stall warning horn

[blueldr]

For those kind of prices, I'd install a micro switch that works on a ground test to satisfy a ground test, and the hell with the thing in flight.

If you really need it to know when your wing is ready to crap out, go ahead and spend the money. You're probably gonna die anyway and you can't take it with you.

[Metal Master]

I found this out a long time ago so none of it is doable now. Except for the part about sending it to Safe Flight.
First the stall warning switches are manufactures by Safe Flight in White planes New York. See web sight below. I have sent some really old beat up switches to them. They have a repair station and will make them like new. But the going price is still over $400 dollars for a non heated type. For a heated one the price has been between $700 & $1100 depending on the function & model.

What’s not doable is to replace the micro switch yourself. For informational purposes only. The Switch itself is based on a standard Potter & Brumfield micro switch. It used to be that you could disassemble the stall switch & see the Potter & Brumfield part number right on the switch. This is not true any longer. Even if you see the part number on a switch that you take apart the current production Potter & Brumfield Micro switch will be slightly different. Current manufacture Potter & Brumfield switches have a different diameter lever shaft and the stall switch format is not interchangeable with them. This change happened many years back. I think that Safe flight special orders the switches form Potter & Brumfield with the characteristics they desire. The springs that the lever operates on are calibrated for the lever & particular model of the installation it is going on. (Potter & Brumfield switches of this format are not that Micro) They are fairly large as micro switches go.

If you need a new stall warning switch, contact Safe Flight. Send your old switch to them. They will send you a quote on what it costs to repair it. Expect to pay around $500 I used to keep one of every type in stock ( For Cessna’s) . And exchanged them with the people who needed one. I charged them the price it cost to repair the one I had in stock plus a 15% mark up. The next person paid the price of the next one so on & so on. I do not have this deal going any more.
No I did not repair these switches my self. I am just curious about the way things work & took one apart to see if I could fix it. I found that I could not. That is how I came across Safe Flight and found that I could send one to them for repair.

http://www.safeflight.com/

[GAHorn]

Yeah, those are good words MetalMaster.
I visited with the Safe Flight fellow a couple of years back. He's a pilot himself, with a C-172.
He explained that outright purchase of the switch from them requires they charge the same price as Cessna. But if you send your failed sw. to them they'll "rebuild/overhaul" it. (Amounts to sticking your switch's serial number on a new switch and mailing it back to you along with the Invoice for repair.)
SafeFlight also built the horn/light assys. (I have repaired those when they failed quite simply. The most usual failure is the capacitor that bridges across the terminals internal to the canister. When that capacitor dies the buzzer will not activate. A generic capacitor from the local electronics shop has saved several of them that I know.)

John,.... before you get too far along in this matter.... the MOST COMMON FAILURE of the switch is simply a defective ground. The switch sits at the END of the circuit and when the switch closes (when the vane lifts the lever) it merely provides an electrical GROUND for the horn/light assy, which is always powered when the Master Switch/Battery Contactor is energized. Mine fails regularly. All I have to do is remove the short lead from the switch to airframe ground (it's out there in the wing behind he switch and may be accessed via a nearby inspection cover).... clean the terminal and the grounding point, and re-assemble it. It will then work just fine for another six months when I must to it all again. Takes about 5 minutes.

And, YES! When I purchased my airplane it had it's genuine Potter & Brumfeld microswitch replaced with some look-alike that took too much air pressure to lift the vane and close the switch. (The clue was my full-stall landings that never gave me a stall horn.)
I found a used one for sale online at a price I thought reasonable (from a salvage shop up in Canada actually) and bought it and installed it.

(It was later that I tripped over the NOS set, with which I plan to fund my 401K)

[Bruce Fenstermacher]

Metal Master I appreciate your experiance and sharing it with us.

I'll keep it in mind as I tear into my switch if I should ever have to. What do you mean I CAN'T FIX IT. In fact I think I'm going to rip into mine and it ant broke! Them's fighting words.

[GAHorn]

An additonal "for what it's worth"... the incorrectly "repaired" switch on my airplane would pass the usual preflight/annual inspection procedure of raising the vane to see if it would activate the warning..... but it would not work in flight due to the excessive pressures required to activate the improper microswitch. (It took too much spring pressure.)
Relying upon memory here... I believe Safe Flight advised me the vane should activate the switch with 5 grams of pressure. How to measure that? Simple. I used a cheap postal scale. But a reloading scale would also work, I'd imagine. Or, in a pinch, purse your lips and BLOW!

[jrenwick]

Thanks for all the help on this! George, I'll go back over the ground connection and hope that fixes the problem. I think that could very well be the problem -- more likely than an intermittent P&B microswitch, I would think.

Lastly, FWIW, while searching around I found this source: http://hobbytech.ca/template_itemair.php?iditem=5. The price is a little closer to Safe Flight's charge. Maybe in Canada they don't have to match Cessna's prices?

[GAHorn]

That would still be a charge of $1200 for the repair, John. I think it'd be better to send a failed sw to Safe Flight for overhaul.

Nigel Craig
Technical Services Manager
Tel: (914) 220-1137
Fax: (914) 946-7882
Email: ncraig@safeflight.com

[GAHorn]

Bruce, are you aware of Safe Flight's Powerline Detector for helicopters?
http://www.safeflight.com/mmain.php?px=1&cm=2&cs=19

Downloadable brochure:
http://www.safeflight.com/imgs/photos/P ... 202_16.pdf

[futr_alaskaflyer]

The earlier airplane did not list the stall warning system as required equipment. The B-model lists it as required equipment. Unless "MEL'd" (inoperative per an approved Minimum Equipment LIst) the B-model's stall warning must be 1-operational, 2-installed per Cessna's dwgs (not out of sight under an instrument panel), 3-or in "other, approved status" (removed per STC, etc..)
I do not know of any other basis of approval for an inoperative stall warning system on a B-model. (The ragwing's square wing planform stall characteristics are such that significant, natural airframe buffet occurs prior to stall, thereby providing the pilot with ample warning of impending stall. The A-model's higher stall speed also has a well-defined buffet and "break", providing the pilot with defined stall warning and/or identification. The B-model's wing and flaps produced a slower stall that was judged less abrupt and therefore less identifiable. It was described as an "onset of high sink rate devoid of significant buffet or pitch-over tendancy" so that the CAA decided it needed a stall warning to warn the pilot of the event. (Translation: Due to increased dihedral, more effective flaps, and other minor wing improvements the onset of the B-model's stall is sufficiently masked (gentle) and warnings so insignificant a pilot might not recognize it in all configurations, therefore the mfr was req'd to give the pilot a completely seperate method of determining when a stall was imminent. This can be demonstrated with full flaps, when a B-model's wings can be held level with rudder alone, while the airplane demonstrates a high rate of sink without an obvious "break" and pitch-down.)

Resurrecting this thread because I found George's post above in a search. Yesterday afternoon I was flying 77C for the first time in a long time, after an engine overhaul and some upgrades. While orbiting the airport at 5000 agl burning $$$ and looking for post-overhaul problems, I decided to do some stalls. My CG was right at the forward limits for my weight (which was 1960lbs.) As advertised there was little buffeting and no break either power-off or power-on. I'm not used to having a CG this far forward since most of my flights (even "practice" ones) usually have a CG further aft because of the loads of "stuff" I usually carry. These were certainly different than previous experiences. All that happened at idle and full up elevator was an insidious sink that slowly accelerated to about 500 fpm with a tiny bit of porpoising. Power-on attempts: I really was hanging on the prop and again no break at all but I have a 51 pitch prop and 20 more horses than stock. Sink rate that steadily increased. Tried them with about 15 degrees of bank - same thing though I leveled the wings with the rudder once the sink rate was obvious.

Anyway, what disturbed me was that I had a devil of a time getting the stall warning horn to activate. Mechanically it appears to be working perfectly fine - it seems to be sensitive enough when I manually manipulate it on the ground. It only activated momentarily in the power-off stall when I "horsed" the controls a bit, otherwise I was in full sink for many seconds and nada bleeping. I could have landed 5000 feet later on the airport helipad and I don't think I would have gotten a horn activation once Power-on it finally activated as the stall matured but jeez I was pointed at the sun.

Very seldom does it activate during three point landings either. Yes, I am stalling it on touch down

Ideas? Only mods to the wings are VG's.

[jrenwick]

It might be that the switch in the wing is making contact intermittently. Mine used to behave as you describe: I could make the horn beep on the ground by lifting the vane, but it would not sound on landing. It eventually got so bad that I couldn't reliably make the horn sound on the ground, so I looked into it. The switch is some standard "Micro" brand switch, I don't know what number. It has a metal case riveted around it, and the case includes a pivot point for the vane, which pivots on a small steel shaft. If you drill out the rivets, you can take the assembly apart. I found a cocoon of some kind under the vane, and cleaned it out. With the vane removed, you have access to the switch, and I was able to exercise it manually, enough to get it working again. It's been just fine ever since. I put the assembly back together using screws from Ace Aircraft Supply in place of the rivets. Probably not a legal repair, but it sure saved me money! It worked because the Micro switch wasn't actually bad, but probably just had a little dirt in it that needed working out.

[GAHorn]

The mounting of the "lift detector" (switch) is adjustable. There are slots in the mounting plate, and there is room in the hole/cut-out in the wing to adjust the detector to give proper warning (3-5 mph minimum) prior to actual stall. You might loosen the screws, and adjust the lift-vane/detector UP/FORWARD (higher on the leading edge) which will increase warning speed, then test-fly it again.

[futr_alaskaflyer]

The mounting of the "lift detector" (switch) is adjustable. There are slots in the mounting plate, and there is room in the hole/cut-out in the wing to adjust the detector to give proper warning (3-5 mph minimum) prior to actual stall. You might loosen the screws, and adjust the lift-vane/detector UP/FORWARD (higher on the leading edge) which will increase warning speed, then test-fly it again.

Thanks, will try that tomorrow morning.

[futr_alaskaflyer]

Adjusting the location of the lift detector a millimeter or two upwards did the trick. Thanks Although after testing the adjustment at altitude I landed in a normal three point attitude without thinking much about it...the blaring of the stall warning startled me so much I bounced