Breaker Breaker!

[Tom Martin]

I have one Bendix mag and one Lightspeed EI on my rocket. The other day I did a mag check before take off and found that I had lost my electronic ignition.
A little trouble shooting pointed to a bad switch.
The switch that I chose, 750 hours ago was a Potter Broomfield breaker switch. This combines a breaker and a toggle switch in one unit and I used it in an attempt to lower my parts count. Documentation states that it can be used as a switch. I estimate that with starting, and mag checks over the years the switch probably had 2500 to 3000 cycles.
I replaced the unit with a separate breaker and toggle switch and all is well. In fact things are better then they were. Over the past year or two I have had some little glitches, minor things, that were not traceable. At higher rpms there was a bit of a vibration that went away at my normal 2200 cruise rpm.
Today that was all gone, and it runs smooth at all rpms. Keep in mind that it was just a tiny bit uneven, nothing that a passenger would note.
When I got home I took the old breaker switch apart to see what was going on.
Clearly it can be seen, at the pointer, that the little braided wire is broken



Now I flipped the switch to the closed, or ON, position and you can see that the broken braided wire actually makes contact with the other side of the broken wire. This explains a lot of my symptoms, the little glitches, etc. I have had two, one hour flights since changing this breaker switch to a new system and the plane is running better, no doubt about it. If the switch had not actually failed that one mag check then it is hard to say how much longer I might have flown with this faulty switch.



When testing the breaker switch before removal I would get continuity when it was closed but only partial voltage, around one volt, rather then the 12V in the system. This confirms the symptoms of intermittent contact.
Based on this type of failure I can not recommend breaker switches for use in our aircraft.

[Bob Axsom]

I use straight circuit breakers and switches mainly but I do have a few combination units because of panel space. Most of them are light circuits that I seldom use but I will have to review them for critical applications.

Bob Axsom

[airguy]

I have quite a few of those breaker switches in my panel - but I am aware of the AD pulling them from service in some aircraft and I don't have them on frequent-use circuits. They should be considered an expendable item and spares carried onboard, IMHO. 5 minutes to replace a failed one and let's go.

[dutchroll]

I have two of these in my cockpit. One is a system used only post-flight on the ground (once per flight), and the other is a non-essential system which is again normally only used once per flight.

I'd personally never use one of these for a mag switch.

[BillL]

Great information Tom. This kind of definitive information is very helpful to understand the experienced guys comments of .. uhhhh, I don't think you should use those . . .

Now we know exactly why.

I don't have any of these, but it does bring to mind careful thought and selection of hardware for critical flight systems.

[jjconstant]

I have those exact breakers, for that exact purpose...power to my 2 Emags.
Hmmmm...only 150 hrs or so on them after replacing LASAR ignition. Change soon or buy some spares and replace when they fail?

[airguy]

Originally Posted by jjconstant

I have those exact breakers, for that exact purpose...power to my 2 Emags.
Hmmmm...only 150 hrs or so on them after replacing LASAR ignition. Change soon or buy some spares and replace when they fail?

I would change them now, and not use them on critical items or items that get used frequently. Sometime after 1000 cycles is when problems start showing up with that type, if my fuzzy memory of the AD/SB is correct. Use your own judgement but I don't have any on critical-for-flight items or items that get would get more than one actuation per flight.

The primary failure mode seen on these is failure to connect when turned on, but there have been rare reports as refusing to turn off or failing to trip on an overcurrent situation.

EDIT: - I found the SAIB (supplements the AD for Cessna and Beech aircraft with these switches) here - http://rgl.faa.gov/Regulatory_and_Gu...E/CE-13-22.pdf

They recommend (not require) removing the breakers at 2000 hours for normal use and 600 hours for high-use (described in the document as 1 cycle and 4 cycles respectively, per flight hour). The document also says that of the switches analyzed, 2% had a failure at 2000 cycles and 50% had a failure at 6500 cycles, with 90% failure at 10,000 cycles. Don't use them for anything that gets frequent use, or at least plan on changing them occasionally.

[newtech]

Years ago, before Potter & Brumfield was bought by Siemens, I was the liaison engineer between the Gainsville, Georgia P&B plant that made the thermal circuit breakers and the test equipment lab that built all of P&B's test equipment. I spec'd and actively worked on the design, construction and programming, with the Test Equipment Lab, on the computerized test equipment that tested the W-28 circuit breakers. I don't have any P&B circuit breakers in my plane.

[Tom Martin]

I guess I should have been a bit clearer in the original comments. The real problem is not that it failed but how it failed. By that I mean that it was working but not supplying a complete connection at all times. This can not be good for any equipment on our aircraft. When I originally installed the breaker/switch I was expecting a shorter life then a regular toggle but I had not anticipated this insidious type of failure. One which is very hard to diagnosis, or to even detect, until that one time it happened to failed a mag check.