What's new

Stall Warning Horn Adjustment

NorCalPilot

Soloed!!
Joined
Oct 31, 2021
Messages
188
Location
Northern California, Central Valley - 1968 172I
1968I has flap gap seals that were installed in the early 70's. When I test the stall warning horn and the taste of the bugs... ;-), the horn works fine. But the stall warning horn never goes off. ~27 knots doing power on stalls this morning and nothing, not even a peep. Do any of you have a recommendation on how I should start trying to adjust this?
StallWarningHorn.jpeg
 
Last edited:
I learned in a plane that doesn't have a stall horn, so I hardly even notice when I am flying a plane and the stall horn quits working. Had a couple checkrides at different times through the years where the check pilot noticed and I didn't, had to get it fixed after the flight. Those were also Cessna horns, not sure how it is possible they can work on the ground but not in flight.
 
If the stall horn works on the ground but not in flight, two possibilities. First, it's working in flight but you're not hearing it because it's too quiet. Second, it's adjusted incorrectly. You can loosen those 4 screws and slide the stall horn opening up and down. Sliding it up makes it more sensitive, down less sensitive.

This is due to how it works. As AoA increases, that orifice rotates upward toward the low pressure area above the wing. When it gets high enough, that low pressure sucks air through the orifice activating a kazoo-like reed.

... When I test the stall warning horn and the taste of the bugs...
Think of it as a healthy protein snack!
 
Thank you for the replies. I think there was an assumption on our part that it had been adjusted after the flap gap seals were installed and that we were just not getting it slow enough to activate the warning horn. After the power on stalls this morning, we are hoping than an adjustment takes care of the problem.
 
Thank you Brock. You are definitely the Boss ;-)

16-53A. PNEUMATIC STALL WARNING HORN.(See figure 16-10.) The system is composed of an adjustable plate on the left wing leading edge that is connected to a reed type horn by means of plastic tubing. The horn is actuated approximately 5 to 10 miles per hour above stalling speed as a negative air pressure area at the wing leading edge causes & reverse flow of air through the horn. By moving adjustable plate (7) up, actuation of the horn will occur at a higher speed, and moving the plate down causes actuation to occur at a slower speed. Center the adjustable plate opening in the wing leading edge opening upon installation, then flight test aircraft, observing horn actuation during stall. Readjust the plate to obtain desired results if necessary. Approximately 3/32 inch adjustment of the plate will change speed at which horn actuation occurs by 5 miles per hour. To test horn operation, cover opening in plate (7) with a clean cloth, such as a handkerchief, and apply a slight suction by mouth to draw air through the horn.
(I used Live Text on iPhone to grab the text above. I made correction(s) where I found them - there may be other errors)


WarningHornManual Medium.jpeg

Somewhat ironically, perhaps not, I recently grabbed this photo from a local shop, I think they said an H or I model.

WingHorn Medium.jpeg
 
I’ve seen aircraft that had the cuffed leading edge of a STOL Kit installed

and the new location of the wing port is questionable.
 
When I began this thread, I mentioned flap gap seals but I intended to first state that it has a Horton STOL kit and flap gap seals installed circa 1971. magman brought up something about the leading edge and wing port location could be questionable which makes me wonder about the original installation of scoop/plate.

First off, I can't imagine that this plane has not had a working stall warning horn since the STOL install. The plane was used by a flight school in the late 70's & early 80's and I just can't imagine that getting by everyone.

After I remove the hose from the adapter and the screws from the leading edge, I am only able to move the entire Scoop and Adjustable plate as one unit from side to side and a little up and down but I not able to get the Adjustable plate to adjust up and down nor am I able to pull it out to take a look at it.

The screws thread into Tinnerman clips. I am guessing that the two holes on the original leading edge that can be seen above the Scoop are the old mounting holes for the Scoop & Adjustable plate.

Does anyone have any ideas or a recommendation on how I might be able to adjust the Adjustable Plate? Thank you.

IMG_9226 Medium.jpeg
This 1/2 of a spring type hose clamp was just sitting on the top of hose. The other piece must have rusted and fell off? I felt around for the other half but no luck.
IMG_9223 Medium.jpeg
 
Last edited:
The saga continues... I am chronicling this adventure just in case anyone else ever runs into something like this...

If anyone cares to comment, do you think it is possible that this stall warning horn has not been operable for ~50 years?

I reached out to Curry Aviation who purchased Horton Stol-Craft from Horton Inc.. Excerpts of that email follow.

I'm seeking guidance on an issue with our 1968 Cessna 172I, which has a Horton STOL kit installed (circa 1971). The stall warning horn fails to sound during landing or power-on stalls, despite passing the pre-flight test with light suction on the leading edge of the wing. The image taken from the adjacent access hole suggests that the adjustable plate was modified to accommodate the Horton STOL installation. Additionally, when the screws are removed the scoop and adjustment plate move as one - not separate pieces. Also, it seems that the device is not easily removable from its position between the original leading edge & Horton leading edge.

Yesterday evening, I attempted to simulate negative air pressure using a shop vacuum's blower, and found that airflow around the wing from above and below triggers the stall warning horn - so it would appear that under that particular condition, the stall warning horn works.

Our A&P mechanic is stumped and will be performing an annual inspection in a couple of weeks on an M-model Cessna that has the Horton STOL kit installed. He's offered to take a closer look at our issue again after he takes a look at that particular installation.

I've attached two images and a short video link for your reference. If you have a moment to review and notice anything unusual or have a suggestion, we would greatly appreciate your input.


They got back to right away asking for my cell-phone number. He states that he shared this info. with his engineer who will get back to me noting that the engineer is fairly slammed right now.

HORTON STOL-CRAFT - Webpage announcing sale
Curry Aviation, LLC

magman's comment got me to wondering about the location of the installation and while I did not find anything particular to our issue on the WWW, someone had a unique situation and came up with a tab.

PPRuNe forum: Modifying a Cessna stall warning system

On my second attempt, I have came up with what you see in the photo (the first tab was half the height and did nothing). This "modification" causes the stall warning horn to go off ~40 knots.
StallHorn.jpeg

At this point, we will wait to hear from Curry Aviation and our A&P.
 
Yesterday, I received a call from Brian, Curry Aviation's engineer. Brian was extremely thoughtful and he was as helpful as he could be. Earlier today, they followed up and emailed me the installation instructions. It appears that magman's comment is/was seemingly prophetic as the instructions are not precise and direct the installer to: "Relocate stall warning transmitter in new leading edge in same angular position."

The journey continues...
 

Attachments

  • IMG_9400 Medium.jpeg
    IMG_9400 Medium.jpeg
    80.1 KB · Views: 5
Suggestions:
Remove the screws entirely to enable the inner plate to slide up & down
Set the initial position with the hole slid/rotated higher up on the wing
Duct-tape it into position for test flights
Go flying, test at what speed the horn begins to sound, and adjust up or down as necessary
When you find the right position, drill new holes in the backplate so you can screw it into that position

Remember: sliding/rotating up higher makes it more sensitive, sounding at a higher airspeed. And vice versa.

PS: my suggestions are entirely about getting it to work properly. How to make it legal is a whole 'nuther thing.
 
I think this should have been addressed PRIOR to issuance of the original STC.

No way the Installing Tech can verify proper operation prior to flight.

This I NOT the first.
 
I just made a repair (replacement) to the scoop assembly on my 1979 172N. I'm a little confused -I don't see the inner(adjustable) plate that the scoop assembly. attached to. The photograph shows a square outline of what looked like was a once-there plate, now with rivets where the plate would normally be. It looks as if the scoop is attached directly to the inner leading wing edge, with the inlet to the scoop lacking the slot, which is how you adjust it. The plate has vertical slots where the screws go in, and by moving it up & down, you change the position of horizontal slot on the plate. Leading edge plate IMG_1087.JPEG
 
The piece with the slotted hole is the actual plate. Including an image of the part below. Do you have a STOL? The issue with ours is that when they attached the new leading edge ~50 years ago, they cut our adjustable plate to fit and it is no longer adjustable - currently.

StallAdjustablePlate.jpeg
 
Top Bottom