Ethiopean 787 fire at Heathrow

[DWS]

FAA to issue AD ?

Boeing 787 Dreamliner: FAA to issue 'airworthiness directive' in wake of fire | Business | guardian.co.uk

Boeing 787 Dreamliner: FAA to issue 'airworthiness directive' in wake of fire

Regulator to order mandatory inspections of 787's emergency beacons just two months after lithium-ion battery problems

..
The Federal Aviation Authority was due to order mandatory inspections of the emergency beacons aboard Boeing's ill-fated 787 Dreamliner as early as Monday after a fire broke out aboard one of the planes in London.
The FAA "airworthiness directive" comes just two months after the 787 was certified to fly again following a global grounding triggered by problems with its lithium-ion battery system.
A fire aboard an Ethiopian Airways jet at London's Heathrow airport earlier this month forced the closure of both runways for more than an hour.
The UK's Air Accidents Investigation Branch (AAIB) concluded that the fire started near the aircraft's emergency locator transmitter – a distress beacon used to help rescuers find a plane if, for example, it is forced to land on water or in polar regions. The focus of attention appears to be whether a pinched wire under the battery cover triggered a short circuit.
The FAA spent the weekend telling other aviation safety regulators around the world of its concerns. It is expected to formally issue the directive early this week, possibly as early as Monday.
The beacons, made by Honeywell, are powered by a lithium manganese battery, which could have suffered a short circuit. Last week the FAA said its inspections would call for operators to check "proper wire routing and any signs of wire damage or pinching, as well as inspect the battery compartment for unusual signs of heating or moisture."
Robert Mann, an aviation expert at consultant RW Mann, said it was unclear whether the latest issue was caused by installation, quality control issues or design issues specifically related to the 787. Honeywell had issues with its emergency locators in 2009 that led to the beacons failing to send out signals. But Mann said the issue could also be specific to the Dreamliner.
The 787 is the world's most technologically advanced passenger jet. Its use of lightweight materials means its uses 20% less fuel than its peers. Aluminum wiring and Teflon coating are used to save weight. "We got rid of aluminum wiring in the US in the 1980s because of the tendency of aluminum oxide to cause problems," said Mann. "And there have been reports of fragility with Teflon insulation." ...

[Ian W]

Quote:
This may be one of the reasons that a hermetically sealed blue box was designed for the 'fix'. To keep combustion gases in and to keep water vapour out.
More than that.... Their "solution" of putting a battery in a box, which looked like avoiding a problem they didn't understand.... included "drain holes" for the battery....

http://assets.sbnation.com/assets/23...on-English.pdf

Maybe Boeing had worked out what the real problem was and that's why they were so confident that it wouldn't happen again.

Perhaps they could buy some more boxes to put electrical stuff in.

The battery box was to meet the new RTCA standard for LiIon batteries, the new Cessna Citation has a similar box and had to have similar tests.

[simple-simon]

The battery box was to meet the new RTCA standard for LiIon batteries

Could be. I think it's a great solution to the problem.... Here's the scenario:

1) Plane flies high in the sky.... Airframe gets very cold....
2) Airframe made of composites has much lower thermal conductivity than aluminium. Nett result... airframe takes longer to get cold on the way up.... once cold... takes longer to warm up on the way down...
3) Plane lands, doors open, passengers get off. Cabin fills with air far more humid that the 15% talked about on this thread.
4) The moist air, inside the aircraft, getting close to the airframe, forms condensation.... Since the airframe is cooler than we see in aluminium airframes.... we see more condensation....
5) Water and electrical circuits don't go too well together...
6) Issues that previously were unlikely to occur, become more likely, in the more moist environment.

So by putting the battery in a sealed box.... nicely ensures that the condensation doesn't get to the battery... It wasn't poorly made batteries, or overcharging.... it was a short caused by the battery being covered in condensation.... Oh and meant to say.... the technology behind the battery wasn't the problem and this is why it didn't make sense to change to conventional batteries...

I could be wrong of course, just a theory.

[Volume]

Airframe made of composites has much lower thermal conductivity than aluminium. Nett result... airframe takes longer to get cold on the way up.... once cold... takes longer to warm up on the way down...
3) Plane lands, doors open, passengers get off. Cabin fills with air far more humid that the 15% talked about on this thread.
4) The moist air, inside the aircraft, getting close to the airframe, forms condensation.... Since the airframe is cooler than we see in aluminium airframes.... we see more condensation....

Not necessarily. Airframe made of composites has much lower thermal capacity as well, so less "coldness" stored in the airframe.
GA experience: Aluminium airframes parked outside have water accumulation inside in the morning, GFRP airframes parked outside do not (or significantly less). Might be an issue of the sandwich construction as well, so for a monolithic dreamliner it may be different again.

[HDRW]

Here's a thought: The new battery-box in the 787 is vented to atmosphere, so at altitude it's going to be at low pressure, and maybe quite cold (I don't know if the box is insulated, or how much of the aircraft's warmth will penetrate). On descent the box will "breathe in" the outside air, and if it's humid, will the cold battery have a tendency for condensation to form on it? I also wonder if they had to strengthen the battery's case to withstand the low pressure around it?

[kenneth house]

An interesting theory...

@ simple-simon post #666

but could you condense it down to a simple version?


Looking at the pattern of the charred area it appears that the longitudinal struts (also a composite structure?) shielded the skin or acted as a heat-sink to pull heat out of the surface.

i wonder if the coeff of thermal expansion (CTE) of the composite is much greater than aluminum and may allow more aircraft growth and contraction such that wire chaffing might occur?

[fenland787]

The new battery-box in the 787 is vented to atmosphere, so at altitude it's going to be at low pressure,

You are presuming no overpressure valve or rupture disk in the vent? I don't know either way but would have assumed the opposite. I cant believe those cells would like to spend most of their flying lives at unpressurised Ps & Ts?

[Speed of Sound]

The new battery-box in the 787 is vented to atmosphere

It's not.

It is only vented to atmosphere once the pressure inside the box ruptures a disc in the vent. This protection is set to rupture at a much higher pressure than the differential at normal altitude.

[Karel_x]

i wonder if the coeff of thermal expansion (CTE) of the composite is much greater than aluminium and may allow more aircraft growth and contraction such that wire chaffing might occur?

Maybe twice as much as aluminium, I am not sure about type of resin:

Aluminium 22.2 . 10-6 m/mK
Epoxy, castings resins & compounds, unfilled 55 . 10-6 m/mK

That means ca 1 mm per 1°C for hole long fuselage. Fuselage is getting shorter at FL so wires are not tighten but some movement surely exists.

2 simple-simon: Interesting hypothesis...

[LASJayhawk]

I think you are all reading way too much into this. If they shorted one of the battery leads under the cover, it would only take time to make an ELT immolate.

[awblain]

CFRP thermal expansion?

CFRP typically has a very low thermal expansion, at most about 6 times less than Aluminum at 300K. If designed right, it can have almost zero. Many sophisticated structures (that are not limited by weight) use it, despite its cost, to take advantage of this property. If there's any chafing it would probably be because the metal wiring changes size while the composite structure does not.

[olasek]

Maybe twice as much as aluminium,

Your data is wrong.
Thermal coefficient of expansion of CFRP is about 5 times smaller than aluminium. (2.3 e-5 per degC for Al and 0.5 e-5 per degC for CFRP).

[fenland787]

Maybe twice as much as aluminium, I am not sure about type of resin:

Aluminium 22.2 . 10-6 m/mK
Epoxy, castings resins & compounds, unfilled 55 . 10-6 m/mK

Is 'unfilled resin' the right number to use? I thought carbon composite is much better than that - because of the fiber, optical tables made of it have a COE typically 1/10th that of steel for instance, so I thought the number is lower than Al, not higher.

The hugely dominant effect is flexure in flight, have you seen the wings on takeoff?

[Cathay Boy]

If condensation is the cause of battery fires, and Boeing "knew it", it can be easily tested in test scenario. Are you suggesting conspiracy that Boeing is trying to hide 787's condensation issue as it effects not just the battery but the entire electrical systems on the 787s (which is a lot), and may even force Boeing to redesign the entire plane?

[olasek]

If condensation is the cause of battery fires,

There are a lot of "ifs" in your statement. I could bet my home that condensation did not play any role here. But even if it did you wouldn't have to redesign electricity - you simply turn down humidity to much lower levels. There is nothing in the design of A/C-ventilation in 787 that says you MUST have higher humidity.

[syseng68k]

In light of the discussion re: composites and aluminium alloy in the
presence of fire, I wonder if someone (amicus ?) could comment:

In a composite structure, serious fire at one side of the sheet could
cause serious delamination and strength reduction before it burnt through,
due to the insulating properties of the composite materials. Aluminium
alloy, on the other hand, even though thinner, conducts heat very well
and has a high melting temperature.

Seems to me that, in the same way that an aluminium pan doesn't melt when
exposed to a gas flame, so long as it doesn't boil dry, aluminium should
be safer, especially at 30k feet, lots of air cooling the panel and air
temperature of say, -50 C ?...

[lomapaseo]

There is nothing in the design of A/C-ventilation in 787 that says you MUST have higher humidity.

Wasn't that a selling point of this aircraft to the passengers?

[olasek]

Wasn't that a selling point of this aircraft to the passengers?

Yes, it was ONE OF selling points. It doesn't mean that higher humidity is mandatory.

[amicus]

syseng68k,
Thank you for your kind request and I trust that I can make a sensible and accurate response to your good self regarding conduction and insulative properties of both aluminum alloys and CFRP, whilst simultaneously attempting to correct some misunderstanding by others and make some suitable, accurate and helpful remarks regarding relevant differentials of Coefficients of Thermal Expansion (CTE) and other composite properties.
First, please let me address those who claim that CFRP quote "chars'" and that it thus self extinguishing. Sadly this is not the case for epoxies, I again repeat to those that will hear, the fact that all epoxies have been banned from aircraft interiors for around the last 30 ears and for excellent reasons, and it was not merely the flammability and combustability of such epoxies, but primarily the Fire, Smoke and Toxicity (FST) emisions that occur.
And, yes, a char develops. but only as a result of the epoxy having first burned with deadly initial combustion, flaming, smoking and emitting copious FST hazardous products in the process. Incidentally, that is what we used to employ way back when to make carbon/carbon, but that process was a controlled, pressurised and in an enclosed chamber. We used phenolics, primarily, as they have a greater carbon yield than evil epoxies. However, even that process , required over seven re-denisification and burn cycles to come close to the desired theoretical density of carbon/carbon.
The epoxy itself,and I have both performed or witnessed many 100's or 1000's of OSU ( Ohio State University) tests as developed back in the 60's re testing flammability of epoxy interiors, seat cushions, fittings,overhead bins and the like which became a semi-standard interior burn test over the decades to prove this point. However, and it's a very large however, the OSU test was designed around a mere burning cigarette or lighted pipe scenario, with a 50 watts level, far from 28V electrical fires and shorts and fuel fed fires in a survivable crash which requires 200-250 watts per square metre and their ilk. This point is cited and detailed in my paper as some have read on this board.
But the charring of epoxies does not, repeat NOT, prevent burn-through which is why the FAA and Boeing developed a whole new set of high wattage burners to try and adequately simulate and test for burning fuel fires and resulted in the FAA, over Boeing's weeping, wailing and lobbying to stipulate that the lower half, and, most regrettably only the lower half, of the 787 fuselage be expensively and heavily insulated with a special anti-burn through insulation to allow a five minute escape window in case of a wheels-up or off runway survivable crash.
However, and here is yet another huge however, the FAA and Boeing still had to assume and design the burn-through test only for a totally intact, all doors closed, no slides deployed no doors opened and no fuselage fractures, given the FST loaded epoxies employed by Boeing. Clearly, in most survivable crashes, this is not the case and I supplied the FAA, Boeing and Airbus with over 150 such commercial airline survivable crashes since the 1980's as my paper cites. All were survivable crashes with fuselage fractures and/or opened doors. The simple and clear fact is that Boeing and the Northwest FAA Office did not have a prayer of certificating the 787 unless an intact and closed fuselage was assumed as a basis for fire testing in the realm of 200-250 watts as is the case in fuel fed fires. Specifically, EADS also presented totally independent, but similar findings re the A350 two or three years ago.
Now , allow me to wander back, please, to the char that some have cited. All such claims are nonsensical, tendentious and worthless twaddle, to put it mildly. A "CHARRED" CFRP is no longer CFRP, it is merely a residue of strands of free CF, either woven or UD, but it is no longer a composite structure capable of meeting any design loads or criteria at all. So 'char' is a silly diversionary tactic , pure and simple,with the emphasis on simple to my mind and has nothing whatsoever to do with any structural load carrying ability or any chance to sustain any pressurisation loads. I hope that I see no more silly inputs re"char"and would note that the non-structural and useless char would be blown away by the 500 plus mph airflow.
Next, please let me outline briefly the relative CTE's regarding Aluminum and CFRP and epoxies. Let me work in old fashioned degrees F units, please. just to humour me.
Typically an aerospace alloy will have a CTE of 12-13 x10 to minus 6 inches/inch /degree F, and epoxy will have (in its pristine state epoxy only state it will have 22-23 in same units). But not in a CFRP composite whose CTE is controlled by the CF with a modulus of 40 MSI vs epoxy at a mere MSI of less than 1.
MSI matters, just as loads follow stiffness (same degree F units,of course). This differential generates internal stresses in the composite, but they are usually of a second order concern, but some of we composite folks like lower cure temperatures to minimise such internal and residual autoclave induced curing stresses. For CF itself, as a unidirectional material, it has an expansion close to zero (typically 1 or less re CTE units quoted, which is why it is employed for space mirrors encountering wide night/day temperature swings. This is also why INVAR is typically used as Boeing and other aerospace companies as tooling material to minimise problems involving thermal distortions. Boeing and others typically employ a quasi-isotropic layup, with, I note, emphasis on the quasi, as there are some significant differences between a true isotropic material and a quasi-isotropic material. Such CFRP structures typically have CTE's in F units of around 3-3.5. I hope that this clarifies and settles the CTE issues previously discussed in this thread.
And, prior to getting to my final points regarding thermal insulative and conductivity properties of aluminum alloys and CFRP's as I promised that the outset of this overly long input, let me give some relevant metallic vs. CFRP cross plied properties for others to mull over. CF is widely touted, and oft over-touted, regarding its strength, but let us be very careful here. A decent CFRP will have a tensile composite strength, if a UD material at 60 % fiber volume, of around 280- 320 KSI,( note, this is the sigma 1-1 equivalent to metallics).
A quasi-isotropic composite in the same CFRP will have a tensile value of around 90-120 KSI. Now, let us look a couple of weaknesses without boring you all with the hygroscopic nature of that nasty epoxy.
The shear strength of metallics is typically 60% of the tensile, so for a decent steel, for example, say in the 240 ksi range we will have a shear strength of around 145 KSI. Now look at composites and up pops that nasty epoxy again. Whereas Steel will have a SBS or ILSS strength of 144 KSI, a quasi-isotropic composite will have an ILSS of 8-10 KSI static on a good day and, if we allow for fatigue et al, we are down in the 4 KSI area. And finally another key nasty is the short transverse tensile strength ( equivalent to metallic sigma 3-3), which is entirely epoxy dependent and no CF failure is involved, there we find on a good day around 3-4 KSI with fatigue knocking that down to around 1-1.5 KSI which is close enough to zero in this composite engineer's mind and no tensile Sigma3-3 asallowed by an decent and competent comosites engineer.
Achilles only had one heel , lucky for him, but for we in composite design and analysis,there are plenty of other heels to fret about. This is not an anti-composite rant of any ilk, but rather to emphasise why composites need to be analysed far differently and much more closely than most metallics Further, large scale repairs become much more demanding and possibly questionable in contrast to the blithe assertions of some posters on this board. And, as a final point,why has nobody in the composites community,after a full half a century of designing and using CFRP, has yet developed "A'" basis allowables for composites as is so common for metallics? Why and we are still operating with "B" basis allowables with significantly lower probabilityand confidence levels?
Now, belatedly back to the original question that I was requested to respond to and, perforce, my final zinger for today for those seeking to minimise the magnitude of the 787's inherent epoxy based FST et al difficulties.
Yes,CFRP is indeed an thermal insulator compared with aluminum alloys, but also exhibits significant differences depending upon fiber orientation and layup. This, in turn, leads fires, heat and the like to be concentrated in one local area rather than with aluminum's high thermal conductivity, which lowers and significantly lowers and dampens peak temperatures in event of fires from whatever the source.
I trust that this helps the ongoing discussion.
And now I promised you a zinger and here it is; for all proclaiming and advocating the efficacy and safety and non -flammable nature of CFRP, let me issue this very simple challenge.Go out and buy (or borrow from your missus or similar)some aluminum frying pan. Then please cook something requiring 600-650 degrees F on a gas stove. Next, just to humour me, reproduce that same test, but this time first build a CFRP frying pan of similar size and construction and cook the same food with 600-650 degrees F gas cooker. There is only one stipulation, as I do not want you or your loved ones or pets to suffer or die in such a foolish attempt, please do this test of your cooking skills on an outdoor barbicue well away from all humans and pets and let me know the results. In closing we all know, I would hope, why there are no CFRP cooking utensils on the market and,just as a clue,it is zero to do with cost and all the do with flammability and FST.
Any takers, 787 defenders and "it only chars and is better than aluminum" folks, please put your frying pan where your mouth is?
Cheers and apologise for such a long post, but I hope is deemed of some value.

[Walnut]

Amicus's post is very interesting, I note the Asiana 777 crash at SFO burnt through its upper fuselage, but not the lower, I wonder whether that a/c (777) has any upper insulation.? If composites burn more readily then maybe 787's should?