Help Diagnose Engine Problem - M20K TSIO-360-MB

[Guest]

12 minutes ago, Z W said:

Not to my knowledge.  How would those get changed?  The prop has not be reinstalled.

Just trying to eliminate other items which could have an effect.  No one mentioned the air box repair to you, so who knows  what else may have been done.  If you have a McCauley it’s a mute point.

[Z W]

On 12/31/2022 at 12:53 AM, Will.iam said:

Are you sure the prop governor control arm is set to hit the stop in the govenor before reaching the end of the arc motion of the arm? My A&P made sure of that and that it reached that limit with another 1/8 inch of travel left in the control knob in the cockpit to make sure i could get the control arm full travel to the stop. If you adjust the rpm screw and get no changes then your control arm is limiting or something is not setup internally correct. 

Depends on what you mean by the "stop in the governor".  The control arm swings up and stops against the RPM adjusting screw.  Better picture here:

We wondered if there's some internal stop inside the governor, because the arm rotating does not seem to bottom out.  It's stopped by the screw, which is now all the way out.  However, this is how the governor came back from the repair station, approved as in limits and supposedly making 2700 RPM on their test bed.  So it doesn't seem likely that it's incorrectly set.

The arm swings smoothly and firmly to the stop, with 1/8" of travel still left.  Firmly pressing in the prop cable does not give any more RPM.

I do still suspect the governor.  I don't understand how giving it more travel did not increase the RPMs at all.

Edited January 10, 2023 by Z W

[Z W]

8 minutes ago, M20Doc said:

No one mentioned the air box repair to you, so who knows  what else may have been done.  If you have a McCauley it’s a mute point.

Hah, good point.  To be fair to them, we knew they repaired the airbox due to metal fatigue cracks.  We've been waiting for the Mooney factory to make us a new one since May. They declined to release the shop drawings to let us have it made.  So it was re-welded by a local welding shop.  I just did not know they'd also added the rubber to the intake boot.

It's a McCauley prop.  I'll put checking the blade angles on the list of possibilities.  Thanks for your help.

[Guest]

8 minutes ago, Z W said:

Hah, good point.  To be fair to them, we knew they repaired the airbox due to metal fatigue cracks.  We've been waiting for the Mooney factory to make us a new one since May. They declined to release the shop drawings to let us have it made.  So it was re-welded by a local welding shop.  I just did not know they'd also added the rubber to the intake boot.

It's a McCauley prop.  I'll put checking the blade angles on the list of possibilities.  Thanks for your help.

It’s a real brain tease and must be frustrating for you. The blade pitch angles should be included with the STC, and should be something like these from the original Mooney Type Certificate.

[EricJ]

5 hours ago, Z W said:

What seems odd about the EGTs?

It's hard to guess exactly what was going on during the runup in the last run in the last data set, but it looks like there's a mag check at the end of the "runup" where the EGTs go up, but they don't come back down before the bigger prop pull.    This suggests that perhaps it was left on one mag?    Like I say, it's hard to tell what was going on, but the expected behavior is that the EGTs go up when on one mag and come back down when back on both.

[PeteMc]

5 hours ago, Z W said:

Also in the latest data we were cycling the prop a lot, both at low and high power settings, trying to see if flushing some oil through the prop / governor would make any difference.

I don't see any advantage to cycling the prop at high RPMs and putting that unneeded stress on everything.  Cycling at 1500ish (prob. will vary with different models) will get the oil flowing.  If it didn't then you wouldn't see the prop change.

 

[Ed de C.]

Here's what comes to my mind based on my experimental experience with a governor problem.  I'm not familiar with the turbo engine setup in Mooneys (only what I had in my Lancair and even that was a long time ago) so let's have to Pro's check my thinking.  I'm pretty sure (but not certain) that the governor has two RPM limits: one set internally by the governor shop (2,800 rpm I think you said) and the second being the external set screw adjusted to suit your installation).  If I'm wrong about that, some of my narrative below is wrong.  Sorry for the length of this, but if just one sentence helps you, it'll have been worth it.

25 years ago when I was preparing my Lancair 360 for its first flight (Lycoming IO-360 with a governor that I purchased from Lancair) I goofed up the governor installation.  For some reason I cannot remember I had removed the cable attachment arm from the splined shaft and put it back on a couple of splines off.  That would have changed the relationship between the governor's internal workings and the stop screw.  

The test pilot did not like how the static runup RPM was only 2450 when 2700 was the MT 3-blade propellor's limit.  We adjusted the governor (set screw I believe) to get to something like 2650 on the ground.  When he took off on that first exciting flight, the prop over sped to 3,000 (he thought) before he caught it and dialed it back.  When I called Lancair they confirmed that the original 2450 static RPM was about right for the 3-blade MT prop (the 2-blade Hartzell would static run higher).  That engine and prop combo was a case where the prop would not lift off the fine pitch stops until well into the takeoff roll.

You know your propellor/governor was working properly 6 months ago, so my story doesn't help, perhaps other to observe there may be another "degree of freedom" here which is the cable arm to shaft orientation. 

What I can't figure out is once you effectively moved the stop screw out of the way, why you aren't getting more RPM.  I'm suspicious of the governor shop's inspection confirmation, but you said the low rpm problem started before the governor shop got their hands on it.  (You also mentioned the governor was overhauled not that long ago.)  If your RPM is not increasing as you accelerate on the runway, then the prop must be off the fine pitch stop and the governor is controlling your rpm but at too low a level.  It sounds like your prop operates off the fine pitch stop normally since you used to see 2,700 at the start of your takeoff roll. 

Another way to think of it:  during the runup, as you advance the throttle and as the MP rises, RPM should rise obviously.  Which one stops rising first should tell us a lot.  If your A/P is right then MP will stop rising first, indicating the engine isn't developing enough power to lift the prop off the fine pitch stops.  If the RPM stops rising first, then the governor is doing its job controlling RPM but at too low a level.  Then, with full throttle, and the prop control full forward, slowly pull the prop control back and note what happens.  Based on your stop screw being backed all the way out, I would expect you have some dead travel on the prop control before the RPM begins to drop below 2550.  This confirms your max RPM is being controlled internal to the governor, not by the stop screw and suggests the governor is set too low.  Make sure you can control the prop down to a reasonably low RPM.  This should convince your A/P that a test flight is or is not necessary.

Someone else alerted to the need to make sure the prop control cable is adjusted so that the governor stop screw is the stop, not the control cable itself.  It seems you've confirmed that already.  You said at one point that max RPM was moving around.  That's curious and could have been related to the control cable.  The broken control cable end is also curious, although you've now fixed that.

The solution to the MP problem ended up being fixing the crushed airbox tube.  Our diagnostic instincts went to fuel and intake leaks, we didn't think of intake restrictions.  Had we known about the airbox work, we might have asked.  I bring this up because I wonder similarly about the RPM issue.  It was working before the annual, and then it wasn't.  I would catalog everything that the shop did (like the airbox work) that could conceivably have affected the governor, propeller, control cable, and anything else related to RPM.  We probably are missing a piece of information, it's not that our powers of deduction, logic or engineering have deserted us.  Unfortunately, the loss of your original A/P is a setback.  

Since it feels like we've run out of ideas, you need to do something different at this point.  I see 3 options for you:

1.  Do what your shop suggests, and test fly the plane.  At the end of the runway and at full power, confirm the prop control is controlling the RPM by pulling it back a little.  If you see a max of 2,550 pull it back to 2,400.  You have plenty of power.  Inquire about governor failures and what will happen if it breaks so that you are ready.  Once in the air, advance the RPM carefully to see if you have control up to 2,700 rpm.  If you don't and it stops at 2550 then you and the shop know the problem is not solved.  If it does control up to 2,700 then you may conclude the governor is OK and it's something else that changed from when your plane went into annual.

If he'll do it, take the A/P up with you for a second set of eyes and brains.

2.  Don't fly the plane and replace the governor (or call the governor shop, describe the problem and see what they say to do).  Maybe the prior overhaul had something to do with your trouble (but it doesn't seem likely).

3.  Get a second opinion about the whole situation and then probably take the plane to a different shop.  I wonder what the Savvy guys would say about the RPM problem.       

Another troubleshooting challenge I had on the Lancair that might be useful:  Early in its life, I was descending from altitude well into the yellow arc.  The winter air was smooth and clear.  Suddenly, there was a loud and low tone coupled with a slight vibration.  Instinctive reaction kicks in almost before the adrenaline starts to flow.  I chopped the throttle and eased the stick back to arrest the descent and reduce airspeed.  After a few seconds the tone stopped.  Flutter was my main concern and was a popular discussion topic in Lancair forums.  The stick had been smooth the entire time (the vibration was not coming through the controls) and obviously no surfaces had departed the aircraft, so I began to doubt flutter and think towards something else. 

It took a year to find it.  I quickly suspected a flimsy panel was hitting some sort of resonance.  I could duplicate the tone at high speed (I had convinced myself I wasn't being reckless) and determined it was purely airspeed related, not RPM or throttle related.  A big moment came when I cracked the emergency gear extension valve (the gear is actuated by hydraulic powered by an electric pump, so you pull the breaker and open the manual valve to let the gear fall free).  I cracked the valve just enough to change the pressure of the gear doors being held against the wing and fuselage.  The tone went away.  I was then certain that one of the gear doors (there were 5, two attached the main gear, 3 actuated by their own hydraulic cylinders) was the culprit.  But which one?

Here's where I wasted a lot of time.  I started hacking up gear doors that didn't seem to fit well, thinking aerodynamic interaction must be it.  Nothing worked.  The nose gear door had a particularly bad fit, so I removed it entirely, no luck.  I changed preloads all around and found this affected the airspeed at which the tone appeared, but not in a way I could make sense of.  I taped things in front of the doors to change airflow.  Finally, I put down the tools and decided I needed to confirm which door was vibrating.  I realized I could remove the interior and see into the main wheel wells from the cockpit.  And I could put my hand on the hydraulic cylinders attached to the inboard gear doors.  Sure enough, when the tone started, the right-side inboard door was singing away, and the attached cylinder vibration confirmed it. 

Now, how to fix it.  I looked at that door and it's attaching piano hinge and kicked myself.  The hinge was loose.  When attached to the actuating cylinder, the looseness wasn't obvious, but when disconnected, it was.  It was also too short (not enough wheelbase compared to the left side).  I replaced the hinge, and the tone never came back, at least as far into the yellow arc as I was willing to go.  Thinking back at the time, the high-speed descent (possibly the first time I had flown that fast) placed a lot of load on the gear doors under the wing.  The hinge wasn't up to it.  In hindsight, it all made sense.  But until the end, I was working with not enough information to isolate the problem down to the offending component.  

I think the moral of this story is never give up in the troubleshooting process.  The culprit is just hiding, maybe in plain sight.  

 

 

 

[N231BN]

I don't think the governor is the problem as the RPM increases on the simulated takeoff runs. I think it is time for you to bring in another mechanic or at least have them contact someone at Continental for guidance. It is hard to troubleshoot this way when we don't know what exactly has been done by that shop.

Do they have a differential pressure gauge to set the metered fuel pressure or are they using the totalizer? It is OK to use the totalizer but it must be accurate. Your FF seems very high, especially since you aren't making rated RPM. See attached chart for the correction factor.

Check the intake and exhaust for obstructions. Make sure the wastegate is controlling the manifold pressure and not the "pop-off" valve.

[Ed de C.]

1 hour ago, N231BN said:

I don't think the governor is the problem as the RPM increases on the simulated takeoff runs. I think it is time for you t

Ah, I missed that in the thread.  If RPM rises on the takeoff roll, then that certainly suggests low engine power (or the prop was adjusted, which I think is ruled out.)  

Back to the engine:  Fuel, Air, Spark for each cylinder.  What the instruments say is happening, and what is actually happening.  I think you confirmed MP and FF (at least according to the instruments you and the shop have).  Are the mags, wires, and plugs confirmed OK?  If you adjust the mixture during the high power runup does anything interesting happen?  I recall that on cranking one magneto's timing is retarded for easier start.  Any chance that retard is stuck?  Timing check should have eliminated that possibility.

As N231BN said, you want to escalate this to higher powers.  You've done a lot.  Someone's going to have the answer for you.

[Z W]

18 hours ago, N231BN said:

I don't think the governor is the problem as the RPM increases on the simulated takeoff runs. I think it is time for you to bring in another mechanic or at least have them contact someone at Continental for guidance. It is hard to troubleshoot this way when we don't know what exactly has been done by that shop.

Do they have a differential pressure gauge to set the metered fuel pressure or are they using the totalizer? It is OK to use the totalizer but it must be accurate. Your FF seems very high, especially since you aren't making rated RPM. See attached chart for the correction factor.

 

That makes sense, about the governor maybe not being the problem.  Bringing in another mechanic is not really an option, that I'm aware of, we'll have to take the plane to them. 

I'm getting more comfortable with that option as we rule out causes.  With the air intake issue resolved, it's running strong and reliably again, for the first time since it went in for annual in May.  A lot of my concerns were caused by it running differently (and sometimes roughly) every time we fired it up.

The shop ordered differential pressure gauges to set the fuel flows, and we had to wait a couple weeks for that, so I believe it was done per the engine setup instructions.  I'm now wondering if it was done with the air intake blocked off and the engine running on alt air, though.  I think it almost had to be.  Maybe re-performing the engine setup with an unrestricted air intake would be warranted.

Thanks to everyone for your thoughts.  Your help means a lot.

[EricJ]

4 hours ago, Z W said:

The shop ordered differential pressure gauges to set the fuel flows, and we had to wait a couple weeks for that, so I believe it was done per the engine setup instructions.  I'm now wondering if it was done with the air intake blocked off and the engine running on alt air, though.  I think it almost had to be.  Maybe re-performing the engine setup with an unrestricted air intake would be warranted.

That's not a bad idea, since one big variable in making static/takeoff rpm is the engine making sufficient power.

[Z W]

Plane is fixed. 

Thanks so much to everyone here who chimed in.  I don't think we'd have figured it out without Mooneyspace.  Cheers.

Edited January 10, 2023 by Z W

[N231BN]

How frustrating, congrats on getting it fixed.

[EricJ]

Glad you got it sorted out!