Hight current?
 

Why would a control-bus carry a high current? This would be a major safety issue. I cannot believe anybody would do that.

Very true, but this would almost certainly fry the IC which would usually lead to an open circuit somewhere along the path to ground. A dead short of the battery via a damaged IC is also likely to result in an open circuit condition. I'd also be surprised if there wasn't a fuse or diode to protect the battery itself from a short circuit condition.

An internal short circuit within the battery is a much more likely cause of a high current/high temperature condition.

DWS,
Sort of depends on what the control-data circuit wiring was shorted TO external to the ELT --for example like a 32 vdc bus bar or any power circuit or sneak circuit.

Speed of Sound,
... this would almost certainly fry the IC which would usually lead to an open circuit somewhere along the path to ground. A dead short of the battery via a damaged IC
is also likely to result in an open circuit condition. I'd also be surprised if there wasn't a fuse or diode to protect the battery itself from a short circuit condition.

I'm trying to think through the implications of these interesting posts, while remembering that there are two batteries involved in the above scenarios. [One feeding the bus,
the other within the ELT.]

A hypothetical situation is that a fault in the wiring causes the plane's battery to fry the ELT's IC, raising the temperature within the ELT.

In a related scenario the plane's battery might be able to feed current into the ELT's battery via the damaged IC.

Regards, Peter

Bloomberg reporting a wire smashed under the beacons battery cover.

Boeing 787 Probe Said to Focus on Pinched Beacon Wire - Bloomberg

:ugh:

I was not aware of this '32VDC bus', can anyone enlighten me?

However - and this is making the assumption that the ELT was indeed the cause and not a victim - all the 'evidence' we have to date makes the most likely cause internal to the ELT. Given the way these things should be protected, for instance if there was a failure mode that could lead to data lines 'back-feeding' the battery a diode would be fitted, likewise a non-resettable fuse in the battery pack to protect against overcurrent, it almost sounds as if the wire that got 'pinched' was within the pack itself and upstream of the protection. That would not be good but I can't see how it would have got through a design review - it's the sort of thing one focuses on big time in battery powered equipment!.

As someone posted earlier, is this the first ever incident of this kind in this ELT? If so, and it was on a 787.......................what odds?

Yeah, what a lousy way to win a lottery!

The AAIB stated that there have been 'no significant events' with this unit previously.

The 'pinched' wire, if more than just rumour, puts the ball back in Honeywell's court as the ELT is a customer specification which is simply fitted by Boeing during assembly.

So I take it the we are to assume an internal short circuit is the "fault"?

Then why didn't it show up during unit assembly or testing??

[However it could also be chaffing/fretting of wire to expose conductors in a longer timescale, but I find it hard to believe an ELT Tech working at a bench in the factory wouldn't spot this during assembly]

I've been there and you can bet your life those questions, and then some, are being asked right now!

However a pinched wire would not necessarily be visible from an external inspection and could well not have created a short straight away. My guess is the combination of a pinched wire followed by the temperature cycling eventually ruptured the wire insulation and if, as I speculated earlier, the fact the wire was pinched also compromised the hermetic seal and allowed the damp in, well - there you go!

That nothing like I posted.

And assuming this is what occurred, do you think this will this event lead to tough questions about the thin insulation?

if this turns out to be a pinched wire in an ELT, what are the odds that it happens on the ground...(some operators plan to have the 787 flying up to 18 hours per day...)

so far I dismissed all the speculations about the 787 having more than it's fair share of teething problems, but the way this is developing, I think it's legitimate that some people are starting to ask questions about quality and testing processes, particularly those who built the plane should :oh:

And not a mention of the K [TM] word.

Sorry - I must be missing something here. If - and it is still only 'if' at this stage - the problem that this thread is addressing turns out to be an internal manufacturing fault in a piece of tested, certified kit delivered by a third party who supply it to most people who build airplanes, how is that added to the 787 'teething problem' list?

I agree there are teething problems, just this isn't one of them, or did I misunderstand your post?

The Main & APU batteries are not manufactured by Boeing either.
Quality control?


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I was as just irritated by the odds of this happening in a 787, and expressed myself in a misleading way. I dont think this is necessarily Boeings responsibility, and a 787 issue, from the little we know so far. but in the end they put their name on the airplane, and if I was them, I would be asking some questions, and I am pretty sure they do.

I've read through all of these posts since this thread was created.

There are three things that may, or indeed may not, be related to one another.

A mention of sparks seen well before the fire though to be from an aircon unit. If that was correct could that have affected the humidity in the area on that day?

The pinched wire discussions as a possible cause to failure of the ELT unit.

The attachment of the cables from the aircraft to the ground cart. Now it is stated that the power was disconnected and observed to be so by the flight engineer, however my question is whether the connection of those cables presented a passive electrical characteristic of some kind. e.g. a ground connection that was significantly related to a fault condition on that day. Note I've said 'fault' because normally it should not be a problem.

I'm not an avionics engineer, btw, but I have experience of EMC and have tested military gear. Some of EMC work is passive where you listen to signals emitted from a system and decide whether they are supposed to be there or not. Other tests involve introducing electrical conditions to see if anything changes.

If you mean the ship's wiring - no not at all - because it is the internal ELT wire that got pinched, or so we believe.

If you mean will Honeywell be looking at the insulation of whatever wire they used in their box? Well they may, but much more likely they will be figuring out how to re-route the wire so it can't happen again!

I would be VERY wary of reading anything into what has been "released" so far....

Boeing 787 Probe Said to Focus on Pinched Beacon Wire - Bloomberg

:hmm: "said the person, who isn’t authorized to speak publicly." :hmm:

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

Glad Rag,
My apologies, I jumped ahead of myself in my reply. You asked how come it was not picked up during assembly and test, my point is that as it is thought from the reports that the outer battery cover did the pinching, there would be nothing to see during the assembly process and once it is assembled, inspection and test is all that is left to you. It is possible that inspection would not show anything and if the insulation was not compromised at that time then the unit would pass it's tests too.

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.

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.

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 . .

I wonder how they would find moisture on something that had been through a fire. Unless some idiot had sprayed water on it. :8

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

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

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.

Albeit after the fuse has punctured at 33000 feet.

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?

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.

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

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'.

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.

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

Placebo, anyone? ;)

olasek
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? :confused:

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.

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.

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.