Shunt (auto-reset breaker for T&B and Stall warn)

[Lee]

Hi ... redoing some (!) of the wiring on our 54B (N1967C) ... while chasing down wires off of the Safe Flight Stall warning, I noticed one wire goes to what appears to be a shunt mounted in back of the instrument panel in the approximate middle. Any idea of what the shunt is for? I am fabricating a new sub panel with push-pull CBs and I was also trying to figure out how what size CB I needed to protect the wiring into the stall warning indicator. Thanks ... Lee Robinson (MEM)

[Bruce Fenstermacher]

I don't think that is a shunt your looking at. I believe what you are looking at is a self resetting circuit breaker. The location you describe is exactly where this is located on a 170.

[n2582d]

Lee, The Klixon self-resetting circuit breaker was a 2 amp. circuit breaker.

[Lee]

Got it ... I was looking at the Electrical System manual while reading replies, specifically the electrical schematic and noted that the turn and bank as well as the stall warning are jointly fed into the breaker. I might add that my main source of information I am using on the wiring project is an article in Second Quarter 170 News. I found wires that were either just old, had become ‘warm’ if not hot at some point and the bus wires soldered to the fuses were in less than ideal shape. My airplane has dual Nav/Comms, audio panel, etc; radios are a KX-155, Narco MK12E, Narco AT-50, PMA 6000M audio panel and had a Northstar M1 installed. I removed the M1 and the At-50. I am installing an Appareo Stratus transponder, Transcal altitude encoder and leaning towards replacing the MK12E with a GTN 650 or possibly a 750. Farming told me that they would only support the 430/530s until parts run out. I have been flying a -208B part time out of Memphis after retiring from FedEx last year which has both the 430W and 530W I installed ... the 530 has a bigger display that is easier to read. That said, the GTN 650 would work ... use an iPad for a larger moving map. Main project now is laying out two sub-panels (main bus and audio bus) and figuring out how to arrange the CBs. I did install a 50 amp alternator. I am going all electric at some point ... for now installing EI combination oil temp/psi, volt/a meter and tach and Appareo Power Plug to charge iPad etc. The bird already had an EI fuel flow indicator installed. I installed LED landing/taxi lights and thinking about going with LEDs for instrument and cockpit lighting. The current strobe will become a low intensity beacon and the current Nav lights will be Whelen LEDs with strobes. Interesting the way the airplane was wired 30 odd years ago and the way changes have been over the years ... reminds me of the wiring issues we had on the J Cobras I flew in the Marines ... wires everywhere and avionics would go nuts trying to trace them down. The Association has been and is of tremendous help in this project ... thanks much! R/Lee

[Bruce Fenstermacher]

Got it ... I found wires that were either just old, had become ‘warm’ if not hot at some point and the bus wires soldered to the fuses were in less than ideal shape.

True story Lee. I was flying my 170 to commute to work. One day I'm flying along thinking how great it is I could fly to work when I had a stinging sensation on my inside right angle. My immediate though was what the heck is a bee doing stinging me on my angle. I quickly reached down and not seeing a bee thought it must be in my sock and I smashed it with my hand. That is when my hand and my angle now burned like heck and it wasn't stopping. Of course all this time I held altitude perfectly.

I looked down at my sock and there was a blob of solder on it. I thought how in the heck did I get a hot blob of solder on my sock? And then it hit me. I immediately turned off all electrical power. As it was severe VFR and I did not require power to complete the flight, and I needed to get to work, I continued my flight. While in flight I carefully felt down under the panel in the location of the master fuse holder. Yep it was hot. It got so hot it melted the solder which dripped on my angle. Never did it smell of electrical burning.

Needless to say prior to returning home I borrowed a large solder iron and resoldered the master fuse holder wire joint. Later I inspected all the others but found them all good. I suspect the master fuse holder wire joint solder connection failed over time or perhaps the failure was assisted by someone noodling around under the panel and moving the wire causing the joint failure. After resoldering, I never had another issue with it.

[Lee]

I ought to post a few pictures of the fuses I had and their condition ... I think at least some of the wires showed evidence of what I think was excessive heat. I wanted to go with push-pull CBs anyway and the condition of the "bus" wire and fuses convinced me it was worth the effort. I do have a question though: what I thought was a shunt and was corrected by one of our members indicating it was an auto-reset CB still puzzles me. I re-read the electrical manual I got from the Association several years ago and the manual says (pg 12) under "Stall Warning System" is:
1) the stall warning system is required on the B, which I have

2) "The system consists of a current limiter (shared with the turn-and-bank indicator) [which I have] a warning horn/lamp indicator ...." ; further on,

3) "Electrical power (12/14 volts DC) is supplied to the warning horn/light via the current limiter (fuse/circuit breaker) whenever the Master switch is ON."

What I am curious about is "why" the current limiter and why is the turn and bank indicator tied in with the stall warning system? It looks like the current limiter is 2 amps based on the electrical schematic ... page for serial numbers 25373 thru 26995 ... mine is 26112. Where can we find a current limiter if one goes bad? Is there such a thing as a substitute? I did note on the back of the current limiter/CB a stamp that had what looks like "+15 A" ... does this mean the limiter/CB is rated at 15 amps? If so, why the 2 amp notation in the electrical schematic?

The other question: What devices (how many) can be safely powered through one CB? Or, is it preferred/better to have a separate CB for each device? (The original sub-panel had a number of devices with a common CB ... Panel Lights Heater; NAV Dome lights etc) I am fabricating two sub-panels ... a main panel and then an avionics panel which will be energized after engine start through the current radio switch.

What a zoo! But, looking at the condition of the wires, the IA I am working under and I both think that a number of the wires should be replaced and I figure that as long as I have the airplane opened up this far, I might just as well go ahead and clean things up. I have had one "interesting" experience with smoke in the cockpit (Citabria ... taxi out ... a capacitor in a radio fried itself ... could not see anything for a few seconds ... we shut down and got away from the aircraft) and a fire in flight is something none of us want to have to face. It is just too easy at this point to spend the tine and effort to try and get the electrical system squared away. Any thoughts, comments and/or suggestions are welcome and appreciated! The fun never stops ... !

R/Lee (N1967C)

[GAHorn]

... under "Stall Warning System" is:
1) the stall warning system is required on the B, which I have

2) "The system consists of a current limiter (shared with the turn-and-bank indicator) [which I have] a warning horn/lamp indicator ...." ; further on,

3) "Electrical power (12/14 volts DC) is supplied to the warning horn/light via the current limiter (fuse/circuit breaker) whenever the Master switch is ON."

What I am curious about is "why" the current limiter and why is the turn and bank indicator tied in with the stall warning system? It looks like the current limiter is 2 amps based on the electrical schematic ... page for serial numbers 25373 thru 26995 ... mine is 26112. Where can we find a current limiter if one goes bad? Is there such a thing as a substitute? I did note on the back of the current limiter/CB a stamp that had what looks like "+15 A" ... does this mean the limiter/CB is rated at 15 amps? If so, why the 2 amp notation in the electrical schematic?

The other question: What devices (how many) can be safely powered through one CB? Or, is it preferred/better to have a separate CB for each device? (The original sub-panel had a number of devices with a common CB ... Panel Lights Heater; NAV Dome lights etc) I am fabricating two sub-panels ... a main panel and then an avionics panel which will be energized after engine start through the current radio switch....

...as long as I have the airplane opened up this far, I might just as well go ahead and clean things up. ...R/Lee (N1967C)

I'll try to address those questions.
Every item installed on the airplane which requires electrical power must be protected by a "current limiter" (i.e., a fuse or circuit-breaker) by certification/regulation as well as for simple safety's-sake. Since BOTH the stall warning AND the T&B are required for your airplane to be certified in it's category...and since they BOTH serve important functions... Cessna engineering decided they should be served by a current limiter which is "self resetting" and which is not accessible to the pilot for disabling unless the aircraft MASTER is disabled.

Keep in mind that current limiters do NOT protect the appliance. (Should be evident considering your experience with an internal short in that Citabria's radio.) Current limiters (fuses/circuit-breakers) protect the WIRE which serves the appliances on the airplane. Their job is to disconnect from the main buss any wire which shorts ...to prevent it from becoming incandescent and contributing to an onboard FIRE.
If your auto-reset C.B. serving your T&B/Stall Warning is rated at 15 Amps... then that C.B. is NOT going to serve the function intended. It should be replaced with the correctly-rated C.B. (I.E., larger gage wires can carry more current. The wires serving your T&B/SW are sized smaller than your 15A CB in that service. I suspect someone has changed your original C.B. to one of the incorrect over-size.)

The actual/original wire used in these 60-70 year old airplanes met the rules in place during their manufacture. Better wire is available now, with better insulation materials, and if you intend to replace your wire, I recommend you use modern wire. The following paragraph (courtesy of Aircraft Spruce) will explain why:
For many years the standard hookup wire in light aircraft has been MIL-W-5086A, which calls for use of tin-coated copper conductor rated at 600 volts and temperatures of 105°C. This basic wire is then coated with various insulating coatings including PVC. There has been much discussion during the past few years about the use of wire with PVC in aircraft. The Air Force does not permit such wire to be used in military aircraft due to the toxic fumes generated in the event of a fire which can disable the crew. Although there is no formal restriction on the use of MIL-W-5086A wire as of early 1991, ...we will now be supplying MIL-W-22759/16 wire which complies with current military and anticipated future FAA requirements.


As for your comment "...as long as I have the airplane opened up this far, I might just as well go ahead...." BEWARE! You may end up in a years-long project. That is one of the most common statements made by those who end up in TOTAL RESTORATIONS!

Hope this helps.

[Lee]

Hi, George ... just got back from the hangar and chasing wires. A few things/questions and would appreciate comments etc ...

1. I mis-wrote regarding the current limiter ... I should have written that the stall warning is what is rated at 15 amps (I think ... trying to read the back of the indicator with a mirror)

2. I removed what I thought was a current limiter and found out that the device is actually a Klixon thermostat ... data on the back (a little hard to read) : Type NAF 1131-1.
- there are two wires are attached to the thermostat: dome light and stall warning.
- I traced the dome light wire to a 10 amp fuse which also powers the Nav lights
- the other wire goes to the stall warning indicator ... it is attached to the post which is labled as +15Amp

3. Stall Warning Indicator ... 2 posts
- the one labled as +15 amps has two wires:
- one wire goes to the turn coordinator (current style) ... not the older style turn and bank indicator
- the other wire goes to the aforementioned thermostat

- the wire off the other post goes out to the stall warning actuator in the left wing

Question(s):
1. Why the thermostat? I have never seen one of these before ... is this the auto-reset function which would open the circuit if overheated? Why did they not use a fuse or circuit
breaker?
2. Oddly enough, a lot of the newer wires appear to be in good shape ... the only thing I can't figure out is why those wires that were really long were not shortened but merely doubled
over and secured with non-aviation zip ties ... the older wires are showing signs of insulation breaking down and some cases, a little evidence of overheating and chafing ... at least
I think so.
3. I take your comments about newer wire seriously. Good point (has been mentioned in the Electrical Manual for the -170 as well) that the purpose of the fuse/CB is to protect the
wire ... for obvious reasons I take that to heart! The main problem now is figuring out the number of CBs, ratings and wire sizes, some of which is being STC driven.

Finally ... I was rolling about your comment about how opening the airplane up can lead to a total restoration! I am looking forward to getting the bird back in the air but, I have to admit I am enjoying solving the problems and working on the airplane ... a nice time of the year to be be doing so! I could not have done this, or even attempted it, with out the help of this Association ... I mean that very seriously and sincerely! R/Lee

[c170b53]

I have to admit I am enjoying solving the problems and working on the airplane

Yep, I'd say your unknowingly, well on you way down the well traveled path of acquiring another plane/ project. Not that there's anything wrong with that.

[DaveF]

A thermostat is the same thing as a current limiter / self-resetting breaker. The contacts open when the device gets hot. In thermostat mode, the heat comes from the environment, but in current-limiter mode the heat comes from current flowing through the device.

It's tough to have this discussion without pictures. The pic below is Fig.72 from the 170B parts catalog, applicable to serial numbers 25373 through 26995. Should be the same one as in your owners manual. It would help if you sketched what you've got so we can compare them. Tracing wires is not easy, especially when they disappear into bundles. Because everything is ultimately tied back to the bus, you can't use an ohmmeter to trace the wires, you have to unbundle them and actually see where they go.

The schematic shows wire 78 coming off the main bus and going to a 2A breaker (note the different symbol than a fuse), and then to both the stall warning unit and the turn and bank. That 2A breaker must be the thermal breaker behind the panel.

there are two wires are attached to the thermostat: dome light and stall warning.
- I traced the dome light wire to a 10 amp fuse which also powers the Nav lights
- the other wire goes to the stall warning indicator ... it is attached to the post which is labled as +15Amp

Hm, that sort of agrees with the schematic, but not exactly.

If you have the standard panel-mounted Safe Flight Instrument stall warning unit with a buzzer and a red light, then it should have a label that gives the current draw as 0.5A. No way it draws 15A.

Thank you for starting this thread. I now understand why someone added a breaker for the turn and bank instrument in my airplane. During the panel re-do in the 1990s, they must have removed the thermal breaker, and instead of running both the stall and T/B from the stall breaker, they installed a separate one for the T/B. Makes sense now, thanks!

[Lee]

Hi ... I tried to submit a picture of the thermostat ... no joy. That said, I have been referencing both the IPC and the Electrical System manual I got from the Association in ‘07. Interesting ... would really like to know the “why” of the thermostat vs using a standard fuse or better yet, CB. I intend to remove the thermostat/limiter and go with CBs for all of the powered devices.

Another question: I have a heated pitot ... I think I read that the stall warning actuator is also heated? Significant in the size of wire and amperage each draws.

The fun never stops! TNX R/Lee

[DaveF]

I'd guess that they used the thermal breaker because of some characteristic of the current draw of one of those two devices that made fuses unsuitable. Inrush current, or something. Was the goal an extremely slow response or was it the self-resetting feature? No idea, but I'm pretty sure that if we wait long enough, Gary will post a Cessna service bulletin from 1950 that explains the whole thing!

[c170b53]

I think that their thinking would be along the lines of present day design of hot battery busses or if you prefer essential busses.
When I replaced my wiring, I replaced the self resetting unit because the windings were corroded. I bet the majority of those units still in service likely are in the same state and probably wouldn’t function well.

[n2582d]

... I'm pretty sure that if we wait long enough, Gary will post a Cessna service bulletin from 1950 that explains the whole thing!

Dang, I've struck out! Sorry to let y'all down. Looks like Klixon CA-2 breakers are still made though.

[Arcticmayhem]

Man, I'm hearing echos from 2 years ago when I did the same thing to my plane. I removed the panel, cleaned up all the junk wiring and abandoned crap, traced back everything to its place and relanded it all on its proper breaker, removed the ADF, cut a new panel and generally overhauled the whole dash. It took me 3 months of cold evenings working outside in the dark, but it looks a lot better now and I feel safer flying behind a panel that I know is properly secured. I found several dead end wires just hanging live, and a scotch-lock wire tap that is usually used for really poor quality trailer wiring installs. One thing that I did that helped my IA to sign it off without having to check my work too much was to make a wiring diagram. I went to Digikey and used their free wiring diagram program to lay it all out. I mostly copied from the 170B and early 172 wiring diagrams, but added a radio master switch that energized a sub panel. If it would help your project, let me know and I can send a copy to you.