Can Stall Warning Occur at Incorrect Speed?

[MBDiagMan]

I’ve been trying to get proficient with the Mooney again after being out of it for a while due to an engine rebuild and flying my other plane a lot. Having trouble with my landings so I’ve been doing some pattern work.  I have had the stall warning go off on final while seeing 80 MPH of air speed.  It was IFR certified only a few months ago and the air speed readings seem reasonable.

Can the leaf type stall warning sensor on my ‘67 F model indicate stall at the wrong air speed?

[MMsuper21]

Was is windy?

[Shadrach]

Stall is a function of AOA. It can happen at any speed. It is common to get a horn bleat in the chop when there are temporary load excursions above one G.

Edited December 8, 2021 by Shadrach

[philip_g]

The vane can be adjusted. On the newer ones you loosen the screws a d the whole mechanism slides. Don't try to bend it.

[Yourpilotincommand]

What you probably heard was the LG (still up) alert when you pull back the throttle. It sounds similar to the stall warning.

[MBDiagMan]

The gear horn is what I thought I was hearing at first, but gear went down on downwind.  It does sound the same or similar to the gear horn.

It was about a 7 knot crosswind and a little bumpy, but I don’t think wind was a factor.

Thanks for the replies.  Please keep them coming.

[Tommooney]

1 hour ago, MBDiagMan said:

I’ve been trying to get proficient with the Mooney again after being out of it for a while due to an engine rebuild and flying my other plane a lot. Having trouble with my landings so I’ve been doing some pattern work.  I have had the stall warning go off on final while seeing 80 MPH of air speed.  It was IFR certified only a few months ago and the air speed readings seem reasonable.

Can the leaf type stall warning sensor on my ‘67 F model indicate stall at the wrong air speed?

I had stall warning sound at 190K 11K' high above the lost coast CA on long xcountry OR to SoCal. Loud noise took over a minute to diagnose. Seemed like 10 minutes. I told SEA center I'm turning off radios / power IFR because can't hear anything. I finally pulled the circuit breaker. The whole stall devise had come loose from the wing and went inside. It took minutes IFR at high speed / altitude to diagnose the loud noise coming in headphones to me and PAX. I check it at every pre flight now.

Take Care

Tom

[N201MKTurbo]

On a good smooth day go do some straight ahead stalls and see what speed it comes on at. If it is right then it is what others have said. If it is wrong, get it adjusted. 
 

The only way to know it is adjusted correctly is to do stalls. 

[skydvrboy]

As @N201MKTurbo said, why guess or rely on internet answers from people who've never seen the plane.  Go up and test it, then you'll know for sure.

[philip_g]

27 minutes ago, Tommooney said:

I had stall warning sound at 190K 11K' high above the lost coast CA on long xcountry OR to SoCal. Loud noise took over a minute to diagnose. Seemed like 10 minutes. I told SEA center I'm turning off radios / power IFR because can't hear anything. I finally pulled the circuit breaker. The whole stall devise had come loose from the wing and went inside. It took minutes IFR at high speed / altitude to diagnose the loud noise coming in headphones to me and PAX. I check it at every pre flight now.

Take Care

Tom

I one of our k had a little lump of shit break free inside the switch and did this when it jammed under the vane. No way to kill it in flight that I found 

[PT20J]

91.411 checks (what you probably meant by “IFR certified”) do not normally check calibration of the airspeed indicator. So the first thing I’d do is check that the airspeed indicator is reading correctly. Then adjust the stall warning if required. Procedures should be in the service manual.

[Shadrach]

On 12/7/2021 at 7:15 PM, MBDiagMan said:

The gear horn is what I thought I was hearing at first, but gear went down on downwind.  It does sound the same or similar to the gear horn.

It was about a 7 knot crosswind and a little bumpy, but I don’t think wind was a factor.

Thanks for the replies.  Please keep them coming.

If you’re getting a constant horn while level at 80mph in smooth air then it is misadjusted or malfunctioning (this seems unlikely to me). If you’re  getting intermittent horn bleats on a bumpy day while bouncing down final at 80mph, that is normal. I’ve had the stall horn go off above 100MIAS in turbulence.

Edited December 9, 2021 by Shadrach

[Mooney Dog]

On 12/7/2021 at 5:45 PM, MBDiagMan said:

Can the leaf type stall warning sensor on my ‘67 F model indicate stall at the wrong air speed?

The short answer is yes

the long answer is it depends on a lot of factors. 

[MBDiagMan]

I did four more landings a few days ago in smooth air, light wind.  Never heard the stall warning until close to the ground when you would expect to hear it.

[Dieselsnplanes]

A stall warning is a warning, it does not mean you’re in a stall. Primary training has us look for the stall warning, buffet, then the break. If it’s gusty with a cross wind you will get a stall warning all the time, gusts of wind will flip the indicator around. Remember you can stall at any airspeed, exceeding the AOA causes the stall. Ask a CFI to show and guide you through accelerated stalls. 
 

On cross wind landings it’s usually good practice to allow the plane to crab into the wind on a stabilized approach to develop maximum lift, then straighten the plane with rudder on short final. This is covered in the PHAK chapter 8 for reference.

 

i can tell you coming in to KEYW you learn this well. Always a crosswind and in the summer with afternoon T storms you’ll get a crosswind pretty close to max acceptable. Stall warning horn will blare its head off at 8* nose up the whole way on the approach. 

[N201MKTurbo]

As a wing cuts through the air, there is a horizontal line where the airflow separates into the air that goes over the wing and the air that goes under the wing. When the stall vane is below this line it is blown down. When it is above this line it is blown up sounding the alarm. As you increase the angle of attack this line moves down on the wing. When the line is aligned with the stall vane, it takes very little wind to coerce it one way or the other. This is why the slightest gust will make it chirp when near the stall AOA.

[Immelman]

To answer your question yes the stall warning can come on too early. If the vane gets bent it will switch over to turn on the horn at a different angle of attack. Similarly, the vane is not rigidly bolted in a fixed location but can be adjusted up or down if the screws are loose and thus calibrated. I would expect that would be something done at the factory but a lot can happen over the years. Was a repair made? Did the airplane get painted, and the vane removed/replaced in a different orientation? Did something bump it? Lots can happen in 50 years.

Finally, its useful to know that the tones the stall warning make and the gear horn make are slightly different. Go flying, gear up, retard the throttle.... now you know what the gear horn sounds like.

If it were me playing detective I would fly the airplane into the stall warning, noting airspeed/weight/configuration/gusts, and continue to decelerate to a stall buffet, again noting airspeed..... keep in mind if you are flying below maximum gross weight the wing will stall at a lower airspeed than what is painted on the ASI in unaccelerated flight. If not comfortable with this, its a good excuse for an instructional flight.

Edited December 12, 2021 by Immelman