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TYCO W31 Circuit Breaker Switch in Flight Failure

chuckwn

Well Known Member
Flying along fat dumb and happy when all the radios started flashing off and on, eventually failing off (Day VMC fortunately). This drama was accompanied by a little electronic smoke smell. Mostly I was seeing $$$$ as the GTN650 was subjected to this power cycling event.

Trouble shooting determined that the avionics circuit breaker/switch had failed in the open position. Luckily all the $$$$ avionics were not damaged.

This CB/switch is about 8 years old and has maybe 500 hours of use. Interestingly the failed switch is the newest of a whole plane full of these CB/Switches. The older ones have 2550 hours over 19 years without problems. The older trouble free switches were made in Mexico, the failed one was made in China.

I found this online:

https://rgl.faa.gov/Regulatory_and_Guidance_Library/rgSAIB.nsf/dc7bd4f27e5f107486257221005f069d/469f896b8a88d2a886257b2d0066ed6c/$FILE/CE-13-22.pdf

Attached are some pictures of the culprit.
 

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Last edited:
what Bob said

sure glad nothing was fried but your experience reminds me why Bob Nuckolls advised years ago not to use an avionics master, which is to avoid a single point failure.

I don't if that's current prevailing wisdom but it still makes sense to me. I am, however, so often wrong ...
 
SSCB Fun

I hate it when safety devices start fires. FAA guidance somewhere in AC43.13 recommends not using a circuit breaker as a switch, but if’n I recollect rightly, it did not address using switch style circuit breakers that way. We have a few of these things in our RV-6 but I’ve never liked it. Well, it was like that when we got it. Limited panel space forces us to keep some of them but I have managed to eliminate a couple, replacing them with pull-type breakers and toggle switches. One of these days, when I redo the panel, the rest will going away, too. Besides, the switch action feels kind of limp and rattle-y.
 
Circuit Breakers/Switches

In General - Unless you must use a circuit breaker as an ON-OFF switch, I would never recommend using the integrated circuit breaker/Switch in your design.
When reliability is count, mechanical parts are much less reliable then any electronic devices. Therefore, turning On-Off frequently of the circuit breaker, when it is used as a switch, destroy (defects) the mechanical function of the device and you can expect very short MTBF.
 
I had one of these go T.U. on me in IMC some years ago; I have been gun-shy of anything more than light IMC ever since (unless there's suitable electrical redundancy). My -7 will definitely not have an avionics master for the same reason!
 
I had one of these go T.U. on me in IMC some years ago; I have been gun-shy of anything more than light IMC ever since (unless there's suitable electrical redundancy). My -7 will definitely not have an avionics master for the same reason!

Having an avionics master has nothing to do with the failure of a piece of **** brk/sw which has had known issues/AD's for many many years.
If you choose to use crappy inexpensive products the results are predictable.
 
These combo breaker switches do at first glance seem like a simplification by combining a breaker and a switch however, their past performance has been bad. There have been many past failures.

I'm glad you didn't have any issues related to the failure though. Smoke in any form in the cockpit has to be exciting.
 
Just as a data point, I’ve used these switch breakers for 20 years and have never had a failure.

Note however that I do not use them for any power or control to the panel. I use them only for “non-vital” loads like landing/taxi lights, nav/strove, pitot heat and such.

I do use small 30amp relays to control power to the panel, but I have four of them. Two connected to each battery, half the panel running on each battery, each panel half can be selected for either battery via a three position toggle switch. This approach provides for redundancy in the event of a failed component, and creates backup modes of operation to maintain the panel for continued IFR flight.

So all you IFR and/or electrically dependent engine builders, look at your power design and determine if loosing any single component (solenoid, switch, breaker, battery, alternator, ground, connection, etc.) is acceptable to your level of risk tolerance. If not, then create a mitigation for that risk.

Carl
 
Having an avionics master has nothing to do with the failure of a piece of **** brk/sw which has had known issues/AD's for many many years.
If you choose to use crappy inexpensive products the results are predictable.

Double thumbs up Walt. I have refused to use these on any projects for years.
 
I have had 3 circuit breaker failures, all were Potter Brumfield, now TYCO which were made in Mexico. They are now made in Taiwan, no problems with these.
I opened the mexican made units after failure, the braided wire unraveled, leaving a few strands which melted through for the failure.

Regards,

Gary
 
Having an avionics master has nothing to do with the failure of a piece of **** brk/sw which has had known issues/AD's for many many years.
If you choose to use crappy inexpensive products the results are predictable.

I didn’t choose anything, this switch was in a type certificated airplane (not subject to an AD or otherwise required to not have a “crappy inexpensive” product in it). I also should have been more clear; my -7 won’t have a single point failure in the form of a single master switch supplying power to the avionics. :D
 
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