Stall horn and annunciator malfunction

[Schllc]

Plane is a acclaim ultra, but the little tab on the leading edge of the left wing is the same device that is on every mooney I have ever seen so I am hoping someone else here has experienced this. 
I took off today in fairly scattered weather and at about 1000 ft agl the Avionic warning voice started to say stall repeatedly, and the stall horn was blaring. 
my air speed was good and all functions were normal, and I was entering imc so I just asked act to level off so I could run through the systems and make sure all was ok. 
long story short we went back into vmc shortly after and I confirmed it was just a malfunction. 
I continued to my destination, landed, jiggled the little tab and it stopped the horn and annunciator.
i took off and flew it again without issue. 
has anyone else ever had this happen?

its baffling because the air pressure required to push the tab back is minimal. 

[Vno]

I have the same recurring problem with my J. The tab had gotten bent. Also there is play in the switch itself. In certain conditions the tab gets pushed up (Near stall) and the tab gets bound up in the wing and won't release. I was able to duplicate this on the ground. I bent the tab a very minimal amount and it seems to not happen as much. That minimal amount was like a millimeter. Stall warning works as advertised now.

Brian

 

[PT20J]

Funny how these things seem to happen at the worst times. I recall departing San Jose CA years ago IFR in a rented Arrow. Just as we entered the clouds, there was a strong odor and the cockpit filled with "smoke." The mechanic had returned it to service with a note that the heater was inop. He failed to mention that he left a hole in the firewall while waiting for a replacement part and then washed the engine down with solvent.

One of the really good things about Mooneys is that there is a circuit breaker for just about everything. So if it happens again, you can pull the STALL WARN breaker to make it shut up after you've determined that it's a malfunction.

Sounds like you have isolated the problem to the warning vane microswitch. It's probably a good idea to exercise the vane a couple of times during preflight. If it were mine, I'd do that and see if the problem recurs. If it does, then I'd have to dig deeper -- probably remove the vane assembly to check the wiring and probably replace the switch. But I generally try to avoid digging into stuff until the problem becomes repeatable.

Skip

[buddy]

I had the same problem once after washing the airplane, I must have pushed the stall warning tab up and over to the outboard side of the wing and the tab got hung up. I found the problem after landing and that was the last time that happened. I now make sure after washing around the stall warning tab that it moves freely.

[larrynimmo]

The stall switch is line $2,200 because it is calibrated...

[Schllc]

It wasn’t stuck on the ground. If it were it would have been chirping on the ground. 
It didn’t start until long after takeoff, that was the bizarre part. 
also, I have to get back to look at the breakers but I’m pretty sure I didn’t see a breaker for the stall warning. 

[carusoam]

Sch,

Do you have the Fiki heated stall warning vane, or just the ordinary one..?

 

There is a thread around here that has detailed the heater element wearing /falling off... Lots of pics to reference...

You might check to see if leftover bits of the heater materials are causing a challenge...
 

Best regards,

-a-

[Schllc]

No, no FIKI 

[carusoam]

In that case...

The stall vane is a piece of sheet metal connected to a stout industrial switch...
May want some cleaning... if it takes more than a light touch to move it up or down...

It may want time to dry, if somehow it became sensitive to water...

 

I test Mine daily... to confirm the tone... mine had gone months without making peep once...

 

Its a simple system in a tough environment...

If it gets bumped, bent, or internally soaked... It may need calibration, or replacement...

 

It can be an important reminder, so don’t leave it off for any too long...

 

PP thoughts only, not a mechanic...

Best regards,

-a-

[PT20J]

It's pretty hard to bend the tab since it is heat treated (I know - I seem to catch a rag on it every time I clean bugs off the wing). The Maintenance Manual has an admonition not to try to adjust it by bending it or it will break due to the increased hardness from heat treating.

Skip

[Schllc]

There is absolutely zero resistance on the tab, which is all the more curious because if there is no pressure to push the switch back, how would it trigger the alarm in flight?

there had to be something to block the contact, which is plausible on the ground , but highly unlikely to occur after takeoff. 
I haven’t been able to find a blowup of the mechanism, but the malfunction mid flight is counterintuitive to how it appears to operate.

[MikeOH]

I believe the system works by the stall switch grounding the circuit.  So, if there is an intermittent ground somewhere between the buzzer (Sonalert) and the stall switch...

[PT20J]

5 minutes ago, Schllc said:

There is absolutely zero resistance on the tab, which is all the more curious because if there is no pressure to push the switch back, how would it trigger the alarm in flight?

there had to be something to block the contact, which is plausible on the ground , but highly unlikely to occur after takeoff. 
I haven’t been able to find a blowup of the mechanism, but the malfunction mid flight is counterintuitive to how it appears to operate.

The tab shouldn't have noticeable resistance but if you push it up slowly you should hear the microswitch click. The blow up diagram you can't find is a Mooney drawing number. Mooney only provides a part number for the entire assembly and not the sub-assembly components. You could take it apart and find out the manufacturer and part number for the switch and replace it if that's the problem. Also, a MSC might be able to look up the parts from the parts portal and help you, or some like LASAR and Maxwell would probably rebuild it for you. But first, you would want to definitely determine what's causing the problem assuming it persists.

6 minutes ago, MikeOH said:

I believe the system works by the stall switch grounding the circuit.  So, if there is an intermittent ground somewhere between the buzzer (Sonalert) and the stall switch...

That's certainly a possibility, but the OP said it persisted after he landed and stopped when he jiggled the stall warning tab which would indicate something with the switch or mechanics of the stall warning mechanism.

Skip

[Schllc]

Cycling the switch on the ground indicated the circuit was working properly. 
carsoam said the switch has significant resistance, but mine had zero resistance, gravity was enough to close the circuit. 
which is even stranger that it would malfunction mid flight. 

[buddy]

My oral stall warning didn’t sound on the ground either only when I got a couple of hundred feet in the air taking off. Move the tab up and wiggle it from side to side and see if it hangs up, that’s what happened to mine.

[PT20J]

At low angles of attack, the stagnation point (point where the air divides above and below the wing) is above the vane. As angle of attack increases, the stagnation point moves down the radius of the leading edge and eventually the upward airflow lifts the vane and closes the circuit sounding the stall warning.

[GeeBee]

I have had some of these Safe Flight units go bad on various aircraft, especially after washing or rain. Often times, deluging it with WD40 cleans it up without having to pull it out. Might try that first.

 

[moosebreath]

DO NOT bend the tab. The tab is heat treated and will break.  You set the alarm speed by adjusting the mounting, the procedure is in the maintenance manual.  Inside the unit is nothing but a microswitch.  A common failure mode is for the switch to fail and the stall warning to sound either continuously or at random annoying times.  On my J you have to pull BOTH the gear and stall warning breakers to make it stop.

A new identical switch is available from Mouser for less than $10.  Buying the unit just to replace the switch is very expensive.  There is absolutely nothing subtle, complicated, or highly engineered about this sensor.
 

Search the archives, this has all been covered before.

[laytonl]

Mine occasionally goes off in heavy rain.  It will stay on for awhile ( I pull the breaker) and apparently dries out and quits wailing.  Lee

[carusoam]

4 hours ago, carusoam said:

 

The stall vane is a piece of sheet metal connected to a stout industrial switch...
May want some cleaning... if it takes more than a light touch to move it up or down...

 

 

I test Mine daily... to confirm the tone... mine had gone months without making peep once...

 

PP thoughts only, not a mechanic...

Best regards,

-a-

 

3 hours ago, Schllc said:

Cycling the switch on the ground indicated the circuit was working properly. 
carsoam said the switch has significant resistance, but mine had zero resistance, gravity was enough to close the circuit. 
which is even stranger that it would malfunction mid flight. 

 

I cut out some of the excess in my post above...

I’m still learning to write...

Its possible that something got on the vane or in the switch...

 

Once in the air.... the force of air is pretty strong...  

as the angle of attack increases.... the split line moves down the front of the leading edge...

When the split line gets below the vane, the vane flips up, actuating the switch...

How and why this happened Is surely an unknown... unless you bumped into some vertical wind gust and forgot to mention it...

You probably would have bumped your head on the ceiling really hard at the same time... 

 

Under ordinary conditions such as landing, the vane gets activated right at the moment prior to the wheels touching down...   some days the vane sticks on for a few more seconds while the speed bleeds off...
 

Gravity vs wind speed....

 

So... test it on the ground to see if it is behaving properly... if unsure, see your mechanic...

Take it for a slow flight demonstration... at altitude... you should be able to turn the switch on during slow flight... if unsure, see your CFI...

 

PP thoughts only, not a CFI or mechanic... so not making any recommendations outside of ordinary flight for a Mooney...

 

Best regards,

-a-

[Schllc]

21 hours ago, PT20J said:

Funny how these things seem to happen at the worst times. I recall departing San Jose CA years ago IFR in a rented Arrow. Just as we entered the clouds, there was a strong odor and the cockpit filled with "smoke." The mechanic had returned it to service with a note that the heater was inop. He failed to mention that he left a hole in the firewall while waiting for a replacement part and then washed the engine down with solvent.

One of the really good things about Mooneys is that there is a circuit breaker for just about everything. So if it happens again, you can pull the STALL WARN breaker to make it shut up after you've determined that it's a malfunction.

Sounds like you have isolated the problem to the warning vane microswitch. It's probably a good idea to exercise the vane a couple of times during preflight. If it were mine, I'd do that and see if the problem recurs. If it does, then I'd have to dig deeper -- probably remove the vane assembly to check the wiring and probably replace the switch. But I generally try to avoid digging into stuff until the problem becomes repeatable.

Skip

No breaker for the stall warning In the ultra 

[carusoam]

Check that twice...

Any wire in an airplane has the opportunity to have its insulation worn off presenting a fire hazard if there is no circuit breaker or fuse...

Another hint... 

There may be warning posted somewhere that mentions that the stall horn won’t work if you turn off all the electricity... which is an emergency procedure...
 

Maybe it is protected by a CB, but labeled ‘other important stuff’ or something similar... or ‘master’
 

Everything gets protected... some circuits are hot all the time, but still protected... clocks and memory devices, frequencies, and fuel used...

I believe the stall warning for the Long Bodies is on with the master...  making it a tough choice if the stall warning is on for the entire flight...

PP thoughts only, not a mechanic...
 

Best regards,

-a-

[ArtVandelay]

No breaker for the stall warning In the ultra 

I assume there’s a fuse somewhere.

[PT20J]

23 minutes ago, ArtVandelay said:

I assume there’s a fuse somewhere.

You'll have to check the schematic to see how it is wired. Mooney usually doesn't use many fuses. But, I'm not sure how the models with the voice annunciation work -- it might require pulling the breaker on the annunciator. On the planes with just a Sonalert, power goes from the breaker to the Sonalert and then to the wing mounted stall switch and then to ground.

Skip

[Schllc]

It’s a g1000 so it’s not really an option to turn off the avionics. 
the only way I could silence the alarm was to pull the audio breaker which prevents me from transmitting me on the radio, but I can hear the radio. 
I went through the schematic in the poh and there is no reference to this annunciation anywhere at all. 
Still baffled as to how it got triggered in flight and stuck in the position. 
I was in a cruise climb and my airspeed had not dropped below 140kts when it started chirping. 
haven’t been able to find a schematic of the switch online, and removing the switch doesn’t appear to be a simple task.  I also don’t know if removing the switch without disassembling it will provide anymore explanation.