Graham -- re your question on fuel gage design:
<< BTW, isn't there a minimum current required to create a spark with enough
energy to ignite fuel vapour. Current limiting should give adequate safety.
>>
Sure there is. Saw it in graphs presented during the
televised, NTSB hearings into the TWA 800 explosion
(tending toward finding of arcing in fuel level
transducers). I remember only talk about millijoules,
and conditions have to be just so -- consider a cantankerous
lawn mower with bad compression or wrong mixture, and
that spark is delivered by many kilovolts. Anyway, Boeing
said (prodded by attorneys?) that they hadn't yet found
a reliable transducer method not involving introducing
electric current into the vapors in the tank; FAA said
their preference is no electricity there at all, of course.
The possible cause on TWA 800 may be not the design
per se, but evidence on another high-time 747 (not
all of 800's transducers were recovered) of
deposition, over time, of metal from the electric
connectors onto insulating material separating them,
increasing the possibility of an arc. If true, means
even the brain-power at Boeing couldn't anticipate
everything.
I raise another issue, concerning introducing anything
metal into a plastic tank in a composite airplane --
lightening strike. There simply are very few metal
paths to direct the strike's current back outside.
This issue was discussed during the hearings, and
indeed lightening has blown up airliner fuel tanks
(twice, I think), but the conclusion was that there's
so much metal everywhere the hazard is very minimal --
sort of a statistical thing. But, the certified Lancair
has metallic stuff laminated into all its skins.
I'm inclined not to experiment with this area. At least,
not without a "chute and a suit" for the testing, the
latter being flame resistant!
Regards -
Fred Fillinger, A063
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