M20J RPM Fluctuation

[JCD]

All,

 

I am a new Mooney Owner and am having an issue with my '78 M20J 201.  While on takeoff and cruise my engine is randomly dropping 20-70 RPM.  This is happening a couple times a minute to once every 15 minutes.  In the cruise config I have abruptly dived and climbed and saw no fluctuation which make me believe its not the prop governor (which was rebuilt in and installed in November 2014 has about 80 hours on it).  I have sumped the fuel and there is no water or debris in it. The RPM Drop happens with or without the boost pump on. On takeoff it will get your attention real fast.  Before you can cycle mags or do any trouble shooting it corrects itself.  It was at the mechanic yesterday 12/1/2015 and they looked it over and everything looks good.  Any ideas or help is greatly appreciated. I have attached JPI data that show 6 seconds before and 6 seconds after an RPM Fluctuation.  This data was take while in Cruise flight on a calm cool day.  I have also attached a video of the RPM Drop. 

 

Thanks for your help

Justin

Flt #342 RPM Anomolies.xlsx

IMG_4709.MOV.mov

[ArtVandelay]

I notice that too, it only lasts for a second, unfortunately the JPI data is a bit noisy, the display must smooth out these variances. I only notice it at cruise.

Here is what my FF and RPM looks like.

[1964-M20E]

OK so you have electronic instrumentation and that rules out the old tachometer cable.

Oil pressure remained in constant.

It was hard to tell but can you feel and hear the RPM changes?  If so then again that would rule out sensor or instrumentation.

Have you checked the cable and linkage to the governor and are all the connections tight and secure without excessive play?

Finally that would leave the governor itself and the overhaul may have an issue.

 

 

 

 

 

 

[JCD]

Yes you can hear the RPM Fluctuation (through my Bose A20) and feel the engine retard.  Before I can react to it (ie Turn Boost pump on when in Cruise, or cycle mags) it corrects itself.)  I see no oil pressure change.  The Fuel flow does seem drop.

[kmyfm20s]

I'm not an Mx but I would definitely rule out oil pressure fluctuations being the cause of it. That one would be a big concern. Fuel flow will dip with RPM so that wouldn't concern me. I would make sure the governor cable and linkage is lube and working properly and if so I would assume you need a new governor. 

[N201MKTurbo]

I would suspect an intermittent MAG. Have you tried switching them off individually and seeing if the fluctuations stop?

[JCD]

1 minute ago, N201MKTurbo said:

I would suspect an intermittent MAG. Have you tried switching them off individually and seeing if the fluctuations stop?

It happens so fast I have not been able to switch the mags.  The only think I could do is fly with one Mag for 15 minutes and see if I see a drop.

[JCD]

19 minutes ago, kmyfm20s said:

I'm not an Mx but I would definitely rule out oil pressure fluctuations being the cause of it. That one would be a big concern. Fuel flow will dip with RPM so that wouldn't concern me. I would make sure the governor cable and linkage is lube and working properly and if so I would assume you need a new governor. 

Linkage is all tight.  I am leaning toward a new Governor as well.  Guess I know what to ask Santa for Christmas.

[1964-M20E]

Maybe it is not enough time to react but the EGTs don't move during the RPM drops at least according to the data points.

 

 

[Shadrach]

7 minutes ago, JCD said:

It happens so fast I have not been able to switch the mags.  The only think I could do is fly with one Mag for 15 minutes and see if I see a drop.

Take off  and climb to your typical cruising altitude. Fly on the left mag for 30 mins then the right.

[ArtVandelay]

19 minutes ago, kmyfm20s said: I'm not an Mx but I would definitely rule out oil pressure fluctuations being the cause of it. That one would be a big concern. Fuel flow will dip with RPM so that wouldn't concern me. I would make sure the governor cable and linkage is lube and working properly and if so I would assume you need a new governor. 

Linkage is all tight.  I am leaning toward a new Governor as well.  Guess I know what to ask Santa for Christmas.

You said you only had 80 hrs on it SPOH? Maybe a call to the shop that did the overhaul and see what they say?

I've got 400 hrs SPOH, mine doesn't do it as often as yours, on a 4 hr trip it dropped about 10-15 times.

[jetdriven]

Don't be so quick to condemn the governor.  An intermittent mag will cause a small rpm surge.  

[JCD]

More Data Points.  Since I just bought the Airplane I am learning more about the history of it.  Just got off the phone with the Governor Repair noted in tth log book.  A New Propeller was installed and the govenor was re-sealed only.  I am also going to post more data from my flight last night where it only did the RPM drop once during take of where it dropped a 100 RPM! and then did not do it again the rest of the flight.  It definitely got my attention last night and had me "Puckerd".

[JCD]

Here is some more data from last night where it dropped a 100 RPM.  Notice the EGT's  could this be a bad Mag?

RPM anamoly 12_1 Take off.csv

[KSMooniac]

Yes it could.  When was the mag last serviced, and by whom?  The -A3B6D is a great engine and mag, but you must adhere to the 500 hr inspection interval.  I'd send it to Select Aircraft in Lancaster, TX for inspection and rework as necessary, even if it has been "checked" by a non-expert shop recently.

I'll also note that if the governor is commanding an RPM change, your oil pressure will also change.  If it is an ignition or fuel issue, you shouldn't see the oil pressure move IMO.

Glad to see you on Mooneyspace, BTW.  You bought a beautiful plane!

[JCD]

5 minutes ago, KSMooniac said:

Yes it could.  When was the mag last serviced, and by whom?  The -A3B6D is a great engine and mag, but you must adhere to the 500 hr inspection interval.  I'd send it to Select Aircraft in Lancaster, TX for inspection and rework as necessary, even if it has been "checked" by a non-expert shop recently.

I'll also note that if the governor is commanding an RPM change, your oil pressure will also change.  If it is an ignition or fuel issue, you shouldn't see the oil pressure move IMO.

Glad to see you on Mooneyspace, BTW.  You bought a beautiful plane!

Thanks,  Really glad to be a Mooney owner.  I flew a lot of Pipers but once I flew a Mooney I was hooked.  I am an Aerospace engineer and from an Engineering perspective a Mooney is a far superior designed aircraft.  I love it.  Thanks for the info on the Mag Shop.  I need to look up when the mag were last serviced.  I will dive into this more. 

 

Wish they had Electronic ignition for my Mooney!!

 

Also - Thanks to you all for your input to my problem. It's nice to know there is a community out there that sticks together and shares knowledge and experience.

[KSMooniac]

I too am an aero engineer and have the same assessment of the Mooney!

I'm not sure electronic ignition is the answer... might make for easier starts but not a lot more efficiency/performance than what we have currently.  If everything is in-spec, the IO-360 starts easily hot or cold.

[N601RX]

 I would lean toward an ignition issue.  Do you know what kind of spark plugs you have? Champion plugs have known reliability problems.

[JCD]

Just now, N601RX said:

 I would lean toward an ignition issue.  Do you know what kind of spark plugs you have? Champion plugs have known reliability problems.

I don't.  We pulled all the lower ones yesterday and they looked good.   I will find out what brand they are.

[N601RX]

Do some research on Champion resistance problems.  Before I switched to finewire Tempest I had Champion plugs fail that had less than 20 hrs on them and still appeared new.  You can't go by looks, measure the resistance of the internal resistor.

[Andy95W]

6 hours ago, JCD said:

Here is some more data from last night where it dropped a 100 RPM.  Notice the EGT's  could this be a bad Mag?

 

RPM anamoly 12_1 Take off.csv

 

1.)  I don't think it is spark plug related.  All 4 of your EGTs went up in an orderly, uniform fashion.  What are the odds you would have 4 plugs all hiccup simultaneously?  (But I do agree that Tempest fine wires are the best, bar none.)

2.)  The thing I thought was the MOST odd was the fuel flow drop from 18.9 to 16.8 gph.  That's a 12% drop in 2 seconds, after which it stabilizes at 17.2 gph.   Manifold Pressure actually rose slightly, which would be consistent with a decrease in rpm while maintaining a relatively stable power setting.

3.)  I think the most common problem that could cause these symptoms would be a magneto that was beginning to go, or a p-lead that was intermittently grounding.  The problem with that theory here is that I would expect this to happen fairly regularly, with increasing frequency.  That doesn't seem to be the case here.  Nor does it explain the fuel flow drop.

4.)  If the rpm dropped and stayed 100 rpm down, I would definitely suspect the governor, but of course that isn't the case here.  If there was a bit of crud in the governor that caused a temporary clog or surge, I would expect to see that happen regularly, and not at one unique and specific flight regime.

4a.)  If the pressure metering system in the governor was sticky at low pressure (fine pitch), maybe it could cause these types of pressure fluctuations in your prop, until the governor settles down and gets the pressure correct.  I would NOT expect to see a change in engine oil pressure, since the governor is using high pressure oil that will not be seen far downstream at the engine oil pressure gauge.  But this does not explain the drop in fuel flow as I mentioned in #2, above.

5.). If it isn't 4a, above, then I may be leaning toward a fuel problem.  Almost as though at the highest power output and fuel flow the pressure regulator portion of the RSA unit (that regulates metered fuel pressure to the flow divider) says "Oops- too much!" and then drops the fuel flow down to where it was supposed to be, or doesn't realize it was supposed to stay there.  This would cause a slight drop in rpm until the governor could compensate.  This would also explain why your EGTs continue to rise until they stabilize as fuel flow stabilizes.

Good luck, please keep us posted.

 

[carusoam]

Typical M20J govenor problems don't usually cause RPM to drop.  If the gov's gear pump stopped or oil was leaking through the control valve, the rpm would tend to increase.

A Missile, modded M20J, would decrease rpm in failure mode.

I am only a PP, not a mechanic or aero-engineer.  I do like Craig's all electric panel though.  It's memorable.

Hope this helps, and best regards,

-a-

[JCD]

All - Thanks for all of your insight.  I flew the plane back to the mechanic this morning.  When I strained the fuel I saw debris in the fuel.  It appears to me fuel tank sealant.  I strained fuel until it was gone.  Had normal taxi and run up but when I got on the runway and throttled up the plane and it ran rough.  It made good power and I only had a 5 min flight.  I never went to cruise and just left mixture full rich and prop full forward.  I did not notice any RPM Drop, again it was only for 5 min.  Once I landed and shut down the Mechanic and I took it over an did an extensive run up.  Initial idling on start the plane ran rough once we ran it up and leaned it out it began running smoother.  The run up yielded no smoking guns, unfortunately.  To be honest it a little nerve racking to fly. 

At this point we are going to dive into the fuel screens and see what we can find.  I looked through logs last night and the Mags were rebuilt 4 Years & 1 month ago with 207 Hours on them.

I will post more when I get more data.

 

Thanks again for all your insight.

[KSMooniac]

Did you catch who serviced the magneto?

[JCD]

47 minutes ago, KSMooniac said:

Did you catch who serviced the magneto?

Aircraft Systems INC.