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1. 09:59 AM - Re: Re: Fuse or CB for largish wires into cabin? (Robert L. Nuckolls, III)
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| Subject: | Re: Fuse or CB for largish wires into cabin? |
>
>Don't put a fuse in series with the starter motor circuit.
>A fuse has a certain amount of resistance which will reduce engine
>cranking speed.
>As for the main power bus feeder, it is normally not fused, but some
>builder do install a fuse.
The FAT wire feeders in light aircraft are
typically never actively protected i.e. breakers
or fuses. In both Part 23 and Part 25 of the FARS,
starter power pathways are specifically
exempt from such protection.
They are demonstrably immune to hard faults
that might be expected to trip active protection.
A robust wire's inadvertent connection with
ground is more likely to be a soft fault that
burns structure rather than the wire itself.
I've cited an example incident waaayyy back when
where a Beech C90 on short final to New Mexico airport
suddenly found the elevator mechanically disconnected
from the control column. They managed to land the
aircraft using pitch trim.
Subsequent investigation found the elevator
cables completely slack.
It seems that during maintenance activity
some time ago, some wire bundles under the floorboard
were mis-positioned for access but not properly
returned to as-manufactured.
One of the wires was feeder to the copilot's
windshield electric de-ice inverter . . . a relatively
fat wire protected at 75A or so. This wire was
being rubbed against the elevator control cables
until insulation was penetrated.
The intermittent arcing was so subtle that the
breaker never opened . . . and nothing untoward
was noticed by crew. Over a period of time
the steel control cable was eroded to failure
while copper wire strands were barely marked.
A similar thing would happen if a rear mounted
battery feeder were to become inappropriately
involved with the edge of a bulkhead lightening
hole under the floorboard . . . A little
insulation might smell bad but subsequent electrical
effects are more likely to burn away the aluminum
as opposed to over-stressing the wire.
This is why most FAT wires are passively protected
by virtue of judicious installation and maintenance
practices as opposed to breakers and/or fuses.
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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