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FAILED AVIONIC MASTER SWITCH

F

flysrv10

Well Known Member
My combination switch/CB failed in flight. Lost both radios, TXP and Audio panel.

I believe the failure is due to fatigue. You can spot the broken wire in the pic. I estimate that the switch has had a min. of 2000 cycles. Fortunetly, I had a spare and the replacement took 45 min.

Perhaps a parallel master switch is in order.

Update:

The switch/CB has been in service for 14 years. I estimated the cycles based on 1700 flight hours. Thank you all for advice and shared experiences.
 

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My buddies -9A did the same thing to him last summer after we stopped at the fuel pumps prior to a pleasure flight. Not easily replaced on his plane due to things around and behind it. We were able to get it out after some uncomfortable positioning upside down on our backs. His back-up solution was to run two in parallel so he would have one for a backup ready to go in case it ever happened again. He is an AP/IA so that might be a viable solution for you also.
 
I had the exact same switch breaker powering my avionics buss in my Bonanza. Stupidly one day, I took off with no option to return due to low vis and ceilings at my home base (we have no approaches). I flew to my first stop of the day and shot an ILS to minimums and landed uneventfully. Not 30 seconds after landing, that garbage Tyco switch breaker failed and all avionics went cold. Thank goodness it happened on the ground as the low vis and ceiling was very widespread and I would have been in a bad spot.

Quick swap of the switch breaker and I was on my way. After completion of that trip, I swapped the Tyco for a Mil Spec Klixon and added a second with redundant power as backup.

The Tyco switch breaker had a broken wire inside just as the OP is describing.

Moral of the story, Tyco is garbage and should not be installed in aircraft.
 
Another data point:
I built my 9A for day vfr flight using Vans wiring harness. My fuel pump switch CB failed after 14 years / 1000 hrs.
I replaced that SCB and the other 3 with SPST switches and a fuse block.
 
I don't like combination switch/circuit breaker.

Many years ago, in another life, the landing light switch/cb on my Mooney failed on short final of my first night landing of the, new-to-me, airplane.
 
I had the same issue twice in my -7. Converted to the avionics master switch kit that SteinAir sells.
 
I use a single throw four pole switch as my avionics master. Each pole input wire comes from a fuse panel. (I prefer fuses rather than mechanical devices for almost all protection)
The switch comes from Digi-key and when you look at the specifications they are qualified for a huge number of operations. A mechanical failure is highly unlikely. Doesn’t meet nuclear standards for protection, but is good enough.
.
 
Perhaps not a condemnation of the avionics master switch concept, but rather, a cautionary tale about Tyco switch breakers.

Picture is a little fuzzy, but it kinda looks like the design flexes the broken copper braid every time the switch is operated. Is it soldered to the stationary bus bar?
 
Someone remind me why we should eliminate the avionics master switch, yet we still install an alternator field switch.
 
DanH said:
Someone remind me why we should eliminate the avionics master switch, yet we still install an alternator field switch.
Click to expand...

No clue. I don’t install either. I do install a pull breaker for the alternator voltage regulator - but open it only when I want to test the standby alternator picking up the load.

Carl
 
DanH said:
Someone remind me why we should eliminate the avionics master switch, yet we still install an alternator field switch.
Click to expand...

I don't have an alternator field switch either... If I ever really need to kill power to the alternator I'll just kill it from the circuit breaker. Switches are for things that need to be switched on and off every flight at least.
 
Ruready said:
Sounds like a heavy duty switch, why the four circuits? Different things on each one? That must be what you are doing.. well maybe not.. maybe if you blow one fuse the rest wont blow at the same time, naa you must be powering different things on each pole, so you are using those four poles with fuses instead of a circuit breaker fot those items.
Click to expand...

Yes, each pole is a different device with its own fuse at the fuse panel. Switch is not heavy duty, same rating as the “normal” on/off, they are just wider. You can feed the devices from your normal or emergency (endurance) buss.

Available with more or less poles if you need.
 
echozulu said:
I don't have an alternator field switch either... If I ever really need to kill power to the alternator I'll just kill it from the circuit breaker. Switches are for things that need to be switched on and off every flight at least.
Click to expand...

Same here. The alternator and battery are switched on by DP master switch, preventing the alternator from being on when the battery is off. There is a pullable CB on the panel for the alternator field and overvoltage protection via B&C LR3 controller.
The Dynon screen and Trig radio head each have their own on/off buttons if needed. There is no avionics bus or master, flying day VFR only.
 
Combo Cb and switch

Wondering if the tyco combo problem applies to the potter brumfield combo switches that I have a number of and was planning on for my panel?
 
I like the avionics master, but

I always wire two, in parallel ---- for the circuit breaker switches, I use them on non-critical applications to save space/complexity.

Ron
 
ibflyer said:
Wondering if the tyco combo problem applies to the potter brumfield combo switches that I have a number of and was planning on for my panel?
Click to expand...

My buddies combo switch that failed was a Potter Brumfield.
 
I used mostly dedicated breakers in my panel for this reason, but I do have some of the combo switches for less frequently used items. My plan was to replace all of them (combo switches) at 2000 hours, or upon the first switch failure, whichever occurs first. Sneaking up on 1300 hours now and no failure yet.
 
combo

I love the switch/breaker combo idea, but I cant bring myself to use the Tyco. Its too bad klixon or honeywell don't make one....do they?
 
Even the Mil Spec switch/ breakers versions from Sensata have lower reliability then separate Sensata breaker and mil spec switch. For properly designed electrical bus system and modern avionics, neither is needed.
 
blueflyer said:
I love the switch/breaker combo idea, but I cant bring myself to use the Tyco. Its too bad klixon or honeywell don't make one....do they?
Click to expand...

The one that failed in my 1968 Mooney was Klixon!
 
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RVDan said:
For properly designed electrical bus system and modern avionics, neither is needed.
Click to expand...
Obviously, you're entitled to your opinion, however I disagree.
Of course this has been hashed out before, not looking to go there again.
 
Walt said:
Obviously, you're entitled to your opinion, however I disagree.
Of course this has been hashed out before, not looking to go there again.
Click to expand...

I'm with Walt on this one, old curmudgeon that I am becoming. I am not a fan of electronic breakers.
 
Walt said:
Obviously, you're entitled to your opinion, however I disagree.
Click to expand...

May be some miscommunication. Which belief Walt doesn't share was not very clear.

Anyway, the switch breaker appears to have suffered a mechanical failure. What sort of load was on that circuit doesn't really matter....it wasn't a failure due to electrons (burned contacts, overheated components, etc).
 
My approach

I like the functionality of the W31 switch breakers, and used them on my RV-10. But thanks to AD 2008-13-17 and SAIB CE-13-22 (March 2013), I'm keenly aware that the design has the failure mechanism that flysrv10 pointed out.

I've tried to mitigate the issue with a design decision and a maintenance action:

Design: On the RV-10 build, I included an always-hot 8AWG "emergency avionics" wire (protected by a 60A fusable link at the battery end) running from my battery to the bus bar. This "emergency avionics" feed is activated by a 50 amp W31 switch/breaker on the avionics bus. This serves three purposes:
- If the alternator fails, I can shut off the master contactor and activate the "emergency avionics" switch to power the radios. This saves me about 5 amps of current drain by eliminating the master contactor current.
- If the avionics master W31 fails, the "emergency avioncs" W31 provides an alternate source of power to the avionics buss.
- If the master relay were to fail (seems very unlikely), the "emergency avionics" switch provides an alternate source of avionics battery power to get me on the ground.
- During avionics ground runs (software updates, mostly), I use the "emergency avioncs" feed to run the radios without the 5 amp draw of the master relay.

Maintenance: At about 1500 hours, I proactively replaced all the high cycle W31 swtich breakers - avionics master, boost pump, landing and taxi lights.
 
Tim Lewis said:
I like the functionality of the W31 switch breakers, and used them on my RV-10. But thanks to AD 2008-13-17 and SAIB CE-13-22 (March 2013), I'm keenly aware that the design has the failure mechanism that flysrv10 pointed out.

I've tried to mitigate the issue with a design decision and a maintenance action:

Design: On the RV-10 build, I included an always-hot 8AWG "emergency avionics" wire (protected by a 60A fusable link at the battery end) running from my battery to the bus bar. This "emergency avionics" feed is activated by a 50 amp W31 switch/breaker on the avionics bus. This serves three purposes:
Click to expand...
I would certainly not be comfortable with an always hot wire in the cabin like that (a long wire on the RV10), a 60 amp fuse link can carry lots of current before it opens. You're defeating the purpose of the master relay by doing this. The cole hearse relays draw something like 1-2 amps, if you want a better contactor that draws much less look at the Gigavac MV series of contactors, I think the one I have is about 100ma holding current.

Makes more sense to me to mitigate the problem in the first place rather than adding a possibly dangerous work around.
(in the event of accident or smoke in cabin you can't shut down the power which is the reason for the master contactor)

I would suggest adding another relay/contactor in the back for your 'always hot' aux power wire.
 
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