Stall warning failure

How to keep the Cessna 170 flying and airworthy.

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CF-HEW
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Stall warning failure

Post by CF-HEW »

I had my plane painted and the stall warning hasn't worked since. Neither the aural warning or the light work anymore. Both worked fine before so a simultaneous dual failure seems unlikely. could it be the wiring? Any ideas out there? Thanks..
Alex Gloutney
'53 B model s/n 25901
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GAHorn
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Re: Stall warning failure

Post by GAHorn »

First, make certain the stall-vane (lift-detector) out on the left wing leading edge moves properly. (Did they paint it?) 8O
Use your finger to lift it. Listen carefully for a faint "click" as the vane operates the switch contained within the unit.

If that seems OK, ... then confirm the C.B. / fuse is still good (Pushed-in/not-blown)

If that is OK,... then open the inspection-cover near the lift-detector and confirm that:
1- The short wire coming from the switch has a proper "ground". This is the single most-common failure point of this system. (Did your paint shop use a pressure-washer to remove paint-stripper? If so... beware! That's a real lift-detector-KILLER.) The screw which connects the ground-wire becomes corroded. Don't just look at it, ...REMOVE it, clean the area where it attaches, use a new "star" washer, and re-attach it, and use an Volt/Ohm meter (VOM)to confirm good ground-connection. Test the system again.

If the system still does not operate:
2- Confirm electrical power to the switch by using a VOM again. With the meter set to the 12-volt range, and the meter''s black probe properly grounded to the airframe, check the long-wire coming from the cabin to the lift-detector for 12 volts. If that is OK, then....

3- Before proceeding further, test the lamp in the cockpit stall-warn unit. Unscrew the red lamp cover and test the lamp and/or replace it with a known operable lamp. Test the system again and determine that the lamp actually illuminates when the master switch in ON and the lift-detector is operated. If the lamp illuminates but the horn does not blow, then it may simply be the horn that has failed. (The horn unit may or may not have a capacitor wired across it's terminals inside the unit. Disassembly may be required to repair the horn portion of the unit. See below.*)
A- If the lamp/horn unit illuminates/sounds:
Note the correct orientation of the wires attached to the rear of the cockpit stall-warn unit (light/horn) and disconnect them (planning to re-connect them afterwards to their proper terminals....so keep them identified.) Notice that the terminals are labled (+) and (-). Apply 12-volts positive to the plus and provide a ground for the minus terminals to again test the light/horn unit. If the unit now operates, then the fault lies in the aircraft wiring. Check that the negative terminal wire (which leads out to the wing lift-detector) is intact and provides a "ground" when the lift detector is operated. (Do this test by using the Ohm portion of the VOM, or use a continuity-tester.) If OK, then check that power is available at the wire which connects to the positive terminal. Make the appropriate repairs to the wiring.
B- If the lamp/horn unit does not llluminate/sound when power is applied directly to it's terminals, then you must repair or replace the unit.
*- The cockpit unit may be disassembled by using the point of a screwdriver to "pop" the sheet-metal retainer-ring from the front of the unit. Now the "grille" which contains the lamp-holder and which has a "circle" of "sound-holes" punched in it may be removed. Be careful. The bakelite case of the lamp/horn unit is brittle and easily damaged. The interior wiring of the unit basically has the lamp-holder and the horn-unit wired in parallel. Confirm the wiring is OK by testing the continuity from the outer-terminals to the interior contacts of those terminals. Again, be careful not to strain/stress those terminals where they penetrate the back of the bakelite case. Make any necessary internal wiring repairs and retest the unit.
The horns were usually units of two differing types. One type is a metal-cannister about 1.25" diameter and 1" thickness, with a 3/4" sounding-hole at it's front. This may be replaced with a similar unit often found in electronics stores and hardware-store buzzers. The other type of horn involves a cannister which has a "capacitor" wired across it's terminals. That capacitor is usually the cause of the horn's failure. Any Radio Shack geek can help you replace that capacitor with one of similar capacity. I've repaired several of these units by that simple procedure.

Good luck. (Maybe I should write a 170 News article and supply pics on this subject.)

These wing lift-detectors are made by Safe Flight in While Plains NY, and they will not supply replacements directly to the end-user, but they WILL "overhaul" your unit for about $350. (Cessna will charge you over $600) Replacing the interior micro-switch can be done using electonic-store parts but usually the unit will only "get you by" the annual inspection where the mechanic/inspector tests the unit by lifting the detector while on the ground. These common store-part-switches usually require too much pressure to actually detect a stall in the air. (The switch should activate with only 5-grams of pressure. Use a postal-meter-scale or ammunition-reloader powder-scale to test them.)
The cockpit units are also made by Safe Flight and I haven't asked them what they charge to repair/overhaul them, but I suspect it's about $1200...half of what Cessna charges for a new unit, as that seems to be the going rate for the comparative lift-detector "overhaul".
These "overhaul" prices are obviously what drive the on-line-auction prices. 8O
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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Bruce Fenstermacher
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Re: Stall warning failure

Post by Bruce Fenstermacher »

Well in a system that is 1/3rd wiring, 1/3rd switch, and 1/3rd indicator there is a 1/3rd chance it is the wiring. As you state it is unlikely to but not impossible for the horn and the light to fail at the same time. That leaves the switch on the leading edge of the wing.

Since the switch and it's operation could have been effected by the painting, like it's painted in position, I'd check it's operation first. Before I got into the switch removing it from the wing for further inspection past simple operation, I'd check with the paint shop and see what if anything they did with the switch. Did the remove it? Did they tape it over? Could give a clue.

It is important to know when trouble shooting the stall warning system that power is applied to the indicator all the time (that the master is on) but the indicator is isolated from ground so the indicator will not illuminate or sound. Closing the switch on the wing grounds the indicator completing the circuit.

So I'd next remove the switch from the wing and remove the wire to the switch. With power on, ground the wire to the aircraft. If the indicator works it is the switch or the grounding of the switch to the rest of the wing. You can check the switch operation with a multi meter and once found to work the only thing left is the switch isn't grounded to the wing. A bad ground at the switch is most likely your problem. Probably from fresh paint.

If when grounding the wire to the wing independent of the switch and the indicator doesn't work then it's the wiring. Either the wire suppling power to the indicator or the wire running to the wing. A wiring problem is unlikely as I'd think just painting an aircraft would have anything to do with wiring unless they removed your wing to paint.
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Bruce Fenstermacher
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Re: Stall warning failure

Post by Bruce Fenstermacher »

Well it looks like George and I were busy typing at the same time. To bad we have no way of knowing this is taking place. Could leave more time for St. Pauli Girl for one of us. :)
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GAHorn
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Re: Stall warning failure

Post by GAHorn »

N9149A wrote:Well it looks like George and I were busy typing at the same time. To bad we have no way of knowing this is taking place. Could leave more time St. Pauli Girl for one of us. :)
Grate mines are deep. :lol:
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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CF-HEW
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Re: Stall warning failure

Post by CF-HEW »

Thanks for the replies guys, I will look into the grounding issue and report back. The switch moves freely and the wing was not removed while painting.
Alex Gloutney
'53 B model s/n 25901
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GAHorn
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Re: Stall warning failure

Post by GAHorn »

CF-HEW wrote:... The switch moves freely and the wing was not removed while painting.
That's not what was meant.

What was meant is: If they stripped the old paint off your airplane and used a pressure-washer to blast the old paint and stripper off...and if they damaged that switch (or it's ground) because they got water in it.... they'll be responsible for damaging your $600 lift detector.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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azmuth1
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Re: Stall warning failure

Post by azmuth1 »

I was having the same stall warning failure problem. I did as suggested in the replys. I removed the switch from the wing, disassembled the switch, cleaned all connections, then reassembled. All worked as it should. My wife says I should do that to my office now and then...maybe I would work better!

J Edmund :)
j3pup
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Re: Stall warning failure

Post by j3pup »

This post also helped me, I put a good ground to the proper wing switch terminal and the alarmed sounded. I am now going to clean the switch and terminals up and reinstall.

Thanks,

Randy
CAVU Mark
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Re: Stall warning failure

Post by CAVU Mark »

gahorn wrote: The horns were usually units of two differing types. One type is a metal-cannister about 1.25" diameter and 1" thickness, with a 3/4" sounding-hole at it's front. This may be replaced with a similar unit often found in electronics stores and hardware-store buzzers.
Well I have looked for the metal-cannister type and have not found one the correct size, only larger. Any suggestions on a supplier? Thanks.
Mark
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Bruce Fenstermacher
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Re: Stall warning failure

Post by Bruce Fenstermacher »

Mark, How big is it? Diameter and thickness. And a picture please. I don't have one to measure and I have a supply in mind.
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GAHorn
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Re: Stall warning failure

Post by GAHorn »

This one SHOULD fit...but you may have to partially disassemble it's outer case:
http://www.alliedelec.com/search/produc ... KU=8540096


Here's the full specs: http://www.alliedelec.com/Images/Produc ... 4-0096.PDF

Another even less expensive which MAY work (and is perhaps even louder): http://www.alliedelec.com/search/produc ... KU=6231955
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
CAVU Mark
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Re: Stall warning failure

Post by CAVU Mark »

Well the little buzzer is 1.5" in diameter and 1" thick. The picture shows it with the retaining ring off, I disassembled it thinking the internal wire broke like before, not the case, it just does not vibrate correctly. I tried to "adjust" the thin metal diaphragm but am not getting it. And now with the second dis assembly the retain ring lock tabs are breaking off.

I found this buzzer at McMaster Carr but it is too large. pn: 5801T13
http://www.mcmaster.com/?m=true#buzzers/=etn497

Any my buzzer is here:
http://www.flickr.com/photos/24686554@N ... hotostream

And here is a close runner up from a buzzer lover. Not sure about this source thought.
http://www3.telus.net/chemelec/Parts/Buzzer.htm
Mark
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Bruce Fenstermacher
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Re: Stall warning failure

Post by Bruce Fenstermacher »

Mark, did you ever have a Lionel Train? That is the source I was thinking about. The diesel engines from the later 50s through the mid 60s and probably later had a horn which looks like this and operates on 12 to 20vdc. I just measured one in my collection and the main round body is 1-25/32" round and the whole body from bottom to top with the lid on is 1". If this would fit I hate to suggest you cannibalize a Lionel engine but there are new replacement parts available which I believe include these horns.

If this horn will fit here are some part numbers

One such horn is part #2367-55 http://www.trainz.com/p-252709-lionel-6 ... gp-79.aspx

Other part numbers are 2034-34 and 2023-34 http://model-trains.trainz.com/search?w=2023-34

SEE WOOPS POST NEXT PAGE FOR MORE INFO about the Lionel horns.
CAUTION - My forum posts may be worth what you paid for them!

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GAHorn
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Re: Stall warning failure

Post by GAHorn »

Mark, his buzzer looks fine to me, and the price is right.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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