Stall Warning MIA

[Conrad]

Yesterday was the first time I've ever practiced a power off stall without an instructor in the plane. Gear was down, full flaps. I had about half fuel in the tanks and I'm not a particularly large person, so the plane was light. I tested the stall warning horn both before and after the flight and it definitely works (the horn engages when the tab is fully up), but I heard nary a chirrup before a wing started to drop around 55 - 57 mph (visibly below the white arc on my ASI).

I'm pretty sure I've heard the horn on landing relatively recently (when more heavily loaded). I assume that when the plane is approaching stall AoA at a higher airspeed there's enough shove on that tab to trigger the horn, but I don't think the scenario I've just reproduced will be unusual in my flying so it worries me. Should the horn trigger before the switch reaches full travel? Should I lubricate it and see if that helps before I go having it adjusted?

The tab isn't bent. It hasn't been adjusted since times when I know it was working as expected. I didn't feel any resistance to pushing on it, but a finger is a rather coarse tool for that evaluation.

Edited March 2, 2017 by Conrad

[carusoam]

Check how the sensor gets adjusted.

Sounds like it is working, but when it is out of adjustment, it won't work when you want/need it to...

Bending it is not the way to adjust it. That just ruins a good sensor.

Best regards,

-a-

[DonMuncy]

You probably need to have it adjusted. There is a factory specification for the speed at which it activates. I think there is some lee-way in where it goes off, so you can ask your A&P to set it at which ever end of the spectrum you desire. As long as it sounds before the stall, you can do some testing so you know whether that particular setting activation means "be careful, you are getting close" or " get the nose down NOW".

[takair]

Conrad

Could you feel the buffet well before the wing dropped?  Were you able to get it to sound with the instructor?

[kortopates]

If it works on the ground but just not in the air and you don't feel any resistance in lifting the vane that would hang it up, I'd start with adjusting it per the service manual. it probably moved. But its no longer airworthy or doing you any good if it doesn't go off at least the number of kts specified in the manual before the wing stalls.  

[Conrad]

@takair I would not say I felt buffet "well before" the wing dropped. Nor did the wing drop very aggressively.

I went back out to the airport today to inspect the switch. It is utterly normal in every way. It has the correct angle, it's clear that its mounting position has not migrated, and it definitely engages the horn a bit before it reaches full travel.

What I did find is that when I pushed the tab against either limit of its travel with just a little force, it had a bit of stickiness. Most of the time for example it would fall right back down from the upper stop, but in once instance it stayed there for perhaps a quarter of a second before falling. Though it's harder to quantify stickiness in the down position with no constant force like gravity to calibrate, I imagine it was similar.

The cause of the stickiness was a thin layer of CorrosionX mixed with dirt coating the tab. Through creative use of fingernails and wiggling the tab around, I was able to remove enough of the coating that there was no longer any detectable sticking. It still seems a little bit unlikely to me that this would cause the tab to stick under normal circumstances, but perhaps circumstances were not entirely normal on the day in question -- temperature or water could have been present on the tab causing extra stickiness or adding surface tension, and the plane was definitely quite light meaning there was less airflow over the tab at the critical AoA.

I'll report back once I'm able to do another test under similar circumstances, and assuming the horn goes off I'll also pay close attention to how much a margin over stall it's actually giving me.

 

[takair]

I did have an intermittent switch that I finally had to replace.  It would appear to work on the ground after one or two flicks, but would not always work in the ear.  A new (used) switch cured the issue.  It typically goes off before the buffet, but when you are light, it can be at a surprisingly low speed and high angle of attack.  Let us know how your next test works.

[MBDiagMan]

I have been flying a rag wing Cessna 140 for many years.  It Has no stall warning and is difficult to feel the buffet leading to the stall, but I have learned what it feels like.  Can you not feel the buffet in the Mooney wing?

[takair]

Can't speak to other Mooney's, but I can get mine to go off consistently with just me and light or heavy on fuel.  Prior to correcting the switch it was intermittent, but that was years ago.  If it is not going off before break, I would still argue that something is not correct.   I don't rely on it, but it is nice to know it is in the background and is intended to help us when distracted.  The buffet on my 64 is noticeable and, in my opinion, is adequate to detect the pre-stall condition, even without the horn.  This assumes standard, straight ahead, practice stalls.  Start doing accelerated stalls and all of the warning times are much reduced.

[Conrad]

Fortunately mine is not intermittent. On the ground it always goes off when you move the switch. It's easy enough to feel for the buffet in training, but I need my stall warning to work reliably. I'm not going to leave an intentional hole in the swiss cheese.

It's good to get confirmation that weight and balance definitely play a role in exactly when (and maybe if) you'll hear the warning.

[carusoam]

We have a European MSer that had video of what the airflow splitting at the leading edge of the wing looked like under various conditions.

As the Cg moves towards the back of the envelope, the AOA changes to increase lift, to hold everything up.  As AOA increases the stall switch is moved closer to the region of the air flow split.

If your stall switch isn't activating unless weight is added in the back... consider having your mechanic adjust the switch so it is activated sooner. It is a small change like a mm or two.

From basic flight training, you learn slow flight with the stall horn continuously blaring.  Slow flight in a Mooney shouldn't be any different.

I am not recommending anyone fly a plane without knowing that their stall horn is working properly.  Use proper altitude and technique if exploring these ideas...

I am only a PP, not a CFI...

Best regards,

-a-

Edited March 8, 2017 by carusoam

[1964-M20E]

Interesting I did not think much of this thread but I went fun flying yesterday just me in the plane slightly above half fuel.  With full flaps and gear up no stall horn, no flaps gear up no stall horn.  I did bring it to a stall with slight wing drop and the plane did not play any bad tricks on me.

I have had it go off before many times on landing as the wheel touch down, and during slow flight and it works on the ground. ????  Maybe W&B have something to do with it but seems to me it should work anywhere inside the envelope and I have never noticed that it did not work when it should have.