nuckolls.bob(at)cox.net
Guest
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 Posted: Sat Nov 15, 2008 8:34 am Post subject: Lessons Learned, Lessons Forgotten, Tools for Sharing |
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At 09:39 PM 11/14/2008 -0800, you wrote:
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Bob,
I didn't build the plane, but I did maintain it for more than 3 years (and
annual condition inspections) without noticing the danger posed by the
proximity of the two. I will never overlook this particular issue again.
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Ahhh . . . the crystal clarity of experience and hindsight!
| Quote: | The builder used a stainless braided line for the brake which crossed
near the battery cable at 90 degrees. The last time I replaced the
battery, apparently I failed to insure separation and it rubbed through
causing the short.
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What kind of insulation was used on the battery
cable? I ask because Tefzel (22759 and cousins)
and welding cable are exceedingly resistant to
touch-erosion. It takes some sharp-edged (read high
pressure) forces which includes grit-laden contaminants
on top of motion eat away at the insulation.
You mentioned an overbraid on the brake line,
if this was steel and finely pitched wires, then
perhaps this was the source of high-pressure
contact.
| Quote: | The current was conducted along the brake line to the gear leg mount where
it was also rubbing, another oversight on my part.
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Steel resistance is much higher than copper so it's
axiomatic that the overbraid would warm up like heater
wires in your toaster. The resulting heat energy would
be conducted into the fluid line that was supposed
to be PROTECTED by the braid . . .
| Quote: | The 5606 was all boiled off and filled the cabin with smoke. My nephew
declared emergency and shut off the master and E-busses while holding the
canopy part open, so he could breathe, with one hand and flying with the
other. He found an airport and landed and yes, the brake was not
functional. The voltage was measured after the fact, as you surmised, Bob.
When I got there 90 minutes later, the brake line was still in contact
with the cable. The cable had a total of 3 broken strands and some
localized discoloration. There was about a half a square inch of
insulation missing. It was returned to service with a few layers of heat
shrink over the scar.
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The relatively low damage level to the copper is
striking. When it comes to an jousting match
between Sir Copper and Sir Steel using electron-
lances it's not an even match. I wouldn't be surprised
that the fault current flowing was much less than
the levels experienced during engine cranking . . .
100 amps is probably more than enough to produce the
conditions cited.
| Quote: | The braided line was limp and I made a replacement in the field out of the
aluminum tubing that Van's supplies. There is now very positive separation
between it and the battery cable, believe me. I have replaced the fluid
with the high temp variant, MIL-PRF-83282. I also replaced the O-rings in
the calipers with Viton for higher temperature capacity.
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The biggest change to your airplane's configuration
is the separation. Of course, upgrading fluid
system components is not a bad thing to do . . . but
when it comes to draining your battery through the
overbraid on a non-metalic fluid line, the contents
of the line and accessories would not have changed
the outcome.
| Quote: | I just wanted to share this story in the hope that people designing,
installing or maintaining systems take a hard look at the way the battery
cable to the contactor is protected from harm. This is pretty much the
only place in the plane where an insulation failure cannot be mitigated by
turning off a switch or automatically by circuit protection.
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Thank you so much for doing this. Interestingly enough,
this incident would not have been classed as an
electrically initiated event. Similarly hazardous
conditions could have presented had this been a
control cable contacting a fuel line where the
fuel line fails first or a control cable rubbing
the battery cable where the control cable would
fail first.
These events grow out of failure to mechanically
separate and secure potentially antagonistic pairings.
I'm sure we're all pleased that this experience
had a relatively benign outcome . . . and we hope
your nephew is not having nightmares about it.
It had to be an exceedingly tense moment. His
willingness and ability to prevail in the face
of such discomfort is laudable.
| Quote: | The other issue, as has been mentioned, is that if 2 electronic ignitions
are hooked to 1 battery, bad stuff you hadn't accounted for could take
them both down at once. I don't particularly like magnetos and I think
that, barring a failure in the power provided to them, an electronic
ignition is probably way more reliable. The thought of having an magneto,
which will almost certainly crap out at some point, as a backup to a more
reliable system offends my sensibilities. My next airplane will have 2 EIs
on board, and 2 batteries also. One of them may wind up being little to
save weight, but I don't want all my eggs in one basket.
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Sure. If the AeroElectric Connection philosophy
of system architecture is reduced to its simplest
terms, it is "failure tolerance". As the airplane
is planned, parts procured, installed, tested,
operated and maintained, the builder/owner/pilot
needs to be constantly considering ways that the
failure of any single component becomes more than
a maintenance event. The happy pilot is one who
never breaks a sweat in the air no matter how much
time he sweats turning wrenches on the ground.
Your willingness to share this experience goes
directly to these deliberations . . . a thought
process that is almost never matched by trying to
dissect a systems issue in "Never Again" or
"I Learned about Flying from That".
Bob . . .
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