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1. 01:03 PM - Re: Re: faston failures et. als. (Robert L. Nuckolls, III)
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| Subject: | Re: re: faston failures et. als. |
At 06:56 AM 7/30/2021, you wrote:
>Bob,
>=C2 Thanks for you're thoughts on this.=C2 The
>picture of the darkened terminal was exactly
>what mine looked like.=C2 My strobes are on every
>flight.=C2 About 300 hours now.=C2 Something I
>will be watching.=C2 One question though.=C2 What
>might total failure look like?=C2 Blown fuse?=C2 Smoke?
No clear/obvious symptom. Flow of energy along a conduction
pathway is not unlike a chain made up of varying metals, tensile
strengths, resistance to environmental effects. The root
cause of any failure begins with an over-stressing of the
weakest link in that chain. The time line can be anything
from milliseconds to years.
We've seen switch failures (infamous switch-breakers
on Beech singles and the Barons) that progressed
to significant destruction of the switch body accompanied
with serious smoke. I've seen Mil-Spec switches by
Honeywell Microswitch that simply opened up . . . failed
to make connection . . . with no obvious signs of
degredation. We've also seen Carling switch failures
that announced their failure by BLOWING the strobe
fuse when the strobes were turned OFF. This one took
a bit of head-scratching to figure out.
Here's a photo montage of the perpetrator:
We started out with a switch that looks like this
http://aeroelectric.com/Pictures/Switches/Carling_Cutaway.jpg
Note the moving contacts mounted at the ends of
a 'see-saw' supported in centrally disposed rotary
joint. The teeter-totter 'beam' is perfectly straight.
Okay, the teeter-totter from the failed switch looked
like this . . .
http://aeroelectric.com/Pictures/Switches/Carling_Failures/BRA_08.JPG
Say what?????
Further examination of the carcass parts revealed
two strange marks on the under side of the toggle
plate . . .
http://aeroelectric.com/Pictures/Switches/Carling_Failures/BRA_05.jpg
These marks were coincidental with where the
ENDS of the teeter-totter would have come in contact
with the surface. A metallic surface which was
grounded to ship's structure.
The electrical contact surface of the teeter-totter
showed us markings that suggested scoring due to
motion at elevated temperatures.
http://aeroelectric.com/Pictures/Switches/Carling_Failures/BRA_03.jpg
The off-side movable contact shows no electrically
or mechanically induced wear . . . which is expected.
http://aeroelectric.com/Pictures/Switches/Carling_Failures/BRA_01.jpg
The on-side movable contact shows some electrically
induced wear . . . expected.
http://aeroelectric.com/Pictures/Switches/Carling_Failures/BRA_02.jpg
Note discoloration of the teeter-totter metal around the
contact . . . definite indication of heating.
Looking at the bottom of the switch we see some
indication of heating on the center -AND- on-side
rivets.
Okay, a picture begins to emerge. The switch was suffering
significant heating while under load. Enough heating to
significantly reduce the strength of the teeter-totter.
When the switch was turned off, the spring loaded over-center
mechanism 'slams' the off-side moving contact against the 'cold'
stationary off-side contact. This dynamic event on a 'hot'
teeter-totter bends it just a tiny little bit.
After many such operations, the bending is so severe that
the off-side end of the teeter-totter becomes so badly
bent that it contacts the grounded frame of the switch
when the switch is turned OFF. Power to the strobe is
conducted onto the switch through the common terminal,
the teeter-totter pivot. This grounding event eventually
starts blowing fuses.
Interestingly enough, had the wires to the switch been
reversed, the fuse would blow when the switch was turned
ON leading troubleshooters to look for faults downstream
of the switch . . . wiring and the strobe power supply.
So, in this case, a weak link in the conductive chain
only manifested in a mechanical distortion of the
switch's geometry. No smoke, no fire, and the strobes
were still working!
Now, almost all of the pieces to this puzzle are plugged
into their associated 'fits' but a couple are still dangling.
Does anyone see 'em . . . and have a theory for where they
'fit'?
Bob . . .
Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"
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