Ethiopean 787 fire at Heathrow

[fenland787]

Thanks fenland! It is just that the reports I have heard of very thin Teflon insulation on aluminium wires make me concerned.

Joy Ride,
You are welcome, I think you will find this 'thin' insulation issue goes back a few years and is in the public domain. Quite early on the insulation thickness on the wires carrying 230VAC in the unpressurized areas was found to be on the limit for incipient discharge (corona) when airframe aging was taken into account so it was increased despite the weight penalty that was incurred.

I'm pretty sure the insulation on other wires is no different from other airplanes and most of it is copper, just the really chunky stuff moved to aluminum.

[glad rag]

Nope! there have been massive developments in the advancement of "alu" conductors and the termination technology to guarantee they match the service life of the aircraft.

[joy ride]

Understood, fenland, but the 787 does seem to be a more risky environment for various proven technologies due to its increased humidity and more easily combustible structure.

[DWS]

Understood, fenland, but the 787 does seem to be a more risky environment for various proven technologies due to its increased humidity and more easily combustible structure.

Uhh right on humidity- WRONG on more easily combustible structure . .

[Chu Chu]

I wonder how they would find moisture on something that had been through a fire. Unless some idiot had sprayed water on it.

I guess corrosion products that are chemically different than oxides from combustion?

[joy ride]

DWS, you're, combustible is the wrong word, damageable at a lower temperature than aluminium is probably a better way of putting it.

[olasek]

You are completely WRONG, CFRP has much higher temperature resistance than aluminium, much better temperature-strength curve. It is simply a much superior material. In terms of fire it has self-extinguishing properties.

[m.Berger]

Albeit after the fuse has punctured at 33000 feet.

[Mk 1]

Question for the gurus (sort of OT) on temp/humidity etc. As SLF I have sat in many airframes on the tarmac with the temp and humidity well above what is found at cruise (many airport in an Australian summer - particularly FNQ) Surely the fact that the dreamliner has x% increased humidity compared to older aluminium fuselages pales by comparison to the humidity all aircraft are exposed to in say Cairns with the doors open and the A/c packs switched off. obviously in this condition the circuitry is likely to be shut down, but surely when the aircraft is started the humidity inside could be 98% and fully powered up?

Surely aircraft are tested in these conditions too? Does this invalidate the x% higher humidity argument on the dreamliner?

[ZimmerFly]

The humidity at 98% on the tarmac is not the problem as the airframe will be at the same temperature and will not cause condensation. At altitude where the outside temperature is well below zero, moisture will condense on colder surfaces forming liquid water around and possibly in components that are not completely sealed and/or insulated from the colder surfaces.

[nitpicker330]

The humidity isn't that high is it??
What IS the humidity in a 787 in cruise??

[olasek]

No, humidity inside 787 is around 15% - by absolute standards still a very DRY air. But everything is relative, comparing to any other jetliner with around 4% you can call it 'humid'.

[Karel_x]

Blog on the topic:
Dreamliner 787 sweaty passengers may be causing fires | Plane Talking

Note: If you calculate it, you get dew point -3°C (fuselage conditions 80kPa, 26°C, 15%). That means that at FL no liquid water condensation can occur but only icing on cold surfaces. It can melt lately, when a temperature of the part increases. But I am not sure whether 15% is valid for air inlet, average humidity in the cabin or for outgoing air.

BTW I believe that you can hardly feel difference between 5 and 15 percent of humidity.

[Speed of Sound]

BTW I believe that you can hardly feel difference between 5 and 15 percent of humidity.

I suspect you are right although many passengers have sworn there is a noticeable improvement in 'cabin air comfort' on the 787.

Placebo, anyone?

[Lonewolf_50]

olasek

You are completely WRONG, CFRP has much higher temperature resistance than aluminium, much better temperature-strength curve. It is simply a much superior material. In terms of fire it has self-extinguishing properties.

Can you expand a bit on the bolded part? I am not good at advanced chemistry, nor in some of the exotic materials used nowadays, so I will ask:

once something starts burning, how does it stop itself from burning?

[Burnie5204]

One way of being 'self extinguishing' is by using/manufacturing a material whose combustion temperature is higher than the temperature of the flame created when it burns.

[olandese_volante]

once something starts burning, how does it stop itself from burning?

Certain materials are said to be self-extinguishing when they will not sustain combustion by themselves, once an external source of flame has been removed.

An example would be a material commonly used for printed circuit boards, known as FR-4, which basically is glass reinforced epoxy containing certain additives, usually bromine-based (and environmentally slightly nasty).

See also: Flame Retardant (WP) for information on the exact mechanisms involved.

[Karel_x]

Placebo, anyone?

The speed of evaporation depends much more on a velocity of air flow. If I direct an air outlet to my head, my skin and my eyes could be dried several times more (e.g. 300%) whilst decrease of evaporation between old and new a/c (5 vs. 15%RH) is about 5%.

There can also exist some kind of psychosomatic effects in 787. If you feel calm and relaxing (thanks to advertising) your breathing is light and your respiratory system is dried only little. When you are feeling uncomforted and upset, or you worry about it, both heart action and breathing is much higher and your respiratory system is dried more.

[Coagie]

once something starts burning, how does it stop itself from burning?

"Self-extinguishing" material stops burning, because it's already burnt part doesn't shed or become porous, blocking oxygen from the non-burnt part underneath it, so the fire is smothered. Think of a fire in your fireplace. When it's about to go out, you take the poker, and knock the burnt part of the wood off, exposing the unburnt part to oxygen, and the fire increases. In self-extinguishing material, the burnt material tends to stay on and block the oxygen, and, there's no one with a poker, knocking the burnt part off.

[daikilo]

That is one explanation and the other, closer to the sponsored research, is that, like a candle, with heat applied the material gives of vapours which can burn. Remove the heat source and everything cools to being inert again.

The issue in the 787 case is to detremine what was the initial heat source. If it was the ELT itself then it would be a first. If it was something in that area which then caused the ELT to generate enough heat, maybe. But what if the heat source managed to bring e.g. CFRP material up to the temperature where it and/or other local materials gave off combustible gasses which then sustained the burning and then caused the disruption to the ELT?