Cessna 172 pneumatic stall warning question

[David Kagey]

Looking for a part number. I have a C172M. Stall horn works great on the ground when I suck on it, but not so good in the air. I have replaced the horn and reed am it is loud on the ground so that’s not it. Someone suggested that the tubing could be bad….heck, probably 49 years old. Logic was that on the ground I give a harder vacuum and less in the air. We (A&P) have also adjusted the plate on the leading edge per the service manual and again no luck
Been thru the parts manual and no luck in finding the specs on the tubing. Looks like 72 inches long, but no luck on how to order
This is just the latest guess. If anyone had had this issue and fixed, please advise
Thanks

 

[Pilawt]

 

[Bell206]

Been thru the parts manual and no luck in finding the specs on the tubing.

The tube is a standard part item and the same polyethylene tubing used for your pitot and static systems. You can order either through Cessna or buy the equivalent. Since you've changed the big pieces sounds like you have a restriction/debris in the line. Perhaps pull the scoop and horn and blowout the line? It can be a challenge to change the line at times.

 

[Magman]

Did you actually stall the aircraft to test this?

Any possibility the wing may have been repaired or modified. ( STOL Kit?)

This could have resulted in the unit being in the wrong location?

 

[David Kagey]

The tube is a standard part item and the same polyethylene tubing used for your pitot and static systems. You can order either through Cessna or buy the equivalent. Since you've changed the big pieces sounds like you have a restriction/debris in the line. Perhaps pull the scoop and horn and blowout the line? It can be a challenge to change the line at times.

Thanks. Good idea and will try it. I have also heard that there is a gasket in there that could have become compromised.

 

[David Kagey]

Did you actually stall the aircraft to test this?

Any possibility the wing may have been repaired or modified. ( STOL Kit?)

This could have resulted in the unit being in the wrong location?

Yes on the actual stall test. Nothing. You do bring up a good point on the wing. The previous owner had hail damage and when they started to repair they found a little corrosion. They then pulled both wings and completely rebuilt and added the…forgot the name…green paint corrosion stuff. I wonder if they didn’t connect it correctly. That said, it still works when I apply vacuum. Something else to look at. Thanks

 

[Dan Thomas]

That plastic fitting that fits into the leading edge is called a "scoop" and it cracks and leaks and is almost impossible to repair. The sun kills it. There is also a gasket between it and the LE skin that falls apart or gets left out during repairs. ANY leakage will make the stall warning weak or nonexistent.

 

[David Kagey]

That plastic fitting that fits into the leading edge is called a "scoop" and it cracks and leaks and is almost impossible to repair. The sun kills it. There is also a gasket between it and the LE skin that falls apart or gets left out during repairs. ANY leakage will make the stall warning weak or nonexistent.

Thanks Can you see the condition of the scoop if you take the metal scoop plate off from the leading edge? Found it okay (0413028-10) 17261113, but priced at $400, so would like to make sure it's cracked. Also, have the parts manual and don't see a gasket in the diagram or the parts list...page 449 of the parts manual for the M model. Any idea of this is a new add or something? Thanks for the help.

 

[Magman]

IIRC You will not see much from outside the wing.

Before you disassemble it may be wise to check for leakage.

One method would be to plug or cap the line in cockpit.

Could be easiest if horn removed.

Blow in the scoop while squirting soap n water at scoop inside wing.

Bubbles = leak

May be better methods.

 

[Dan Thomas]

Thanks Can you see the condition of the scoop if you take the metal scoop plate off from the leading edge? Found it okay (0413028-10) 17261113, but priced at $400, so would like to make sure it's cracked. Also, have the parts manual and don't see a gasket in the diagram or the parts list...page 449 of the parts manual for the M model. Any idea of this is a new add or something? Thanks for the help.

That metal plate should be inside the leading edge, between the scoop and leading edge. On the outside it could cause turbulence that destroys the boundary layer that generates the suction at the port.

IIRC, the gasket was cemented to the scoop. Probably comes with it. It was a thin closed-cell foam.

 

[Tom Braeunig]

I believe that the thin gasket is part of the Scoop Plate. I've submitted a question to Textron Support but haven't heard back from them yet.
Textron does sell the scoop plate,also referred to as the adjustable plate in the IPC, presumably with the gasket. They show 13 in stock on their website, www.txtav.com, and their price is $132.40

 

[David Kagey]

I believe that the thin gasket is part of the Scoop Plate. I've submitted a question to Textron Support but haven't heard back from them yet.
Textron does sell the scoop plate,also referred to as the adjustable plate in the IPC, presumably with the gasket. They show 13 in stock on their website, www.txtav.com, and their price is $132.40View attachment 113691 View attachment 113692

Thanks. From looking at the parts diagram, I think my scoop plate is in good shape. Firm and no issues. I think maybe the scoop or the adapter. My plan is when she goes into annual in May, have them first blow out the line and see if it might just be plugged up. As I shared, first thing I did was new horn and reed and i know they work from sucking on them. Next idea is just the tubing plugged enough that sucking works but natural airflow won’t trigger it. If that doesn’t do it, then maybe the actual scoop and/or adapter. It really is a pretty simple deal…should not be this hard. Thanks for the help.

 

[Magman]

By any chance do you have an aftermarket STOL kit installed?

Wrong location?

 

[David Kagey]

By any chance do you have an aftermarket STOL kit installed?

Wrong location?

Nope. Good thought though

 

[Daleandee]

Looking at those parts prices makes me want to do the "experimental dance" again ...

 

[Dan Thomas]

Thanks. From looking at the parts diagram, I think my scoop plate is in good shape. Firm and no issues. I think maybe the scoop or the adapter. My plan is when she goes into annual in May, have them first blow out the line and see if it might just be plugged up. As I shared, first thing I did was new horn and reed and i know they work from sucking on them. Next idea is just the tubing plugged enough that sucking works but natural airflow won’t trigger it. If that doesn’t do it, then maybe the actual scoop and/or adapter. It really is a pretty simple deal…should not be this hard. Thanks for the help.

Is that scoop plate inside the leading edge where it belongs? Or did someone put it on the outside where it won't work properly?

 

[David Kagey]

The metal one you showed is good. I think my problem is the inside plastic scoop....PN 0413028-10 Cessna Direct sells it, but $400..so looking at cheaper options Anyone have any ideas

 

[Bell206]

PN 0413028-10 Cessna Direct sells it, but $400..so looking at cheaper options Anyone have any ideas

Check Preferred Airparts and Ebay. Just watch for bogus 3d printed versions which have been popping up for plastic/nylon type parts. If none there start contacting salvage places: Faeth, Wentworth, Dawson, etc. Faeth should have it and good people.

 

[David Kagey]

Check Preferred Airparts and Ebay. Just watch for bogus 3d printed versions which have been popping up for plastic/nylon type parts. If none there start contacting salvage places: Faeth, Wentworth, Dawson, etc. Faeth should have it and good people.

Will do...thanks

 

[David Kagey]

Check Preferred Airparts and Ebay. Just watch for bogus 3d printed versions which have been popping up for plastic/nylon type parts. If none there start contacting salvage places: Faeth, Wentworth, Dawson, etc. Faeth should have it and good people.

Curious...what would be wrong with a 3D version? I guess not legal in certified

 

[Bell206]

Curious...what would be wrong with a 3D version? I guess not legal in certified

Yes, not legal in that manner. There is a way for an owner to produce parts for their own aircraft legally, but it involves more than 3d printing out your part. For this part I think you'll be able to find one at a decent price. Call Faeth and see what they can do for you.