Go Back  PPRuNe Forums > Ground & Other Ops Forums > Engineers & Technicians
Reload this Page >

Ethiopean 787 fire at Heathrow

Engineers & Technicians In this day and age of increased CRM and safety awareness, a forum for the guys and girls who keep our a/c serviceable.

Ethiopean 787 fire at Heathrow

Old 3rd Aug 2013, 22:16
  #801 (permalink)  
 
Join Date: Jun 2009
Location: UK
Posts: 66
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
Jetstream67 is offline  
Old 3rd Aug 2013, 22:39
  #802 (permalink)  
 
Join Date: Apr 2009
Location: Oakland, CA
Age: 67
Posts: 427
. 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!
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.

Last edited by olasek; 3rd Aug 2013 at 22:43.
olasek is offline  
Old 3rd Aug 2013, 23:56
  #803 (permalink)  
 
Join Date: Jan 2011
Location: Seattle
Posts: 613
There is no way anything other than a Honeywell part could be used and maintain certification I would hope?
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.
EEngr is offline  
Old 4th Aug 2013, 15:31
  #804 (permalink)  
 
Join Date: May 2013
Location: UK
Age: 51
Posts: 67
Originally Posted by Jetstream
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
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.

Last edited by Interested Passenger; 4th Aug 2013 at 15:36.
Interested Passenger is offline  
Old 4th Aug 2013, 20:43
  #805 (permalink)  
 
Join Date: Jun 2008
Location: Cambridge UK
Posts: 83
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?
Peter H is offline  
Old 4th Aug 2013, 23:54
  #806 (permalink)  
 
Join Date: Dec 2005
Location: No. Cal, USA
Age: 67
Posts: 112
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.
grumpyoldgeek is offline  
Old 5th Aug 2013, 07:47
  #807 (permalink)  
 
Join Date: Dec 2001
Location: what U.S. calls Žold EuropeŽ
Posts: 925
I imagine it should not be tough to test this theory, they don't need the whole 787 for that, just a small section of a fuselage with all the relevant tubing/insulation/electrics, etc.
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...
Volume is offline  
Old 5th Aug 2013, 20:36
  #808 (permalink)  
 
Join Date: Apr 2009
Location: Oakland, CA
Age: 67
Posts: 427
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.

Last edited by olasek; 5th Aug 2013 at 20:38.
olasek is offline  
Old 5th Aug 2013, 23:36
  #809 (permalink)  
 
Join Date: Jun 2009
Location: UK
Posts: 66
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
Jetstream67 is offline  
Old 7th Aug 2013, 16:47
  #810 (permalink)  
DWS
 
Join Date: Jul 2013
Location: redmond
Age: 83
Posts: 61
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

Last edited by DWS; 7th Aug 2013 at 16:48.
DWS is offline  
Old 19th Aug 2013, 03:39
  #811 (permalink)  
 
Join Date: Jul 2013
Location: Fairfield County, CT
Posts: 1
This has really gone quiet. Any further developments/rumours swirling?
TomOfCT is offline  
Old 19th Aug 2013, 15:20
  #812 (permalink)  
 
Join Date: Apr 2011
Location: USA, Vermont
Age: 74
Posts: 15
http://www.pprune.org/7994995-post124.html
RCav8or is offline  
Old 9th Sep 2013, 20:58
  #813 (permalink)  
 
Join Date: Jul 2007
Location: United States
Posts: 48
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.

Last edited by amicus; 9th Sep 2013 at 21:00.
amicus is offline  
Old 9th Sep 2013, 21:23
  #814 (permalink)  

Dog Tired
 
Join Date: Oct 2001
Location: uk
Posts: 1,645
Holey moley.

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

Sorry.
fantom is offline  
Old 9th Sep 2013, 21:37
  #815 (permalink)  
 
Join Date: Mar 2002
Location: Florida
Posts: 4,992
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.


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.
lomapaseo is online now  
Old 9th Sep 2013, 21:41
  #816 (permalink)  
 
Join Date: Jan 1999
Location: north of barlu
Posts: 6,131
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.
A and C is offline  
Old 9th Sep 2013, 22:07
  #817 (permalink)  
 
Join Date: Jul 2007
Location: United States
Posts: 48
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.
amicus is offline  
Old 9th Sep 2013, 23:13
  #818 (permalink)  
 
Join Date: Apr 2009
Location: Oakland, CA
Age: 67
Posts: 427
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
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.

Last edited by olasek; 9th Sep 2013 at 23:27.
olasek is offline  
Old 10th Sep 2013, 00:45
  #819 (permalink)  
 
Join Date: Jul 2009
Location: Not far from a big Lake
Age: 76
Posts: 1,456
Originally Posted by amicus
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!
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.
Machinbird is offline  
Old 10th Sep 2013, 06:44
  #820 (permalink)  
 
Join Date: Jan 2008
Location: Reading, UK
Posts: 9,795
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?
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).
DaveReidUK is online now  

Thread Tools
Search this Thread

Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service

Copyright © 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.