Carb heat lock knob "popped out"

[hish747]

On the last day of my plane's annual inspection, the AP/IA called me to say as he was finishing up the carb heat lock knob "popped out." He doesn't seem too confident about putting it back. Does anyone have experience with this type of locking knob and what needs to be done to fix it?

 

[NordicDave]

There is a small ball bearing that provides the lock. When someone pulls the knob out too far that bearing rolls out. Usually happens when the other end detaches and no longer provides any limit to knob travel.

The notion it "just popped out" with normal cycling is questionable. If this carb heat cable was not fastened tightly to the carb heat actuator and the knob was pulled, the cable lock rod will pull through and the small locking bearing will pop out.

Replacement cables from McFarlane are not terribly expensive. They ship them will 2 extra bearings. worth saving them in case the above happens.

BTW, the cable is not attached to that knob in hand. It's attached to the other round outer ring still in the panel.

 

[texasclouds]

There should be a solid music wire looking cable attached to the knob shown. Looks like it broke. Search online for “—my plane model— carb heat cable”. Or ask him to order one.

 

[Dan Thomas]

There should be a solid music wire looking cable attached to the knob shown. Looks like it broke. Search online for “—my plane model— carb heat cable”. Or ask him to order one.

No. Look closely at the end of that plunger. It has a ramp machined into it, and the locking ball rides on that ramp. A spring under the knob pushes it out to lift the ball. The ball fits into a hole in the wall of the main plunger (still in the housing in the picture) and pushes against the cable housing, the part that fits into the panel. The main plunger has the wire attached to it.

The mechanic likely had the wire disconnected at the carb and pulled the control out too far and the ball fell out and got lost.

 

[hish747]

If by some miracle I found the bearing rolling around on the floor, could I place it into the ramp and try to force/finesse it back in? Does anyone sell the bearings by themselves?

 

[texasclouds]

What model plane is this?

 

[Clip4]

I agree the loss of the ball was caused by the mechanic. Unfortunately the ball is not going to be a listed part for the cable. It is likely a standard size if you could locate an assortment of ball bearings.

 

[Dan Thomas]

If by some miracle I found the bearing rolling around on the floor, could I place it into the ramp and try to force/finesse it back in? Does anyone sell the bearings by themselves?

Yup. As long as you still have the spring. You'd have to disconnect the wire at the carb air box and pull the control out until you see the hole. You push the lock knob's plunger in and push it all the way against the spring, with the ramp lined up with the hole, then drop the ball in. Keep holding the lock knob in while pushing the plunger back into the housing. The the control gets rerigged appropriately. The mechanic can do that.

 

[hish747]

I guess miracles do happen. Now I have to figure out how to get it in or approach the A&P to ask him to do it.

BTW, for the one who asked above, this a Grumman Cougar (GA-7).

 

[Half Fast]

I guess miracles do happen. Now I have to figure out how to get it in or approach the A&P to ask him to do it.

BTW, for the one who asked above, this a Grumman Cougar (GA-7).

Look closely at that BB and read the part number. Are you certain it's really the one that popped out?

 

[Gary Ward]

I guess miracles do happen. Now I have to figure out how to get it in or approach the A&P to ask him to do it.

BTW, for the one who asked above, this a Grumman Cougar (GA-7).

Good job finding it. I hope you can get it fixed with no cost.