Broken prop control knob

[jaylw314]

Oooohkay, so I was flying around today and the prop control broke.  The push button on the handle stuck in when I did my GUMPS checklist, and I heard it pop.  Afterwards, the knob just spun around without changing the RPMs.  I could still push and pull the knob to control it, I just couldn't make small adjustments.

After I landed, I futzed around with the button and was able to pull it out, at which point spinning the knob worked again.  However, pressing the button again just stuck in the "in" position until I could tease it out with my fingernails.

 

So my guess is, it's just a problem with the knob, not the control cable.  There must be some kind of spring that pushes the button out that broke.

 

Is this a common problem?  Are those knobs repairable, or do I need to replace the whole cable?  I don't want to risk grounding the plane while ordering parts with my instrument checkride next week.  On the other hand, I'm guessing it would look pretty fishy to the DPE if I was frantically trying to pull out the button after the runup...  I think the plane is still safe to fly--fine prop control is really a safety issue, and the prop control is stiff enough it won't change on its own.

[KSMooniac]

What plane, and how old is the cable? If over 20 years, I'd replace it, or frankly all three. You can get them pretty quickly from McFarlane. These are huge safety items that are often overlooked.

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[Bob_Belville]

10 minutes ago, jaylw314 said:

Oooohkay, so I was flying around today and the prop control broke.  The push button on the handle stuck in when I did my GUMPS checklist, and I heard it pop.  Afterwards, the knob just spun around without changing the RPMs.  I could still push and pull the knob to control it, I just couldn't make small adjustments.

After I landed, I futzed around with the button and was able to pull it out, at which point spinning the knob worked again.  However, pressing the button again just stuck in the "in" position until I could tease it out with my fingernails.

 

So my guess is, it's just a problem with the knob, not the control cable.  There must be some kind of spring that pushes the button out that broke.

 

Is this a common problem?  Are those knobs repairable, or do I need to replace the whole cable?  I don't want to risk grounding the plane while ordering parts with my instrument checkride next week.  On the other hand, I'm guessing it would look pretty fishy to the DPE if I was frantically trying to pull out the button after the runup...  I think the plane is still safe to fly--fine prop control is really a safety issue, and the prop control is stiff enough it won't change on its own.

According to McFarlane control cables should be replaced on about the schedule of engine overhauls. I notice that quite a few vintage Mooneys still have the original prop, mixture, throttle controls. $1000 will replace all 3.

 http://www.mcfarlaneaviation.com/Products/?ID=40599046&CategoryID=219

[Guest]

There is normally a small spring under the inner knob.  If you remove the screw and inner knob you'll find it.

Clarence

[jaylw314]

4 minutes ago, M20Doc said:

There is normally a small spring under the inner knob.  If you remove the screw and inner knob you'll find it.

Clarence

Anyone know where to get the spring?  I'll check tomorrow, I suspect the "pop" was the spring breaking...

The engine was a factory reman and is only about 150 hours old...

[KSMooniac]

Think about what happens when that mixture cable jams...

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[Bob_Belville]

1 hour ago, jaylw314 said:

Anyone know where to get the spring?  I'll check tomorrow, I suspect the "pop" was the spring breaking...

The engine was a factory reman and is only about 150 hours old...

It is unlikely that the cable was changed when the engine was.

[takair]

1 hour ago, jaylw314 said:

Anyone know where to get the spring?  I'll check tomorrow, I suspect the "pop" was the spring breaking...

The engine was a factory reman and is only about 150 hours old...

Depends on the type and vintage of the cable.  The newer ones aren't as easy to take apart.  If it is old, it's possible that the ball is worn and not fitting correctly.  The button moves this ball in and out of the threads that allow for vernier control.  If things on these cables start to break, it's probably best to start looking at a replacement....other things....like the cable are likely worn too.  The one good thing with the prop governor is that it is spring loaded to low pitch, unlike throttle and mixture that end up wherever  vibration takes them if the cable breaks.

[carusoam]

Time to know all you can about knobs... If the knobs were changed at OH, There will be a record in the airframe log...(?)

Knowing How they fail and what no longer works after the failure is important.

Having the RPM knob stuck in a low rpm position can make a go around impossible. Depending on conditions.

Some controls have a high rate of wear.  High enough that the recommendation to change them out while OHing the engine makes a lot of sense.

If all it is is a broken spring...  know that something didn't wear or break the spring.  Or that the whole thing is original to the airframe with several Khours...

If the knob button is stuck does the control still work.  If it works does it stay in position or slide out over time...?

Avoid being stuck at a low rpm setting when you are needing a high power setting... this would be bad.

While checking on the one knob, you might want to review the other knobs and controls... they have a tendency to wear and break over the timespan of several decades....

PP thoughts only...

Best regards,

-a-

[mooneyflyfast]

Remove the screw that holds the button on.. There is a nut inside that may have come off or loose. That's what happened to mine a few years ago. Easy fix. I doubt that the spring broke . It is a small spring not highly stressed. 

[mooneygirl]

My prop cable was getting harder and harder to pull out when cycling the prop on run up.  One day it just failed.  I got the new McFarlane cable actually through Mooney as they had it in stock. It was $30 more than McFarlane direct, but didn't have to wait the 10 days for it to be made.  I did get a message from a Mooney mechanic that said that

"Are you aware as to why the cables break? I have seen many pilots shove the cable in all the way to full high rpm, and then continue to try and go further by screwing the knob in. This has the effect of unwinding the braided cable over time and makes it wear out inside the cable housing."

 

Hope this is helpful. 

[Bob_Belville]

4 hours ago, mooneygirl said:

My prop cable was getting harder and harder to pull out when cycling the prop on run up.  One day it just failed.  I got the new McFarlane cable actually through Mooney as they had it in stock. It was $30 more than McFarlane direct, but didn't have to wait the 10 days for it to be made.  I did get a message from a Mooney mechanic that said that

"Are you aware as to why the cables break? I have seen many pilots shove the cable in all the way to full high rpm, and then continue to try and go further by screwing the knob in. This has the effect of unwinding the braided cable over time and makes it wear out inside the cable housing."

 

Hope this is helpful. 

Maybe, but color me dubious.

[jaylw314]

Broken spring in the knob as expected.  The "button" simply unscrews when you hold the knob still.  The small loop at the end broke, allowing the spring to slip off the button's shaft.  I called my A&P, he has a junk control that he can give me the spring out of.  I confirmed the cable was still attached at the governor and the cable controls it through its full range.

[carusoam]

Great follow up, Jay!

We all learned a new one today.

Add that to the list of springs that have failed near their ends.

Thanks for sharing the details.

Best regards,

-a-

Edited May 17, 2017 by carusoam