Light switches disobey no-smoking sign

Jaybird180

Jaybird180

Final Approach
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Jaybird180
Today I decided that I should do a proficiency flight. Go up, do a few maneuvers then spend some QT in the pattern. All that was cut-short as I headed back to the airport due to decreasing ceilings and the thought that the freezing level was coming with it.

On the 45, I decide to make myself more visible to airplanes in the pattern with external lighting. A few seconds later (maybe 10-30), I see puffs of smoke coming from the Landing Light switch.:hairraise:

Turn that off. Better yet, UNICOM ground advisory: turn all the lights off. So I turn off every light on the airplane. I'm thinking about turning off the transponder, gps, radio 1 & 2, etc but decide otherwise due to traffic and also the fact that I'm inside the DC SFRA.:fcross:

Now I'm thinking about traffic and getting it back on the deck ASAP weighted with the possibility of electrical fire.

After making it back safely, it was relayed to me that maintenance said 'they do that when they're end of life, but it's not a safety of flight issue'.

Can someone who knows C-172s (or general airplane maintenance) confirm or debunk? Doesn't make sense to me.:dunno:

I'm still reflecting on lessons learned, but I'll remember today for a long time.
 
Jaybird180 said:
After making it back safely, it was relayed to me that maintenance said 'they do that when they're end of life, but it's not a safety of flight issue'.

Can someone who knows C-172s (or general airplane maintenance) confirm or debunk? Doesn't make sense to me.:dunno:

I'm still reflecting on lessons learned, but I'll remember today for a long time.
Click to expand...


They do that. Those rocker switches (I assume this is a 1986 or earlier 172) are among the least expensive (and also cheap) parts in the whole airplane. Like any mechanical switch, they have contacts that oxidize with age and get burned as they open and make a small arc, and the wire terminals are push-on affairs that also oxidize (both the terminal and the stud on the switch), and the wire crimped into the terminal oxidizes, too. Oxide represents resistance and resistance causes heat, which accelerates the oxidation and increases the resistance. These little switches are expected to handle 19 amps in the single 250-watt landing light installation and so the potential for lots of heat is right there. Smoke isn't uncommon.

Now, a couple of years ago Cessna instituted (via Service Bulletin) a four-year life limit on those switches. Too many reports of smoke, I suppose. We change them out as recommended, and also get a good look at the terminals. If they're discolored they get cut off and replaced, too. No sense having hot terminals heat a brand-new switch.

Dan
 
Why didn't the Circuit Breaker pop first? Wouldn't that be a better design?
 
Jaybird180 said:
Why didn't the Circuit Breaker pop first? Wouldn't that be a better design?
Click to expand...

Circuit breakers pop when the current increases too much - bad contacts in a switch increase the total resistance of the circuit and reduce the current. The problem is the heat generated in the swtich in the process...
 
The single rocker switches are pretty cheap even through Cessna - less than $5.00 if I remember correctly - but the same switches, non-aviation, are only about $1.50 each. The dual switches are more.

I've taken the switches apart after failure and seen the problem. The contacts are bare copper rather than being plated, and the spring making the contact is relatively weak with a very soft snap closure. This results in the pitting and burning of the contacts. I've also seen the terminals on the rear get hot enough to soften the nylon body and the terminal pulls away from the interior contacts making the switch permanently open.

It would have been nice if they had spent a couple of bucks more and gotten decent switches. There is a Cherry rocker switch very much like the single ones that Cessna used and almost exactly the same size but with plated contacts. But, of course, that switch is not approved.
 
No matter what maintenance says, I ain't takin' no smokin' switches for an airplane ride.

BTW, did you consider pulling the fuse or c/b for the circuit controlled by that smoking switch?
 
Ron Levy said:
BTW, did you consider pulling the fuse or c/b for the circuit controlled by that smoking switch?
Click to expand...
How does one "pull the breaker"? On the Pipers that I fly, the breakers are pushed in flush with the panel so that you can't grab them. When the breakers open, they pop out. (Makes it very easy to check breakers in by running your hand over the fuse panel.

Yes, some breakers designed to be pulled have longer shafts that stick out of the panel when the breaker is in the closed position. But for the other breakers... how do you pull them?

-Skip
 
Sadly, the answer is that you have to buy a less cost-sensitive aircraft. You can't replace a breaker either- that was made clear by an FAA maintenence enforcement action recently, in which the operator had his mech change out some of the breakers to the "pull" type....--->"that's unairworthy". HMOG.....
 
bbchien said:
You can't replace a breaker either- that was made clear by an FAA maintenence enforcement action recently, in which the operator had his mech change out some of the breakers to the "pull" type....--->"that's unairworthy". HMOG.....
Click to expand...

What the?!?!?!!!!!!
:incazzato::mad2:
 
bbchien said:
Sadly, the answer is that you have to buy a less cost-sensitive aircraft. You can't replace a breaker either- that was made clear by an FAA maintenence enforcement action recently, in which the operator had his mech change out some of the breakers to the "pull" type....--->"that's unairworthy". HMOG.....
Click to expand...

Someone has their head square up-and-locked with that one. I was hoping to someday retrofit the bird away from the much more dangerous "flush" breakers with the "pull" breakers...

Sigh.
 
Skip Miller said:
How does one "pull the breaker"? On the Pipers that I fly, the breakers are pushed in flush with the panel so that you can't grab them. When the breakers open, they pop out. (Makes it very easy to check breakers in by running your hand over the fuse panel.

Yes, some breakers designed to be pulled have longer shafts that stick out of the panel when the breaker is in the closed position. But for the other breakers... how do you pull them?
Click to expand...
If you have flush breakers, all you can do is wait until the breaker pops (and hope the device doesn't catch fire first) or turn off the master. That's why I hate those flush breakers.
 
bbchien said:
Sadly, the answer is that you have to buy a less cost-sensitive aircraft. You can't replace a breaker either- that was made clear by an FAA maintenence enforcement action recently, in which the operator had his mech change out some of the breakers to the "pull" type....--->"that's unairworthy". HMOG.....
Click to expand...
While you can't just do that on an A&P's signature alone, I know people who've obtained field approval to replace fuses or flush breakers with the pullable type.
 
The checklist for lowering the gear manually on the C-320 I flew had "Landing Gear CB - PULL" as a step, but the CB was the flush kind. :confused:
 
Really? You cannot replace the flush Piper breakers with the nice pull type? About half of my breakers were fixed with the pull types. Where can one findthis memo from the faa?

Sent from my Milestone using Tapatalk
 
bbchien said:
You can't replace a breaker either- that was made clear by an FAA maintenence enforcement action recently, in which the operator had his mech change out some of the breakers to the "pull" type....--->"that's unairworthy". HMOG.....
Click to expand...

Cite, please? I don't somehow believe that.

THanks,

Jim
 
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