M20C - Throttle linkage coming disconnected

[jmbaute]

Long time lurker, first time poster. I'd like to relate a story that has happened to me now with two separate aircraft I've flown. The scenarios are so similar that it doesn't seem like it could be a coincidence, but I'd like to see if anyone has ever heard of this happening.

 

I had a 1968 M20C with several partners about 6 years ago. After we did an overhaul and had flown about 40 hours, one of my partners was flying at about 6000 feet when he noticed the throttle un-responsive. The linkage had become disconnected at the carb, presumably because the bolt either didn't have a castle nut or the pin came out. Long story short, he made a safe landing (dead stick) at the nearest airport.

 

Fast forward 6 or 7 years. I have moved to another state and am in another group with a 1974 M20C. We did an overhaul earlier this year, and when I did the first flight with the new engine on the plane, I made sure with my own eyes that this connection was secured, had a castle nut and cotter pin. All went well for the first 20 hours, when pretty much the same thing happened to my new partner. While flying a practice approach with an instructor, they found the throttle unresponsive and the engine was stuck at 15" MP. This time the results weren't as good- high speed on a dead stick landing resulted in a porpoise, prop strike and all the fun stuff associated with an engine tear down, but everyone walked away.  Upon inspection the bolt was missing and they found a couple washers inside the cowling, but everything else was gone.

 

This is just too weird to be a coincidence in my mind. These are two different planes, with work done by two different shops in different states, but the situation is nearly identical. I'm not sure how a nut with a cotter pin in it could come loose. I can't be sure that the in first incident that the fastener even had one, but in the second, I saw it with my own eyes and everything looked good. 

 

In the first case, the work of the mechanic has other reason to be suspect, but the second guy is really good. However, neither has a bunch of Mooney experience. 

 

I'd sure love to hear from anyone with similar experience, or more importantly, solutions!

 

Thanks,

 

John B

74 M20C KPSK

[Mooneymite]

There is a service bulletin and factory kit for replacing the throttle linkage on the older Mooneys.

 

Do you know if either of yours had this up-grade?

 

The older style had two chances of disconnect....the barrel nut and the nut that attaches to the carburetor arm....the new style is simpler and looks to be sturdier, but anything can come loose under the right circumstances!

[jmbaute]

Thank you for the reply. I have no idea for either aircraft. I just searched the Mooney Service Bulletins on the website and couldn't find it. You don't happen to know any more about this SB, do you?

[Mooneymite]

I couldn't find a service bulletin either.  Maybe there was only a retrofit kit?

 

 Don Maxwell wrote me that he had:  retro kit Pn. 660051-005, that includes a heim bearing, bolt, 2 spacers, flat washer, castle nut and cotter pin for  $132.82.   Contact:  dmaxwell@donmaxwell.com

[Hector]

Unfortunately I worked an aircraft mishap that resulted from a missing cotter pin (not installed during last maintenance action). If you saw that cotter pin installed you may also want to check it is the proper size and properly installed. I have seen installed cotter pins that are significantly undersized for the application (too thin). Combined with aggressive installation and you can pull the pin through the bolt hole. Depending on the application and some bad luck and the cotter pin can fall out.

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[jetdriven]

Our mixture linkage recently fell off, but it was at a cruise LOP setting and we made it back to land at the airport. It had the older style linkage that fits on a ball. The end of the linkage has a plug secured by a cotter pin.  I installed this personally myself when I put the new engine on and I know there was a cotter pin in it; I put it there. All I can think is the sharp edges of the plug cut the cotter pin over time with vibration. We removed this thing and installed a Heim bearing with an AN3 bolt. Its fixed for good now.

[chrisk]

I'm not a big fan of ADs, but this certainly seems like something that should be considered in this case, especially if there is a known fix. 

 

And for everyone who wonders why I go to a Mooney service center, this is why.  They might catch this sort of issues.  A non-Mooney mechanic has no hope.