at 2500. After that, the rpm did not increase again.

My observations based on phases of flight:

Engine start and taxi – no change from normal

Run-up – no change from normal including the prop check x3

Takeoff roll – no change from normal. I check full power ~2700 rpm every time.

Climbout – rpm increases over 2-3 seconds back up to 2700, I pull the prop control out farther and the rpm remains at 2500.

Cruise, approach, landing – no change from normal. Propellor behaves normally when power reduced to bring rpm below governor control. Propellor maintains 2500 with no hunting in cruise.

Hypotheses:

Tachometer incorrect reading: Likely false, my engine power indication was normal during initial power application. I also performed a test in cruise: slowly increase prop to full, reading was still ~2700 rpm.

Propeller control cable slipped: Possible. After that flight, I inspected the cable mount and governor attachment point for defects and had someone work the control in and out. No slipping, looseness, or other issues observed. I did however notice the following behavior on my in-flight test: When I slowly increased the prop to full forward, it would momentarily behave as it did before, before slowly slipping up to 2700 rpm again, requiring me to pull the control out farther as described before. I did also observe that I was no longer able to push the control in quite as far as normal, about ¼ inch.

Oil shortage or contamination: Possible, but I did change the oil then fly 1 lap in the pattern, behavior remained.

Now I suspect the governor, but I do not know enough about potential governor failure modes to know how they could present in flight. I always thought of governor failures as sudden loss of governor function. I’m also wondering what the community thinks about safety of flight given this issue. I am due for annual in July-August timing and would love to not have to keep it down until then, but if this could be a serious issue then of course, not worth the risk. Checking through the logs, the governor was last overhauled in February 1998 (I always thought it was later than that) but only about 400 hours of flight time (plane didn’t fly much before I bought it. I’ve put about 80 hours/year on it for the past 2.5 years I’ve owned it). Thank you in advance for any advice!

[EricJ]

Do you have an engine monitor?   What is the oil pressure doing during these events?

Any new oil leaks?

[N201MKTurbo]

It sure sounds like a cable problem. Did you push and pull on the cable housing where it is secured at the governor?

[PT20J]

I would try to see if I could duplicate it on the ground -- makes troubleshooting easier.

There might be an issue with the vernier mechanism. I would try adjusting the rpm without using the vernier to see if it exhibits the same behavior.

[joemoriss]

I do not have an engine monitor, oil pressure remains constant (as constant as I can tell with my original oil pressure instrument). No abnormal temps ever.

The cable as well as the housing are as far as I can tell firmly fastened to the mount and not slipping

[joemoriss]

5 minutes ago, PT20J said:

I would try to see if I could duplicate it on the ground -- makes troubleshooting easier.

There might be an issue with the vernier mechanism. I would try adjusting the rpm without using the vernier to see if it exhibits the same behavior.

That's a great idea, thanks! I'm nervous about having the engine so high on the ground but I think it's worth it for a test. Agree, that makes a lot of sense, next time I'll try just using the vernier

[takair]

Last post you mentioned just using the vernier next time. Do you otherwise not use it?  It is possible the ball is hanging up and not quite locking. The spring force in the governor then has the opportunity to pull it forward, at least one “detent”. Good advice to try that first. How old is the cable?

[joemoriss]

8 hours ago, takair said:

Last post you mentioned just using the vernier next time. Do you otherwise not use it?  It is possible the ball is hanging up and not quite locking. The spring force in the governor then has the opportunity to pull it forward, at least one “detent”. Good advice to try that first. How old is the cable?

Thanks for the correction, what I meant to say in the reply was next time I will adjust the prop *without* the vernier. I use the ball and pull in/push out on the ground for runup, then while flying I use the vernier only to change the prop speed including my reduction on initial climbout, up until prop full forward in GUMPS. I also thought it may be cable slippage from the governor spring force, the cable slipping internally one detent makes sense. The cable appears original.

[Pinecone]

Maybe your plane is just trying to tell you that pulling the power back to climb is so 20th century.

[Kytulu]

On 5/17/2025 at 10:34 AM, joemoriss said:

Hello Mooney Brain Trust, I encountered an interesting propeller control issue last week and wanted to gather input and hopefully get a better understanding of what might be going on and what to do about it. I have a 1964 Mooney M20E, I’ve reviewed many other governor related posts and the issues there don’t seem to be quite the same.

 

Initial problem: on climbout, I reduced power to 25”/2500rpm, pitched for Vy as normal. I noticed that my RPM started to increase back to 2700rpm, at which point I screwed out the prop control farther than I normally do to hold the RPM at 2500. After that, the rpm did not increase again.

 

My observations based on phases of flight:

 

Engine start and taxi – no change from normal

 

Run-up – no change from normal including the prop check x3

 

Takeoff roll – no change from normal. I check full power ~2700 rpm every time.

 

Climbout – rpm increases over 2-3 seconds back up to 2700, I pull the prop control out farther and the rpm remains at 2500.

 

Cruise, approach, landing – no change from normal. Propellor behaves normally when power reduced to bring rpm below governor control. Propellor maintains 2500 with no hunting in cruise.

 

Hypotheses:

 

Tachometer incorrect reading: Likely false, my engine power indication was normal during initial power application. I also performed a test in cruise: slowly increase prop to full, reading was still ~2700 rpm.

 

Propeller control cable slipped: Possible. After that flight, I inspected the cable mount and governor attachment point for defects and had someone work the control in and out. No slipping, looseness, or other issues observed. I did however notice the following behavior on my in-flight test: When I slowly increased the prop to full forward, it would momentarily behave as it did before, before slowly slipping up to 2700 rpm again, requiring me to pull the control out farther as described before. I did also observe that I was no longer able to push the control in quite as far as normal, about ¼ inch.

 

Oil shortage or contamination: Possible, but I did change the oil then fly 1 lap in the pattern, behavior remained.

 

Now I suspect the governor, but I do not know enough about potential governor failure modes to know how they could present in flight. I always thought of governor failures as sudden loss of governor function. I’m also wondering what the community thinks about safety of flight given this issue. I am due for annual in July-August timing and would love to not have to keep it down until then, but if this could be a serious issue then of course, not worth the risk. Checking through the logs, the governor was last overhauled in February 1998 (I always thought it was later than that) but only about 400 hours of flight time (plane didn’t fly much before I bought it. I’ve put about 80 hours/year on it for the past 2.5 years I’ve owned it). Thank you in advance for any advice!

 

The last time that I had an issue like this, on a Piper Seminole, the flyweights in the prop governor were overpowering the spring. We couldn't duplicate the issue on the ground, and it only happened during flight. We verified that it was the prop governor by installing a different "known good" governor and sending the plane on a MX flight. We sent the PG out for overhaul. 

[MB65E]

I need to do this Lycoming Service instruction 1462A on another airplane. His will go over redline momentarily occasionally when hot. 

J model at 220 oil temp the prop hunts a bit.

-oil cooler and vernotherm are new.

-Gov is overhauled.

-engine and prop have about 1000h

 

I was looking for low hanging fruit. SI 1462A is next. 
 

-Matt