Stall warning in 177 kt

[Fredrik Andersson]

I was flying back to Sweden from the UK yesterday. Went down from FL190 over the North Sea through a layer of icing in IMC. It was not much, not the complete windscreen and just some mm on the wing. The ice went away and then suddenly out of the cloud and when the ice melted below 9000 ft and speed still was about 177 kt, the stall warning started slowly to scream. The wings had all the ice melted away and after 5-10 minutes it began to scream. I had to take out the circuit breaker. After landing and in warm temperatures, it was still screaming if I enabled the circuit breaker again.

I experienced a problem with the stall warning earlier and I think that had to do with why I couldn't turn off the Avionik Master last year. Suddenly the Avionik Master started to work after I have exercised the stall warning switch, but I was not able to prove it since it had gone then. 

My Mooney M20K 252 has no TKS so I think it is not preheated. 

Does anyone have experience of this?

 

[buddy]

Try moving your stall warning tab up and down and side to side, mine hung up once after washing the airplane. I can feel where the stall warning tab hangs up on one side so I now know to to check the tab after washing that I didn’t hit it with the sponge when cleaning the leading edge of the wing.

[Fredrik Andersson]

I tested it in the hangar, up and down, but it was screaming all the time. I will test it when it is dry someday. It doesn't feel stuck. It moves very freely. Can it be the electronics after, like the summer making the signal that has gone bad? It sounds like it comes from the speaker in the ceiling, so no separate speaker for the stall warning?

[Tommooney]

I had that happen at 10K' cruise. Pulled breaker. after landing put breaker back and ok for a while ( or maybe it had already fell into wing and I didn't catch it at the time ) . Then the stall warning came loose and fell back into wing. Mark at Top Gun put it back together at minimal cost and has worked ever since....I'm not sure what was wrong except it came loose?

Good luck.

[Fredrik Andersson]

This was not ok after landing and even after moving it up and down in the hangar...

[Fredrik Andersson]

I visited my Mooney today and it has been standing dry in the hangar for some days, it is no different situation. When moving the stall vane up and down you can feel and hear it switching. This is without any electrical power on. See movie:

If I turn on the master (and the fuse). The signal is continuous. See the movie below:

Is the vane the problem?

 

[0TreeLemur]

The mechanism is a simple microswitch.  Your contacts are not disengaging when the tab is fully downward deflected.   Most likely out of adjustment.   Perhaps the ice load somehow put enough force on the mechanism to push it out of adjustment.  Perhaps the switch has failed.

It requires servicing.  The mechanism as I've heard it described is very simple.  

Mine will sometimes trigger after I wash the wings and catch a rag on it.   The next time I turn the master switch on, the warning sounds.  The stall horn only goes off if I go outside and push down on the tab.  I'm worried that it is nearing an out of adjustment condition.

Good luck.  Keep us posted so we can learn from your experience.

[N201MKTurbo]

The microswitch has a snap action mechanism that causes a spring loaded arm inside to snap back and forth a relatively large distance compared to the motion of the actuation button the stall vain pushes on. This snap action lever is obviously working, you can hear it in the first video. The snap action lever pushes on the leaf spring contact in the bottom of the switch. That leaf spring contacts are stuck together. Either from contact welding or corrosion. You can try filling the switch with contact cleaner through the space around the button and case. And then actuating the switch until the contacts free up. If you can’t get the contacts to free up, you can replace the switch. You will need to drill out the rivets and use nuts and bolts to put it back together. Is this kind of repair approved? Probably not. Otherwise, you will need to pay the $4500 or whatever they get for them these days. 
 

If your plane is old enough, you are allowed to replace standard parts with OEM standard parts. You could claim the base switch is a standard part and you are allowed to replace standard parts. If you make it work, and do a quality job of replacing the switch, it is unlikely anybody would give you grief about it.

The base switch isn’t cheap. Last time I looked for one, they are hundreds of dollars.