Prop not holding RPM

[toto]

On a flight today, for the first time ever, the prop would surge a tiny bit (about 10 rpm) and then drop back about 20 rpm, so it was set at 2500 and would surge to 2510 and then back to 2490 and then settle at 2500 again. It did this about once a minute for about 10 minutes and was disconcerting. I was watching the oil temperature very carefully, and it ticked up a little bit higher than usual but then settled back into a normal temp in the center of the green arc. This may be unrelated and a non-issue.

I made an uneventful precautionary landing, and now I’m trying to get hold of a mechanic on a Sunday. 

I’m trying to figure out whether this is something that grounds the plane, or “nbd don’t sweat it.”

My RPM is normally rock solid. Set it at 2500 and it just sits there perfectly for the whole flight. So it’s definitely unusual - obviously unusual enough that I landed. 

Any advice would be well appreciated. 
 

Update: Talked to a mechanic, he’ll be over to look at it shortly. He did not think this is an issue that should ground the plane. 

[EricJ]

What was the oil pressure during the surges?   Loss of oil is one thing that can cause this.

[toto]

4 minutes ago, EricJ said:

What was the oil pressure during the surges?   Loss of oil is one thing that can cause this.

Oil pressure did not change. I’m not seeing any unusual drips on the ground or anything obviously indicating a loss of oil.

The oil on the stick is low (showing under 5qt), but the engine is hot, and the stick usually shows low when the oil is still up in the engine. 

Would you expect to see this if the oil is a little bit low, or only if it’s extremely low?

[EricJ]

23 minutes ago, toto said:

Oil pressure did not change. I’m not seeing any unusual drips on the ground or anything obviously indicating a loss of oil.

The oil on the stick is low (showing under 5qt), but the engine is hot, and the stick usually shows low when the oil is still up in the engine. 

Would you expect to see this if the oil is a little bit low, or only if it’s extremely low?

Only if extremely low, in other words, the oil supply to the governor is interrupted.    On a Lycoming, that only starts happening when it's nearly out of oil, so if you have five quarts, that's not the issue.   If oil pressure was good then the pickup or pump wasn't an issue, either.

 

[toto]

6 minutes ago, EricJ said:

Only if extremely low, in other words, the oil supply to the governor is interrupted.    On a Lycoming, that only starts happening when it's nearly out of oil, so if you have five quarts, that's not the issue.   If oil pressure was good then the pickup or pump wasn't an issue, either.

 

Okay, thank you. I’ll check it again when it cools off a bit more, but there was definitely plenty of oil on the stick. 

[A64Pilot]

Could you hear a change in RPM?

Sometimes tachs will do that but RPM isn’t changing, if it’s really surging then it’s most likely the governor.

If the governor fails completely, you can still fly, but it’s like driving a car in first gear, back off to 2700 or maybe 2600 and she will fly slowly, but you will have enough thrust to keep flying.

[toto]

1 hour ago, A64Pilot said:

Could you hear a change in RPM?

Sometimes tachs will do that but RPM isn’t changing, if it’s really surging then it’s most likely the governor.

If the governor fails completely, you can still fly, but it’s like driving a car in first gear, back off to 2700 or maybe 2600 and she will fly slowly, but you will have enough thrust to keep flying.

Yes, definitely heard the change in RPM. It was a minor change, but I could both hear it and feel it when it happened. 

[toto]

Update:  Airplane is back in the barn. Had the mechanic come out, pulled both the upper and lower cowl, and found no problems. Did a run up on the ramp, everything seemed to work fine. Flew the last 30 mins home without issue, and the RPMs were rock solid the whole way.

The mechanic said that he did not believe there was any safety of flight issue, but that I could send off the governor to an accessory shop and have them investigate.

Many thanks for the replies. I thought I was going to end up at a rural airport with no mx and definitely needed all the help I could get

 

[Skyland]

Mine is doing something similar.  Flew to the east coast a few months ago and all was perfect until about an hour from my destination.  Just flying along and the engine started fluctuating a bit as you describe.  Analyzed engine data best I could, saw some RPM variance, oil pressure solid, fuel pressure slight variation but I don't know how solid it should be, CHTs, EGTs all normal.  So is it the prop governor or is the engine power changing and the governor is doing its job to keep the RPMs constant?  The return flights to the west coast were mostly fine with the issue only occasionally occurring.  Just returned from another coast to coast trip and the engine ran perfect.  So I'm still in the watch and see mode but it would be nice to know what's up on those rare occasions. 

[A64Pilot]

Any significant change in engine power, if you have an engine monitor you will see something change, fuel flow, EGT’s,MP, something will change with the power.

If it’s a power drop, maybe RPM will drop for a second, but will very quickly return to set RPM.

‘Next time your flying make rapid power changes of a couple inches and see what the tach does, I don’t think you will see much change, these things are pretty good.

I do think that with all this digital instrumentation sometimes highlights what’s always been there, just with the old slow to react analog instruments you never saw it.

Not saying that’s happening to you, but I wish I had a quarter for every time I’ve seen a digital tach alarm an overspeed because RPM went to 2710 on T/O when of course 2700 is max.

[redbaron1982]

In my M20J the RPMs always fluctuate +/- 20 RPM. I thought this would be normal, as 20RPM in 2500 is less than 1%. Should I try to understand why is fluctuating? I don't hear any change in RPM, I just see the value changing in the EIS.

[A64Pilot]

In my opinion, which is just an opinion, but if you have digital instrumentation highlighting an RPM change that you can’t feel or hear, I wouldn’t worry about it.

But that’s just my opinion.

‘By chance is the min display change 20?

 

[toto]

25 minutes ago, redbaron1982 said:

In my M20J the RPMs always fluctuate +/- 20 RPM. I thought this would be normal, as 20RPM in 2500 is less than 1%. Should I try to understand why is fluctuating? I don't hear any change in RPM, I just see the value changing in the EIS.

In my case, it felt like someone tapped the brakes in the car for a second, or tapped the gas pedal in the car for a second. You could hear and feel an increase or decrease in RPM. 

I agree that a variation in indicated RPM on a digital tach with no other symptom is a nothingburger. 

[MikeOH]

My digital tach (Horizon) has always bounced around 10-20 rpm as does the tach in the Insight G3...and neither agree.  Never heard or felt anything; never worried about it.

The EE in me says the input pulses from the mag pickup are noisy and both designs are poor in regards to suppression of false pulses.

[toto]

12 minutes ago, A64Pilot said:

‘By chance is the min display change 20?

In my case, the minimum is 10. It would go to 2510, back to 2500, then to 2490, then back to 2500. 

On a normal flight, I only care that it’s stable. If it’s stable at 2490 or whatever, I’m happy. 

[redbaron1982]

This is how the GI 275 look like when uploaded to Savvy:

 

It has a "resolution" of .1, which doesn't make much sense as for sure the precision is much less than that.

In my case, different from what the OP mentioned, I don´t feel or hear anything. I agree that most likely is electric noise. Something that does I see is when switching magnetos sometimes the RPMs indicated jump to more than 3000. Again, no feel or hear, that is for sure electric noise. I wonder how the GI 275 is measuring the RPMs. It should average at least one second of pulses to get the reading, but most likely it is not doing that.

 

[EricJ]

23 minutes ago, toto said:

In my case, the minimum is 10. It would go to 2510, back to 2500, then to 2490, then back to 2500. 

On a normal flight, I only care that it’s stable. If it’s stable at 2490 or whatever, I’m happy. 

Mine will move around that much sometimes.   It's a feedback control system that is not perfectly damped.    If you are in turbulence or have the autopilot on altitude hold, there will be variance that sometimes takes the governor a moment or two to adjust.   In my experience it's not unusual.

[Pinecone]

9 hours ago, toto said:

In my case, it felt like someone tapped the brakes in the car for a second, or tapped the gas pedal in the car for a second. You could hear and feel an increase or decrease in RPM. 

I agree that a variation in indicated RPM on a digital tach with no other symptom is a nothingburger. 

That could be due to a misfire for some reason.  I would look into that.

[A64Pilot]

10 hours ago, redbaron1982 said:

This is how the GI 275 look like when uploaded to Savvy:

 

It has a "resolution" of .1, which doesn't make much sense as for sure the precision is much less than that.

In my case, different from what the OP mentioned, I don´t feel or hear anything. I agree that most likely is electric noise. Something that does I see is when switching magnetos sometimes the RPMs indicated jump to more than 3000. Again, no feel or hear, that is for sure electric noise. I wonder how the GI 275 is measuring the RPMs. It should average at least one second of pulses to get the reading, but most likely it is not doing that.

 

This is handled a couple of different ways by other companies, I believe EI has “smoothing” which is I believe averaging over a short time interval.

Piper I was told by their Propulsion DER on their turbines have a “snap” function on their instrumentation, that is when the reading gets close to a set parameter it will snap to the set point and won’t deviate from it until there is a significant change.

I flew a twin engine turbine helicopter for a number of years that had digital instrumentation, it had an averaging display, because if it didn’t then everything would be drifting constantly. Even if engine and rotor speed was digitally controlled, they are mechanical devices, speed is constantly changing some with load demand.

What really bothers me is some instrumentation measures and or displays to digits well beyond its capability to accurately measure, volt meters that display to the hundredths of a volt come to mind, but they sell because people believe if they display it, they are accurate to at least that point.