aerobubba(at)earthlink.ne
Guest
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 Posted: Tue Feb 09, 2010 2:52 pm Post subject: Stuck starter contactor |
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HI Bob-
| Quote: | 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.
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As I recall, about the first indication they had was brown smoke coming
from the nacelle. I believe that's why they felt it electrical in origin.
The generator, starter, and feathering pump are the only fat-wired items
out there. The generators are off line anywhere near idle, and the
feathering pumps had yet to be checked, as I understand it. The starter
was the only of the relevant systems used by that point in time. The volt
meter might have been informative, but the ammeters would have shown zero
under these circumstances. Also, I don't think there was much relevant
airframe left for the feds to look at by the time it was all over. I don't
know how the plane was loaded, but the right wing could have had as much as
500 gals of gas and 50 gals of oil, all of it directly accessible from or
actually in the wheel well. (This airplane did not have starter engaged
lights, but as a result of this event, ours does).
| Quote: |
If somebody 'knew' the starter contactor was stuck or
still energized, I'm mystified as to why the aircraft
ever left the parking place.
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They didn't know the contactor failed until the event was well under way.
| Quote: |
A starter feeder should have been protected from destructive
overload by some means . . . c/b, current limiter, etc.
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The only circuit protection for any of the fat wires on this plane is the
reverse current relay. In fact, the external power socket in the aft
fuselage is hard wired directly to the buss. The crew has no direct
control over the application of ground power to the buss, and anytime the
buss is hot, so is the ground power receptacle as well as the 25 or so feet
of connecting fat wire. Seems odd, especially for a combat aircraft.
Nothing like an arcing fat wire in the bomb bay...
| Quote: | 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.
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That was their reasoning as well. An error in the report is the location
of the J-box. It is actually under the nacelle fairing atop the wing skin.
This j-box contains both the feathering pump contactor and the starter
contactor. Just aft of the J-box under the same fairing is the fuel
transfer pump and associated hard and soft plumbing.
I don't know what type of maintenance regimen that aircraft was on, but
ours is on an AAIP and that inspection is a regularly scheduled item. One
other thing about this particular contactor is that material sloughed off
the contacts can bind the mechanism.
| Quote: | When these contactors experience a high-
resistance overheat, they can fail rather spectacularly.
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That was also what the operator thought. Given what they experienced, they
surmised that the contactor did a kind of 'china syndrome' and burned down
through the wing skin into the wheel well and compromised the fuel selector
valve or related plumbing.
| Quote: |
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.
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Most of the fuel lines are cloth / rubber hoses, and the original wiring
also had cloth insulation. The fuel selector valve is shielded from below
to protect it from materials thrown by the tire. This shielding could well
interfere with extinguishing. Of course, the wheel well is right behind a
radial engine, and contains several hinged and telescoping hydraulic lines,
and I have no knowledge of how clean it was or wasn't.
| Quote: |
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.
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Amen, brother-
| Quote: |
Calling this one a "stuck starter contactor" incident
is a WAG.
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The comment about 'the right generator would not accept a load' makes me
wonder about the reverse current relay, which actually IS in the wheel
well. I'm sure if the RCR stuck closed the battery would get discharged
fairly quickly, but I wonder if the current would be great enough to do
damage as long as the engine was at least at idle? Hmmm...
glen matejcek
aerobubba(at)earthlink.net
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