Time to put out some facts regarding thermal conductivity
Regarding the Ethiopian fire of ELT battery, I have already cited my opinion that the CFRP as an insulator rather than aluminum as a thermal conductor mightily contributed to this fiasco and rendered what might have proved to be a minor incident in metallics into a major and ongoing issue re 787.
First, I am a mechanical engineer specializing in composites for a mere 48 years in aviation and am not a thermodynamicist, albeit I know enough thermodynamics to pinpoint this issue, I believe.
Let me give this thread some thermal conductivity values for various materials and then conclude with the dire effects regarding low thermal conductivity rewiring shorts, arcs and the like.
Some typical thermal conductivity values for metallics are: (all units are in the widely accepted and used W/m/K):
Aluminum and its various alloys = 167
Brass = 116
Copper = 339
Steel = 48
And for CFRP it is 0.13, HA!
Thus, when any short or arc occurs close to CFRP, the skin temperature always peaks higher than the highly conductive aluminum alloys, thereby exacerbating any or all fires besides which, incurring permanent structural damage starting at around 375 degrees F and being flammable with a very low self ignition temperature of 580 degrees F.
Hence, fire damage will always be far worse in CFRP with irreversible structural damage vis-a-vis metallics. This plight, of course, also incurs the rightly dreaded FST products of combustion.
This is the case for any or all electrical shorts, arcs, or any other fire sources not merely ELT's. Hence, any fire adjacent to fuselage skin or wing skin will always be far worse for CFRP versus metallics.
Now, was this accounted for in any FAA Special conditions, was it tested for extensively during certification, and finally has Boeing properly assessed all very low thermal conductivity risks due to shorts,chafing, aging, arcs of all electrical wiring, batteries, et al ?
I do not believe such to be the case and now we have a forlorn, and possibly terminally damaged Ethiopian 787, based upon actual repair costs . This aircraft is still sitting in a remote LHR hangar while Boeing and the insurance underwriters presumably debate its final fate. And what might have been merely a minor ELT shorting incident on a metallic aircraft is looking like a total hull loss or a huge and very difficult repair face-saving exercise.
And this present exercise will be replicated many more times for the 787, particularly as electrics age even assuming that they are correctly wired in the first place, I venture to predict. Some airlines and MRO's might care to ponder this note.
Last edited by amicus; 9th Sep 2013 at 21:00.