Unkown Aural Alarm

[Luke Young M20K]

I have recently been flying a 1979 M20K. At random times we get a warning sound that is not coming through the headset. There is no instrument indicating an anomaly and no lights showing an issue. It happens quite frequently and goes away on its own. Curious to know if anyone has any insight. I know limited information, but any ideas would be appreciated.

Luke

[carusoam]

Welcome aboard Luke…

Got a pic of your panel?

Does the sound come from overhead… like where the stall horn sounds?

Does it sound like the gear-up horn when it blares?

 

It’s possible… that something is sending electricity through one of the piezo electric horns to ground… (aka sonalert)

There is an example of this around here somewhere…

Check both your stall horn, and your gear-up alarm to make sure they are working correctly… 

PP thoughts only, not a mechanic…

Best regards,

-a-

[ArtVandelay]

I would start pulling circuit breakers until it stops, then you’ll have the source.

[Luke Young M20K]

3 minutes ago, carusoam said:

Welcome aboard Luke…

Got a pic of your panel?

Does the sound come from overhead… like where the stall horn sounds?

Does it sound like the gear-up horn when it blares?

 

It’s possible… that something is sending electricity through one of the piezo electric horns to ground…

There is an example of this around here somewhere…

PP thoughts only, not a mechanic…

Best regards,

-a-

It does come from the overhead. Does not sound like the gear horn though.

[KSMooniac]

Could be from the autopilot

Sent from my LM-V450 using Tapatalk

[carusoam]

All sorts of electric generated sounds can be made…

1) GPS altitude alerts…

2) AP cutting out alerts…

3) Stall warning…

4) Gear-up warning…

 

IIRC… somebody was getting a buzz through one of the horns in the ceiling…. The issue was a bare wire feeding the horn…

Old fuzzy memory…

-a-

[Luke Young M20K]

Just from how random it is a bare wiring hitting it would seem plausible. 

[PT20J]

Usually, the overhead sonalerts are landing gear, stall warning, autopilot disconnect. I’d check for a faulty stall warning circuit. 

[carusoam]

Key word for these horns…

As Skip has generously pointed out…

They are known as Sonalerts… a brand named device… 

-a-

[PeteMc]

1 hour ago, Luke Young M20K said:

I know limited information, but any ideas would be appreciated.

Can you give us any other info on the flight conditions, etc. 

Are you new to Mooney's or just this K?  If you don't fly them a lot, are you pulling the power way back to slow down because ATC is dumping you way to high for the airport?  That could easily be the Gear Up warning which is triggered by the throttle position.  And as others have mentioned, stall horn, AP disconnect, etc.  So know what phase you're in on your flight might help.

 

[Luke Young M20K]

Most of the time it is happening when just cruising at 3,000 around 2500RPM 25 MP.  I am new to Mooney's. Safe to say new to everything 20-year-old CFI with about 400 hours total time.

[Guest]

Go up and stall the airplane and listen to that sound, compared to the sound when you pull the throttle back to idle with the gear up.  Both use Mallory Sonalert in the overhead panel.

The stall vane on the wing can be faulty or the throttle micro switch can be loose or failing.  Try wiggling/moving the throttle around to see if you can trigger it in flight

[cbarry]

Based on what  has been mentioned, I would suspect a incorrectly positioned/bent stall tab thus causing cruise flight angle of attack to trip the the stall tab.  It might appear normal during a preflight, but if the tab is bent upward just enough it could trip in flight way before it should—intermittently sounding the alarm .  Someone could have caught the stall tab with a towel while cleaning the leading edge or possibly bumped it while crouching under the wing to remove chocks, tie down rope….a knowledgeable mechanic should be able to troubleshoot…

[gmonnig]

What kind of ELT is in the plane, and does it have a remote switch? Do you monitor guard frequency while flying? My plane had the two remote wires going to the test switch come out of the pins. When the two wires ground out, basically touch anything, they'll trigger the ELT. I noticed because I heard a loud beep coming from the back of the plane with the battery off. I turned on my handheld and heard the ELT going off. The sound from the plane sounded like an autopilot disconnect from STEC or something. 

[anthonydesmet]

Just throwing another possibility out there…..maybe JPI alarm?  Maybe the JPI was reset to different warning temp due to maintenance or have you noticed any engine temp peculiarities?  If you don’t reset fuel quantity after top off is it low fuel warning?  Just thinking of other possibilities…..

maybe an alert you have set in that Garmin navigator?

for stall warning…..has there been a lot of turbulence flying at 3,000 ft?  May also add to stall warning as the issue…

 

can you isolate it to when using the electric trim switch?

 

 

[N201MKTurbo]

Push on the celling plastic and see if it comes on. The Sonalerts have a piece of plastic stuck on the aluminum above them, over the years the terminals on the Sonalerts can wear through the plastic shorting the terminal to ground. One of the terminals will blow the circuit breaker, the other will cause the Sonalert to sound.

If this is the case, just loosen the Sonalert and rotate it a bit, you will have another 30 years of trouble free service.

[Luke Young M20K]

On the last flight. I pulled the throttle to idle, and the gear horn came on and it sounded like the alarm that has been going off in flight. Hopefully we have isolated the issue. Thank Yall.

[Will.iam]

Do you have alt pre-select? I turned on my master to check my stall warning vane the other day and heard a weird alarm. Come to find out on my last flight i had set the pre-select to 1k above field elevation and on power up the alerter was doing it’s job of alerting me to 1000 to go. As soon as i changed the pre-select to a different alt the sound stopped. 

[Fly Boomer]

12 hours ago, Will.iam said:

Do you have alt pre-select? I turned on my master to check my stall warning vane the other day and heard a weird alarm. Come to find out on my last flight i had set the pre-select to 1k above field elevation and on power up the alerter was doing it’s job of alerting me to 1000 to go. As soon as i changed the pre-select to a different alt the sound stopped. 

What autopilot do you have?

[Will.iam]

Kfc-150

[trevttu76]

I have been caught off guard by the ILS beacon alarm.  Worth checking that if near ILS marker beacon.

Easy to mute and check if that's it...

[N231BN]

This is similar to the sonalert operation in my 252, the sonalert comes on when over 36 inches manifold pressure, but not when the gear is down. 
side note...if anyone knows how this is wired, please fill me in, cant find it on the schematics.
mike

I think someone royally messed up your gear horn switch. It should sound below ~14" with the gear up.

[jaylw314]

On 10/2/2022 at 5:52 PM, Will.iam said:

Do you have alt pre-select? I turned on my master to check my stall warning vane the other day and heard a weird alarm. Come to find out on my last flight i had set the pre-select to 1k above field elevation and on power up the alerter was doing it’s job of alerting me to 1000 to go. As soon as i changed the pre-select to a different alt the sound stopped. 

That's weird, I have a KFC-150 with the altitude preselector, and the only time I've never gotten any audio warning from it other than the autopilot self-test and disconnect.

[PT20J]

The later Js and probably the Ks used a tone generator for the landing gear and stall warning and it plays through an overhead speaker.