Phenolic Burning Smell

MBDiagMan

MBDiagMan

Final Approach
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Mount Pleasant,Texas
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Doc
Everyone,

I haven't been here in awhile and am just now starting to fly again after my lovely wife of 42 years passing away in May.

I went up Sunday morning and smelled something like burning circuit board material. No smoke, no popped circuit breakers, no problems. I flew around the airport within glide distance and came back in. I don't smell very well. I called my mechanic Monday morning, who has a key to my hangar and asked him to see if he could smell anything. He did.

So, here's the question. Other than a good visual inspection, what would be a good method for tracking it down? I am thinking of opening maybe half the circuit breakers and seeing I get the smell. If I do, shut down half of those and keep draining the swamp like that.

Any suggestions or methods would be appreciated.
 
First, sorry to hear about your loss. I can't imagine what that's like to lose a loved one after 42 years.

I would not power anything. Sounds like you have a short or arcing somewhere and wire is burning.

If it were me (A&P/IA here), I'd crawl under your panel and begin looking for burnt wires....at or around the buss (circuit breakers).

I had an issue with the alternator wire feeding the bus bar once. The connector crimp went bad and the wire began to get hot and heat (internal arcing from corrosion) near by breakers via the bus bar. Those near by heated breakers were popping. That was a real head scratcher. Initially I thought those popping circuits had issues....then after a few hours of trouble shooting realized the large alternator wire was providing the heat (the wire was brown and slightly burnt at the connector). After I cut off a few inches of the alternator wire and recrimped....all was good again.
 
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Doc, first please accept my condolences about your wife. I did notice your absence.
As for the smell. I have two very uneducated thoughts. 1) Is the fact that you have not flown the plane in so long perhaps a Reston for the smell? Perhaps the oil pooled and the burning smell came from some heat before it circulated throughly. 2) Since you haven't flown it in a while, perhaps since the cold weather, is there a chance the heat or defrost vent was left open? Airplane heat always has that funny smell. 3) The third thought is to have an avionics shop do some testing to see if perhaps there is an issue with the wiring or radios. 4) Final thought is some hidden remnants of a nest deep down in some tight spot.
Good luck, chasing down some of this stuff can be maddening .
 
Thanks for the replies and condolences. I said I havent flown but should have said i havent flown much. My smeller isnt as senitive in my old age but i can still identify smells if they are strong enough so i am confident that it is an elect smell.

I will crawl under the panel first and begin the visual inspection. I had begun disassembling for annual when i lost my wife suddenly. I got it back together June 1 and flew around the patch. I took a friend up for an hour 3 weeks ago and flew six tenths Sunday morning.

Please keep the tips, suggestions and comments coming. I should be at the hangar very earl saturday morning to beat the heat.

Thanks to all,
 
I had a similar issue once...my Cessna 150 had a taxi light and a landing light with a 3 position switch (off, taxi, landing). When switched on, it was possible to get the switch between taxi and landing, and then both would come on...but it would overheat the wires and make a burning smell in the cockpit.
 
Stay close to the field and down low, review your plan in case a fire does break out should all ground testing fail to ID a cause. I get a little edgy about unidentified causes of burning smells.
 
Agree. Overheated electrical is often caused by resistance (corrosion or loose connection, possibly). Heat increases resistance, which can snowball, hopefully causing an "open" and not a fire. Sorry for your loss ;(
 
Well, I thought I would get on it Saturday morning, but agreed to mow the Veterans Memorial, something I do on a volunteer basis, Saturday morning. It will have to be Sunday before I get under the panel. I expect to find something, but don't know until I try. When I do fly it I will stay close to the field, but if I do have trouble I plan on turning off the main switch and flying in under power. Being able to think that part through ahead of time, takes some of the fear out of it.
 
GlennAB1,

Yes, what you describe is called "thermal runaway." Once heat is generated, it results in lower resistance, resulting in more heat, resulting in more resistance and so forth.
 
I have a little device I made to locate elusive odors. It's just a couple of feet of 1/4" nylon tubing and a little rubber "funnel" at one end. The funnel is made of thin rubber sheet rolled up and stapled, and trimmed to fit over my nose. It's taped onto the tubing. Sniff through it while moving the other end over suspected areas; in your case, you'd run it along wires and near switches and breakers and stick it against the small holes in radio cases. Don't forget anything in the ceiling, like lights or switches or dimmers, or ammeter shunts behind the instrument panel or side kick panels. The stink should still be evident even a day or two after shutdown.

I made it to find small fuel leaks. Definitely not safety-approved.
 
MBDiagMan said:
GlennAB1,

Yes, what you describe is called "thermal runaway." Once heat is generated, it results in lower resistance, resulting in more heat, resulting in more resistance and so forth.
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Thank you, I am familiar with "thermal runaway", we had a battery shop at a repair station where I worked in QC. With most conductors, heat causes increased resistance, (not lower resistance), which causes more heat and more resistance. That is also why it's important to keep wiring lint and debris free, for cooling.
 
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If the resistance increases, the current decreases while the voltage remains the same. How does that make more heat? P = I x E. I am confused.
 
MBDiagMan said:
If the resistance increases, the current decreases while the voltage remains the same. How does that make more heat? P = I x E. I am confused.
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In the case or corrosion it's usually not the actual resistance that's causing the heat-it's arcing. Arcing even in small amounts increases heat dramatically. I've pulled electrical breakers out of a panel (electrical panels in buildings, not aircraft, I've never worked on those) that were not "kicked" but melted because the wire was loose and was arcing. It doesn't pull too much current, but it will catch fire...

John
 
older cessna a/c usually have problems at the master switch connection terminals especially those red 2 rocker jobs , next if you are a landing lite on during daylight guy ,it might be that switch or connection
 
jsstevens said:
In the case or corrosion it's usually not the actual resistance that's causing the heat-it's arcing. Arcing even in small amounts increases heat dramatically. I've pulled electrical breakers out of a panel (electrical panels in buildings, not aircraft, I've never worked on those) that were not "kicked" but melted because the wire was loose and was arcing. It doesn't pull too much current, but it will catch fire...

John
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Now this makes perfect sense.
 
GlennAB1 said:
Resistance is a materials opposition to current flow. Heat is a product of resistance.
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The first sentence is correct. The math doesn't work on the second sentence, however. With a fixed voltage if the resistance goes up, the current goes down. Power (Watts) decrease with lower current P = I x E. The E remains the same while the current decreases making for less power (heat) dissipated.

The arcing, is th best explanation of what might be going on.

I think the Master and light switch suggexpstions are two very likely culprits.

Thanks for all the suggestions and comments. Very helpful and I think I have somewhere to go with all this.
 
Early stages of corrosion, or a slightly loose connection aren't going to arc, just produce heat. I would think if you had arcing, there might be some other indication, lights flickering, meter fluctuating, or something. Back to the initial question, yes, a detailed inspection should suffice, however it could be inside a component which you may be able to determine just from smell. Anyway, good luck.
 
MBDiagMan said:
The first sentence is correct. The math doesn't work on the second sentence, however. With a fixed voltage if the resistance goes up, the current goes down. Power (Watts) decrease with lower current P = I x E. The E remains the same while the current decreases making for less power (heat) dissipated.

The arcing, is th best explanation of what might be going on.

I think the Master and light switch suggexpstions are two very likely culprits.

Thanks for all the suggestions and comments. Very helpful and I think I have somewhere to go with all this.
Click to expand...


If resistance in a connection goes up, it generates heat where there isn't supposed to be heat. The resistance cuts the amperage to the rest of the circuit so it might not function properly.

Look for discolored (darkened) crimp terminals on the landing/taxi light switches. Those carry high amperages and as the connections oxidize, they build some resistance and get hot. Some beacons (the old incandescent flashers) draw around ten amps and also offer a place for trouble. The master circuit just controls the master contactor's coil and doesn't carry much current.

But that burning phenolic smell, if that truly is what you're smelling, doesn't come from teflon- or vinyl-insulated wire or the plastic insulation on crimp terminals. That smell comes from circuit boards (avionics) or the bakelite blocks often used for ammeter shunts. Maybe there's a bakelite-bodied switch or breaker or fuseholder that's getting cooked, though a breaker or fuse should have popped if it got that hot. They are thermal devices.
 
Great collection of information guys. I am anxious to make it to the hangar when it's cool enough that I can stay in it. The hangar faces west and is dreadfully hot in the evenings. It is not exactly a refrigerator at Sunrise for that matter. I don't want to wait until the Texas heat breaks, that would lose at least a month of flying.

I will report my findings here.
 
Find someone who has an infrared camera attachment for their smart phone or buy one. They see heat instead of light. Very helpful in finding hot spots, energy leaks in your house, etc.
Do a google search on infrared scanner or infrared camera.
 
There are a lot of EWIS inspection references online. Check out EWIS Best Practices Job Aid provided by the FAA.
 
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Went over it with a fine tooth comb this morning and could find nothing. Removed and inspected radios as well.

Moved it out of the hangar to run and check for smell. Taxi'd around and all was well. Decided to go on out. Shot four landings before putting it away. Was ready to kill master and bring it back in IIf necessary. Never went any further away from the airport than was necessary to stay in the pattern.
 
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