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Although this question relates directly to the NTSB findings on TWA Flt800, I was always puzzled and dissatisfied with the reason that failure in the instrument wiring harness inside the center wing fuel tank(CWT) caused the unvented fumes(a failure in itself) to explode; given that instrument circuits use microamps at 28vdc, resulting in less than .02Joules at best to transmit signals. A probe heater of course would be different, but that wasn't given as the reason. Are there any other instances where wires 'shorted' in an instrumentation circuit could cause sufficient energy to create a spark?

This question is not about "what really did happen" to TWA800; this is about whether or not the NTSB(National Transportation Safety Board) made a correct analysis in implicating Instrumentation Wiring.

Note

A joule (J) is a unit of measurement of electrical work or energy; 1 J is the amount of work done by 1 watt of power in 1 second. The power supplied to the FQIS components through the cockpit fuel gauge (0.02 mJ) is about 10 percent of the minimum ignition energy (MIE) requirement (0.25 mJ) for hydrocarbon fuels, referenced by the American Petroleum Institute Recommended Practice 2003 (API 2003), Protection Against Ignitions Arising out of Static, Lightning, and Stray Currents, fifth edition, December 1991 Taken from here

user2479
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  • there were other factors IIRC, including deposits on the fuel sensor that shortened the spark gap – ratchet freak Oct 01 '14 at 13:21
  • Milliamps is not enough. You need to know the voltage and many other factors such as the spark gap medium, the dielectric function of that medium, the spark gap size etc etc. A very high voltage with tiny current (static electricity) will generate a spark as will a low voltage with a high current (try shorting a car battery). 28vdc at 250 milliamps will happily melt aluminium. 28vdc at a few tens of milliamps will happily create spark across a millimetre or so of a reasonably conducting medium. – Simon Oct 01 '14 at 14:45
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    I feel this is more appropriate for physics.SE (spark gap as function of tension, energy released as function of gap, tension and current, etc.) – Federico Oct 01 '14 at 14:45
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    I don't think there are only signal wiring in those harnesses. There could be power lines to instrument sensors and other circuits in the bundle. – JerryKur Oct 01 '14 at 16:16
  • My initial research indicated that a minimum of 1 Joule(I(2)R) is needed to cause sufficient arc to ignite the combustible Center Wing Tank(CWT) gasses. Honeywell and Boeing stipulated that the fuel probe system at max carries .02J. CWT explosion definitely was the cause; instrumentation wiring(IMO) seems an inadequate answer. – user2479 Oct 01 '14 at 19:43
  • @Federico To those who put this on hold....I'm not asking about the physics of what causes a spark, nor am I asking what temperature can Jet "A" ignite. I am asking whether an instrumentation wire (on an aircraft) can produce enough energy to ignite unvented gasses in a fuel tank-so those nerds who calculate quarks will likely send it back here.... – user2479 Oct 01 '14 at 19:55
  • @user2479 You have two questions there: can the wire spark, and can the fuel-air mixture ignite. Both are physics questions. – fooot Oct 01 '14 at 20:16
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    @user2479 After your edit, it has become opinion based. – Farhan Oct 01 '14 at 20:24
  • @Farhan I'm just using the information contained in the NTSB report(you can find it on pg.34). What's interesting after reading the whole report(ouch-the eyes), even they said it was a 'suggestion'-they had no further evidence. I have worked on A/C systems and have replaced malfunctioning wiring-yes, if you short out canon plugs or route wiring wrong-bad things will happen. I have yet to find where an instrumentation circuit caused a short that 'arc'd', but I could be wrong. – user2479 Oct 01 '14 at 20:33
  • @Simon If you just use 28*.02, you end up with .58w, which isn't Joules, but an energy measurement just the same. The Board said >1J will cause an arc; the manufacturer said their system would produce <.02J. So, what's the answer? – user2479 Oct 01 '14 at 20:40
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    the answer is that you have to compute the Joules, hence is a physics.SE question. – Federico Oct 01 '14 at 21:29
  • @Federico The question deals with aircraft, not physics. I am on the physics SE site also, and it's mostly made up of 'brainiacs' working out advanced calculus solutions to theoretical problems. They 'exclude' engineering answers, as they are about pure physics(ahem). This question is about "whether or not anyone has seen this...in an aircraft", NOT-how many Joules per coulomb does it take to ignite a petrie dish of Jet 'A'. – user2479 Oct 02 '14 at 02:14
  • @Federico Close and delete this question if you insist, however, this question 'may' clear up some 'myths' about insignificantly tiny energy sources taking down large commercial aircraft. Notice, I didn't say what DID take it down, but unless someone here(not the geeks in the Physics Lab) can prove that small of an energy source in the adjacent area of the fuel cell could ignite the fumes, I'll stand on my hunch. – user2479 Oct 02 '14 at 02:26
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    I feel it would be better if you would ppass by chat. Anyway, as it stands now, you are asking whether or not the NTSB made a correct analysis in implicating Instrumentation Wiring, i.e. to redo the analysis, an analysis that involves lots of physics' considerations and computations. – Federico Oct 02 '14 at 05:46
  • @Federico At the end of their lengthy report, they admitted it wasn't conclusive, but based on their probability model(1x10(-5)), they list it as the best scenario. What sticks out also is the Flt Capt.'s final comment on the CVR "...look at that crazy fuel flow on #4..see that", which also makes a compelling reason, although not totally proven one. – user2479 Oct 02 '14 at 17:21
  • @Federico(finally) What was interesting was it seems like they were blaming the heat from the Air Cond. packs, which were adjacent to the CWT and vented hot exhaust next to the access door to the CWT, yet later they ruled it out. – user2479 Oct 02 '14 at 17:29

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