nuckolls.bob(at)aeroelect
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 Posted: Mon Feb 08, 2010 7:58 am Post subject: Stuck starter contactor . . . |
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At 02:00 PM 2/7/2010, you wrote:
Hmmmm . . . interesting. I'm wondering how the operator 'knew'
that the starter contactor had stuck when the FAA inspection
could not identify a source of the fire.
If somebody 'knew' the starter contactor was stuck or
still energized, I'm mystified as to why the aircraft
ever left the parking place.
A starter feeder should have been protected from destructive
overload by some means . . . c/b, current limiter, etc.
Since the fire apparently didn't originate under the cowl
(from a self-destructing starter) then components that failed
to the point of starting a fire had to be things like
unprotected wires, failed insulation, failed contactor, etc.
It's quite possible that the starter contactor on this
airplane looked something like this when new . . .
http://www.aeroelectric.com/Pictures/Contactors/6041_Contactor.jpg
and may well have degenerated to something like this
when failed . . .
http://aeroelectric.com/Pictures/Failures/6041_Contactor_Failure.jpg
A contactor that is "stuck" is generally turned on 'hard'
and is not generating a lot of heat. On the other hand,
this style of contactor (mounted in the wheel-well j-box?)
needs to be inspected (note removable cover) periodically
for degraded contacts or signs of heating off the moving
contact carrier. When these contactors experience a high-
resistance overheat, they can fail rather spectacularly.
The failed contactor I showed came out of a j-box that
routes power to a 130A a/c compressor motor. These contactors
have a relatively high failure rate . . . but being
inside a metal enclosure keeps things from getting
out of hand.
It's also interesting that re-ignition was attributed to
"hot" metal. Things that ignite from hot metal are liquids
with easily ignited vapors . . . it doesn't sound like this
fire went on long enough and with sufficient fuel to heat
aluminum structure or steel landing gear parts to the point
where they become re-ignition hazards. On the other hand,
things that are combustible within the j-box may have
been supporting some glowing coals and been shielded from
the extinguisher. . . or it may be that power was still
present in the j-box.
In any case, it seems unlikely that the real chain of
events for this unfortunate demise of an otherwise
perfectly good airplane are known. Given its age,
it was certainly subject to the effects of time on
electrical system materials that were the best-we-knew-
how-to-do in 1940. One needs to spend an extra effort
to track these materials for continued airworthiness.
Calling this one a "stuck starter contactor" incident
is a WAG.
Bob . . .
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