Persistent Horn 5-10 minutes into flights

[Chad Brubaker]

 I am having trouble with my 1969 M20F buzzer alarm going off 10 minutes or so into flight and will not go off until I land and idle the engine completely down. The sound somewhat fades away as the vaccum pressure light comes on. Then as I speed the engine back up the buzzer will again come back on. I activated the stall horn on the ground and popped the breaker out while it was buzzing and that sound went away. That same breaker does not take away this buzzing sound. So then while this buzzer was going off in flight I slowed the plane down to idle without the wheels down to see if that sensor was the problem. I clearly heard the gear horn go off as it was a little louder but the same sound so I don't know that this is the problem. Whatever it is it seems like it takes about 10 minutes to warm up in flight and it won't go off until the engine is completely idleing.

  Any ideas what I have going on? 

[PT20J]

First thing I would try is to pull breakers one at a time while it is buzzing to isolate the circuit that is associated with it.

[Matt Ward]

1 hour ago, Chad Brubaker said:

The sound somewhat fades away as the vaccum pressure light comes on.

Do you mean a high/low light? Is the suction gauge showing anything abnormal, if you have one?

[Matt_AZ]

I recently encountered this same issue. I tracked it down to my stall switch. It may be due to weather changing but a quick check should be able to see if the stall switch is sticking on yours. 

[carusoam]

Welcome aboard Chad!

This should be a pretty easy challenge to figure out...

Lets get it isolated like they described above...

We have a few horns sounding in the ceiling...

Some are a bit easier than others to test...

 

The usual ones...

1) Stall horn... electrical power on, stall vane lifted... 

2) Gear horn... electrical power on, gear up, and throttle out...

2.5)  Each one gets a different ‘speaker’.  There is a name for these piezo electric buzzer things.... there may be two or three in the plane with two in a tough place to reach above the ceiling panel...

 

3) Often what happens, aged wires get a little wear going on their insulation, allowing power to flow through one of the sound making devices...

4) One of the things you can check... to find how they are wired is the electrical diagram in your POH... it isn’t very complex... but doesn’t tell you very much about where everything is either...

PP thoughts only, not a mechanic...

Best regards,

-a-

 

[Chad Brubaker]

My plane only has a vacuum gauge light it doesn't have a gauge. Is there any horn on this circuit? Again whatever circuit it's on it seems like once something warms up it starts going off. 

[Matt Ward]

4 hours ago, Chad Brubaker said:

My plane only has a vacuum gauge light it doesn't have a gauge. Is there any horn on this circuit? Again whatever circuit it's on it seems like once something warms up it starts going off. 

I am unaware of any horn related to the vacuum; the only two horns I know of are the stall horn and gear horn as indicated above.  Your comment about the light got my attention.  I have a suction gauge, high vac light, and low vac light.  Occasionally on takeoff and through climb, the high vac light will come up.  Once I transition out of climb, figure about 10 minutes, it goes away.  I don't worry about it too much as the instruments work fine and the gauge indicates less than 6 (upper end of green is near 5).  But I'm wondering if maybe you have some sort of non-standard horn wired to a vacuum condition.  That's a wild guess and the other suggestions are likely far more probable.  

[Guest]

Just a guess........airframe overspeed warning!  Mooneys take a few minutes to accelerate to cruise speed.

Just kidding of course, stall warning system seems the likely candidate.

Normally the 2 “Sonalerts” for gear and stall warning are in the overhead console.

Clarence

[Keith20EH]

Just a thought, but I had an older GPS with a aural alert when it lost signal, it would sound the alarm,very annoying.  It ended up on the pile with my other outdated radios, they made the prettiest black smoke..