Airworthiness Concern Letter 9/24/2024 on legacy Control Yoke

[LANCECASPER]

Mooney M20 Control Wheel Shaft Fatigue Cracks 
Notice Number: NOTC3947

 

The FAA is investigating the report of a control wheel separation on a Mooney M20C aircraft due to fatigue cracks in the control shaft or control wheel attachment hub. This failure might result in momentary loss of control of the aircraft until the pilot or copilot is able to regain control with the second control wheel. This may reduce the capability of the airplane or the ability of the crew to cope with adverse operating conditions to the extent that there may be a significant reduction in safety margins, especially in certain phases of flight, such as takeoff and landing.

The FAA’s Aircraft Certification Service recently issued an Airworthiness Concern Sheet (ACS) to highlight this concern and the ongoing investigation including a photograph of the failed part. The FAA is recommending operators visually inspect the control wheel and provide information including description of damage, available photos, airplane serial number, and time in service since installation.

To view the Airworthiness Concern Sheet please select this link or paste the address into your browser: https://www.faasafety.gov/files/notices/2024/Sep/2024-09-25_Mooney_Control_Wheel.pdf

If you have any questions or comments, please contact the Aircraft Certification Service through Jacob Fitch at Jacob.Fitch@faa.gov.

 

[ragedracer1977]

Seems to me that wheel had been cracked/cracking for some time and should have been caught

[Shadrach]

I have theorized for decades that compliance with the AD on the yoke was overly invasive and could create more problems then it solves. I would not be surprised if the initial cracks were caused by trying to remove or reinstall the taper pin.

[Shadrach]

3 minutes ago, ragedracer1977 said:

Seems to me that wheel had been cracked/cracking for some time and should have been caught

Agreed. Seems unlikely that there was not evidence of impending failure long before it actually failed.

[ragedracer1977]

3 minutes ago, Shadrach said:

I have theorized for decades that compliance with the AD on the yoke was invasive and could create more problems then it solves. I would not be surprised if the initial cracks were caused by trying to remove the tapered pin.

I literally just texted 201MKturbo the identical sentiment

[Shadrach]

56 minutes ago, ragedracer1977 said:

I literally just texted 201MKturbo the identical sentiment

Great minds think alike. I will likely share my opinion with Mr. Fitch.

[N201MKTurbo]

That is the copilots yoke. They rarely get used. Why would they fail? And who cares? 

[tony]

7 hours ago, N201MKTurbo said:

That is the copilots yoke. They rarely get used. Why would they fail? And who cares? 

which supports Ross's assertion 

[markgrue]

In the military we called it maintenance induced maintenance.   We broke more things while inspecting them than we discovered already broke.

[LANCECASPER]

Mooney sent this out this morning


Sent from my iPhone using Tapatalk

SL-24-06.pdf

[Shadrach]

I wrote Mr. Fitch this morning outlining my concerns that AD 77-17-04 and the instructions for continued airworthiness therein might contribute to an inspection/maintenance induced failure of the components depicted in the recent Airworthiness Concern Sheet.  I would encourage anyone else that has shared these concerns to send a comment. I think that there are more effective and far less invasive ways to mitigate a potential yoke/yoke shaft failure.

[AJ88V]

One issue is that any crack is potentially hidden under the plastic (powdercoat?) finish which is flaking on mine (looks like a crack, but evidently metal underneath is good).

I talked to my A&P about this and specifically asked if we could adapt with this nice, machined billet ACS yoke

(https://www.aircraftspruce.com/catalog/appages/acscontrolwhl3b.php?clickkey=7575)

That got a hearty NO response.  So it's ok to spend $750 for a 50 year old used part made of cast metal that is known to crack, but we can't substitute a modern part like the ACS which is both stronger and cheaper.

[Shadrach]

3 hours ago, AJ88V said:

One issue is that any crack is potentially hidden under the plastic (powdercoat?) finish which is flaking on mine (looks like a crack, but evidently metal underneath is good).

I talked to my A&P about this and specifically asked if we could adapt with this nice, machined billet ACS yoke

(https://www.aircraftspruce.com/catalog/appages/acscontrolwhl3b.php?clickkey=7575)

That got a hearty NO response.  So it's ok to spend $750 for a 50 year old used part made of cast metal that is known to crack, but we can't substitute a modern part like the ACS which is both stronger and cheaper.

The problem is that the replacement yoke has no airworthiness documentation.  So from a legal standpoint, it's about the same as fitting a set of bicycle handle bars.  It could be done and and I can see no reason why it would fall outside the bounds of a minor alteration, but I know of no IA's that would want their name on it.  Apparently someone was willing to sign it off. There is a comment from a Piper Cherokee owner that is very pleased with the installation.

[N204TA]

I really don’t see the big deal.

[AJ88V]

36 minutes ago, Shadrach said:

The problem is that the replacement yoke has no airworthiness documentation.  So from a legal standpoint, it's about the same as fitting a set of bicycle handle bars.  It could be done and and I can see no reason why it would fall outside the bounds of a minor alteration, but I know of no IA's that would want there name on it.  Apparently someone was willing to sign it off. There is a comment from a Piper Cherokee owner that is very pleased. 

Agreed.  My A&P maintains a lot of antique aircraft and is pretty willing to fabricate a part you can't get to keep a bird flying.  But if there is an available used part we can get (in reasonable time), then that's what he'll insist on using.  Never mind that the 50 year old part costs a fortune and is inferior.  And the only reason the 50 year old part costs a fortune is because there is a limited supply and no acceptable substitute.

I just wish there was some recognition by the FAA that we aren't actually making planes safer this way.  We need something like the acceptance of Basic Med for aircraft, even if it's on a case-by-case basis (ok, 337).

The counter argument is that our Mooney fleet is pretty consistent, unlike antique aircraft (and many homebuilts) where no two are alike.  The consistency of certified aircraft does add value.

I wonder what it would take to get an STC for that ACS yoke on a Mooney?  The part is cheap enough that you could risk destroying one to show it was adequate.  Make a test stand and show it will take loads far in excess of the original part?  I'm sure it would.

[N201MKTurbo]

12 minutes ago, AJ88V said:

 

I wonder what it would take to get an STC for that ACS yoke on a Mooney?  The part is cheap enough that you could risk destroying one to show it was adequate.  Make a test stand and show it will take loads far in excess of the original part?  I'm sure it would.

Somebody doing the paperwork. Why don't you start it? You might get some help from ACS if they can sell a bunch of yokes. I can't see the FAA having an issue with this STC. The engineering would be easy.

There would need to be a PMA also.

You may be able to do it without an STC if you can get a PMA for an equivalent for the Mooney part number. I would call the nice lady at the MIDO and ask her what she thinks.

[47U]

15 hours ago, Shadrach said:

I would not be surprised if the initial cracks were caused by trying to remove or reinstall the taper pin.

My ‘63C has the yoke installed with an AN3-14A bolt, no tapered pin.  My (very brief) research shows the tapered pin shows up starting with production in 1965?

I replaced my control shafts this past summer retiring the 500 hr AD inspection.  Mooney had them in stock.     

[DCarlton]

I'm no expert and I haven't done any mathematical analysis, but it's always troubled me that the yoke rocks on the shaft if the set screw isn't tight.  There's opportunities for stress at the pin that holds it on, and there's more opportunities for odd stresses where the set screw bottoms out on the shaft.  And.... most folks pull with their left hand creating side loads on the whole assembly.  When I'm pushing or pulling hard I try to use two hands (because I'm paranoid; I've played out in my head how I would grab the right yoke and get in the right seat).  Now if I designed an aircraft it would never get off the ground it would be so over designed.  With that said, I've fixated on that yoke attachment as long as I've owned the plane; never liked it.  Gotta wonder how you do an analysis on a cast part like the yoke and what causes failure; is fatigue an issue with castings; how do you ever NDT or QA a casting?  Is there some sort of internal corrosion that occurs?  To expertly analyze this connection between the shaft and the yoke and the safety margin isn't easy.    

Would be nice if Mooney could produce enough yoke upgrade kits to make them "affordable".  

[DCarlton]

15 hours ago, Shadrach said:

Agreed. Seems unlikely that there was not evidence of impending failure long before it actually failed.

Interesting that there's a crack in the set screw hole too.  

[DCarlton]

30 minutes ago, N201MKTurbo said:

Somebody doing the paperwork. Why don't you start it? You might get some help from ACS if they can sell a bunch of yokes. I can't see the FAA having an issue with this STC. The engineering would be easy.

There would need to be a PMA also.

You may be able to do it without an STC if you can get a PMA for an equivalent for the Mooney part number. I would call the nice lady at the MIDO and ask her what she thinks.

Guessing the engineering would be easier on the new machined part if properly fitting to the shaft, than trying to reverse engineer the old cast part and figure out the safety margin and potential causes of failure.  

[N201MKTurbo]

50 minutes ago, DCarlton said:

Guessing the engineering would be easier on the new machined part if properly fitting to the shaft, than trying to reverse engineer the old cast part and figure out the safety margin and potential causes of failure.  

The engineering would be figuring out the required forces, the forces a big human could impart on it during any crazy maneuver and then do an analysis on the new yoke and make sure it is strong enough.

If it was the same size, you would be good with that. 
 

Once you made a model of it, you could do FEA on it to make sure it is strong enough.

I would check part 23 and see if there are any requirements or guidance for yoke design.

[phxcobraz]

15 hours ago, N201MKTurbo said:

That is the copilots yoke. They rarely get used. Why would they fail? And who cares? 

That’s what caught my eye. Perhaps it was frequently used for leverage to get in and out, putting significantly extra force on it by the weight of a person vs just normal flight control forces. 
Another theory I thought of was did this plane frequently use the seatbelt as a gust lock while on the ground, allowing weather conditions to contribute to excess force on this location, being that in that configuration it would be at the stop limit. 

[N201MKTurbo]

I would have to say it is maintenance induced. You can see from the picture there is a crack at the set screw and the area of the yoke where the nut is on the taper pin is deformed inward. These are both signs of over tightening.

[Shadrach]

3 hours ago, N204TA said:

I really don’t see the big deal.

That is an ugly and catastrophic failure for sure. However, if I’m honest, I think the design and construction of the old welded shaft is superior to the new drilled and bolted version. The new design is definitely less expensive to produce though.

[Shadrach]

2 hours ago, DCarlton said:

I'm no expert and I haven't done any mathematical analysis, but it's always troubled me that the yoke rocks on the shaft if the set screw isn't tight.  There's opportunities for stress at the pin that holds it on, and there's more opportunities for odd stresses where the set screw bottoms out on the shaft.  And.... most folks pull with their left hand creating side loads on the whole assembly.  When I'm pushing or pulling hard I try to use two hands (because I'm paranoid; I've played out in my head how I would grab the right yoke and get in the right seat).  Now if I designed an aircraft it would never get off the ground it would be so over designed.  With that said, I've fixated on that yoke attachment as long as I've owned the plane; never liked it.  Gotta wonder how you do an analysis on a cast part like the yoke and what causes failure; is fatigue an issue with castings; how do you ever NDT or QA a casting?  Is there some sort of internal corrosion that occurs?  To expertly analyze this connection between the shaft and the yoke and the safety margin isn't easy.    

Would be nice if Mooney could produce enough yoke upgrade kits to make them "affordable".  

I will have to verify the next time I’m at the hanger, but I do not believe that my yolk “rocks“ on the shaft