olasek
 

True, but that could apply to almost any part of the airplane, regardless of what started it? I would have hoped that kind of thing was done already, though someone may be wanting to revisit their ideas in the light of what they've seen in this incident!

Hmmmm....Sorry "in the light of" might be an unfortunate phrase to use in the circumstances......

I cannot speak for the particular cell pack in the ELT but having dismantled and rebuilt quite a number used outside aviation it is standard to include a fuse or a polyfuse (effectively a self resetting fuse) in a pack. These often turn up as links between the actual cells of multi cell batteries but on single cell designs they are at the cell ends of the leads or on an attached protection circuit board.

Whoever did that design would have been expected to test their design (i.e. apply both increasing and random levels of excess discharge current and finally an immediate short circuit across the battery terminals or wires) to ensure the battery shut down and stayed shut down safely in each case.

From the reports the two leads were shorted together which should have resulted in a very short pulse of current then a drop to a hugely limited current for the duration of the fault thanks to the action of the polyfuse. If the polyfuse failed to work or failed to reduce the fault current enough for safety the cells would then be able to potentially overheat over time running the risk of Cell thermal runaway. If the short circuit was intermittent this might have been the subject of many repeated cycles of short, shutdown and recovery and perhaps this is where the issue arose

Once the cells reach a critical internal temperature (c 135c IIRC) they proceed to break down and self ignite and no fuse is going to help them after that point

Was done what?
I doubt recreation of chain of events had been done in this case, it takes time to set it up, it is much more expensive proposition than fiddling with just ELT in the lab environment. It may not be as hard as NTSB's multiple attempts to reproduce sparks in the 747 main fuel tank but it may be difficult and time consuming nevertheless.

I think more than hope should be involved in a QA program. On the other hand, this battery replacement isn't something Boeing usually deals with and planned for. It was forced on them by the program slide.

polyfuses trip at 125'c either from external heating or internal heating and will stay at that temp while there is a fault. Hope the protection device didn't overheat thebattery it was supposed to protect.

Heat flow?
 

Jetstream67
I cannot speak for the particular cell pack in the ELT but having dismantled and rebuilt quite a number used outside aviation it is
standard to include a fuse or a polyfuse (effectively a self resetting fuse) in a pack. These often turn up as links between the actual
cells of multi cell batteries but on single cell designs they are at the cell ends of the leads or on an attached protection circuit board.

Whoever did that design would have been expected to test their design (i.e. apply both increasing and random levels of excess
discharge current and finally an immediate short circuit across the battery terminals or wires) to ensure the battery shut down
and stayed shut down safely in each case.

I assume that a polyfuse would effectively restrict the current during the "short circuit" to that necessary to keep it at ~125C.

If so, the heat-flow from the polyfuse might become significant. Two extreme examples are:
1) If thermally clamped to the battery, the polyfuse would presumably heat the battery to ~125C at the point of thermal contact.
2) If thermally clamped to the ELT case (via the shorting wire), the polyfuse might need to pass a significant current to maintain
its ~125C temperature.

I assume that (1) is designed out, as it's effects would occur in all short-circuit testing situations. However (2) would only arise if
the short-circuit event also provided a "new" thermal pathway, so might not appear in "normal" testing.

Do you have any thoughts on the polyfuse's likely degree of thermal isolation in this incident; both from the battery and from the ELT case?

Polyfuses?
 

Electrical engineer and PP here. I looked at Polyfuses in a design I was working on about a year ago. After playing around with them and a bench power supply, I decided that they could not be trusted for my application. Went with a time-tested fuse. Looks like it was a good idea.

Well, I remember many fellow ppruners accusing Boeing and its subcontractors to simplify many of their tests too much, missing to simulate real in-service conditions...

Yeah, rather completely different subject.
Boeing (like any other aircraft manufacturer) did exactly what tests were required for the certification of their 787, if you try to blame someone for "simplifying tests" blame FAA. Certainly such tests do not require it simulate ELT fires. But if British (or any other) aviation agency wants to be satisfied as to completeness of this ELT accident investigation they could attempt to do full scale tests which would include portions of the fuselage.

Polyfuses 'blow' in a fairly traditional manner and the fault current falls back to a very low level - basically enough to confirm the fault remains.. After all fault current ceases they fully recover over a number of hours to being fully conductive again. So no they do not sit at 125c in an overload state as a protective resistor might.

The issues are :
* they do not cut off all current entirely
* Their correct operation under all cyclic short and recovery conditions is not generally specified (i.e they are seen as an occasional safety precaution not 100 cycles a day etc.) although it might have been tested in this scenario

But unlike traditional fused they are less sensitive to impact forces while in operation (pretty important in an ELT) and they can recover to allow normal operation after a temporary short circuit is removed. For an ELT the latter two factors might be considered valuable

should be of interest re fires on aircraft
 

Microsoft Word - SAFITA_for_Website - SAFITA_2013.pdf


http://aerosociety.com/Assets/Docs/E...AFITA_2013.pdf

This has really gone quiet. Any further developments/rumours swirling?

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 hanger 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.

Holey moley.

I am an A-team person of many years standing but I would not wish that on the B team.

Sorry.

Repair issues are between the manufacturer, the operator and the insurer.

Damage assessment is the concern of the regulator and the flying public and that is of my interest as well.

If I understand correctly what you are saying, is that given a small area of overheat will lead to a more severe level of damage in a non-metallic structure. That the lower ignition point contributes to this damage and that said ignition and subsequent flammability gives off noxious fumes.

But yet no confirmation is given that the area of damage thus occurred will grow to the point where safety of flight is affected as compared to the same heat source in an metallic skinned aircraft.

Likewise no confirmation is given that the noxious by-products for such level of "structurally safe" damage will be perfused to affect the passengers.

Amicus
 

I don't dispute your opinion for one second about the thermodynamic issues with composite structures, what I do dispute is your opinion on the viability of repair of the structure.

The industry is metalcentric and up untill very recently most people tryed to impose a metal style repair on a composite structure, this was usually expensive, disruptive to the shape of the structure and added weight to the structure, in short just about all the things you do not want in an aircraft repair.

Things have moved on, composite repair is now better understood and even Boeing one of the worst practitioners of composite repairs have moved on considerably. I don't agree with your bleak outlook on the Ethiopean aircraft and think that a repair can be made on an economic basis and not just to save face at Boeing......... Just as long as it is left to those who understand composite repair techniques and the metalcentrics are kept away from the aircraft.

lomapaseo,

Well now, there are a three points to be made and cited. First, all epoxies were banned for aircraft interiors back in the 70's due to passengers being killed by FST. Do you think that edict by the regulatory authorities was unwarranted or without facts?
Secondly, at the LHR Ethiopian incident the fire department reported dense smoke when they entered the aircraft wearing, I would bet, full portable oxygen masks and gear plus full Hazmat. If passengers had been on board would they still be alive?
Finally, why is it taking Boeing and its insurers so long to even decide whether to repair or scrap since July if no safety of flight issue? We are coming up on two months with no actions
You asked for some facts and there they are, I would hope they satisfy you or, at least, give you pause.

The fact that you don't know about it doesn't mean there is no "action".
Boeing stated long time ago that the jet will in fact be repaired but the method of repair will be a private matter between Boeing-insurer-Ethiopian. By the way I see zero evidence (based on aviation literature out there) that this event was "worse" because the fuselage was CFRP. You are entitled to your opinions of course. Ethiopian clearly doesn't share your bleak view of composite airframes - they laud 787's performance, efficiency and they want 8 more.

Yes, CFRP seems to be a pretty good insulator seems to be your point, and will not wick heat away into the adjacent structure, thus the local structural temperature can reach higher temperatures near a thermal event.
What you have omitted to show is information on how rapidly the other side of the structure heats. Heating one side of the skin does not create near-equal temperatures on the other side of a CFRP skin.

I do not think we will ever find molten CFRP flowing back in the airstream in a fire the way we see aluminum flowing during structural fires. It is going to sit there and char and continue to insulate until it is finally burnt through to the other side. If it also has structural loads on it, then it will eventually fail as it degrades and loses strength. In the case of aircraft skin in flight, it may hang on quite a while if the surface can dump enough heat to the local airflow since the layer of char formed near the fire will act as an insulator.

One of the key material issues in a fire is whether or not it will spread a fire. If it self-extinguishes away from the source of heat, then that is what is desired, and I would expect that it could not be certified if it did not self-extinguish.
Fume generation from hot CFRP is an issue, but that should be an engineering challenge that can be solved without making the cabin into a gas chamber.

It is just a different material with different properties. We have to learn what is different about it and adapt our thinking and engineering concepts.

It may have nothing to do with safety of flight - but it has everything to do with public perception, not to mention Boeing's stock price.

To acknowledge that at this early stage in the 787's career the first one has been written off would be a PR disaster (though arguably Boeing are getting used to those).

On the other hand, the thermal conduvtivity of molten aluminum is still about the same as for solid one, while the thermal conductivity of charred CFRP is far less, than the one of intact one. Therefore thick CFRP will protect itself from a localized heat source, while Aluminium will burn through locally. However, this means structural damage and requires a repair. It also means toxic fumes. But especially for thicker material, CFRP stands a local fire better than aluminum from a load carrying standpoint. For thin material both will fail anyway. A hot spot like a shorted wire or an arcing one may do more and more localized damage to CFRP than to aluminum, due to the localized heat which is better dissipated for metals. But when charred on the surface, this effect will quickly compensate the localized heating.
Also remember that for items mainly loaded in plane tension (e.g. a fuselage skin), burning of the resin does not keep the carbon fibres (good for > 2000 °C) from carrying tension loads. Molten Aluminum does not carry anything.
You simply can not compare apples and oranges. And I do not say CFRP is better!

It could just be, given that it seems the Honeywell ELT was a key player in all this, that there are some 'full and frank' discussions going on behind the scenes as to exactly who's insurance company is going to be paying anyway?

Boeing stock is trading within a percent or so of it's all time high (and that all time high occurred after the Ethiopian 787 fire) . I'm guessing the public perception can't be all that bad.

tdracer, you don't understand. The sky is falling, or at least for those that wish to remain clueless about this business. ;)

Lots of aluminum airplanes have been lost or badly damaged due to fires. I'd guess almost all of those fires started small, then found fuel somewhere besides the aircraft skin.

Perhaps there will be a whole new class of fires caused by heat sources in proximity only to composite skins. I personally doubt that will happen, but time will tell.

FAA studied fire hazard of composite skin
 

Sorry I can't find the reference again - I mentioned it a couple of times in the original battery thread - but the FAA commissioned a study at a university on composite material that iirc was specifically that which was intended for the B787. I have no expertise in the area but glanced through the report and at least did not see anything obviously bogus about the tests or conclusions (that the skin would survive fire well.) Of course, they thought the Li-Ion battery system was perfectly safe, too, so some of the points mentioned up-thread here might have been overlooked.

If anyone is interested in the report and can't find it, ask and I'll dig out a link to it.

If you have the link handy, I'd certainly appreciate it, thanks.

There is one here http://www.fire.tc.faa.gov/pdf/07-57.pdf

From Boeing's 787 Aircraft Rescue & Firefighting Composite Structure


http://i337.photobucket.com/albums/n...ps86f4de32.jpg


http://i337.photobucket.com/albums/n...psb4c3e184.jpg

Machaca
Boeing total PR codswallop And if not a problem, why epoxies banned for interiors since 1970's?

Why are you conflating interior fitment and structural epoxies?

Seems disingenuous, particularly when you don't address the specific burn properties of the modern epoxy developed for the 787.

Please enlighten us all by addressing the seven bullet points above.

My read between the lines of the Boeing PR is the major source of toxicity is the interior panels of the aircraft.

Neither the aluminum nor the 787 outer structure contribute significantly except for the time to be defeated by an external fire.

My memory of such external fires is almost solely comprised of ground fuel-pooled fires.

An internal cabin fire is clearly another problem.

I am curious what a Asiana fuselage might look like along the fuselage top after the fire breaks through to inside the cabin, but that's just a structural damage question and not germain to toxicity.

lomapaseo

My concern is that the vast majority of survivable crashes, (in fact the norm) for commercial aircraft result in compromised, fractured, open doors , open slides,etc, thus allowing ingress into the cabin of toxix gases and smoke from the external CFRP fire. I have cited over 150 such commercial airline survivable crashes to both Boeing and FAA since 1970, all involving compromised and fractured fuselages to no avail.
To my knowledge this critical safety condition was never tested for during development and certification of the 787. Specifically, Boeing did burn-through tests via cone calorimeters only on intact and non-compromised panels and the FAA and Boeing both refused to replicate the Air France A340 Malton overrun crash a few years back, which would serve as an ideal and totally reproducible example of such survivable fuel fed fire crashes. This could easily be performed on on of the four now non-flying 787 prototypes and such a test, replicating a survivable fuel fed fire crash with 100% passenger and crew survival in the case in the A340 with a fuel fed ground fire would end the debate on either side of the FST issue.

To quote the old aviation aphorism "One test is worth 1000 expert opinions".

Then I guess, amicus, passengers flying on the CFRP A350 and the significantly CFRP and composite (GLARE) A380 are also pretty much whistling past the graveyard every time they board one of those planes, as well.

In fact, having flown the 787 and A380 a number of times (and with my airlines of choice also choosing the A350), I guess I better make sure my life insurance is up to date. :mad:

And god save us when the CFRP A320 and B737 replacements start entering service in the 2030s... Ralph Nader is no doubt working on a new book to lambast Airbus and Boeing on their decision to move to CFRP. I suggest he go with the title Unsafe At Any Altitude. :D

amicus:
Refused who? Why reproduce an accident from which everyone successfully evacuated?

I assume refused HIM. HE suggested and they refused, how nasty and inconsiderate of them.
Assuming there is a debate. This forum is hardly a place to find out if there is a debate (among real 'experts') or not.

Well if Boeing should destroy one of the 787s in such a test, Airbus should also be required to sacrifice MSN001 of the A380 and A350, as well. :rolleyes:

Now that the -9 is flying and gaining orders, what odds on Boeing slipping out the bad news regarding the Ethiopian 787 being "beyond economical repair".

Wouldnt put the A380 'Glare' in the same league as carbon fibre . Anyone remember the hazards with the harrier composite materials in an accident . Boeing wouldnt be aware of that having responsibility for the AV8B now......!