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PDR
User Rank
Iron
Reliability 101...
PDR   4/23/2013 12:56:46 PM
NO RATINGS
Lots of misunderstandings here (the dangers of a little knowledge, and all that jazz). Firstly the certification data does not cite the RELIABILITY of the battery system, it cites the HAZARD RATE of the battery system - the instantaneous probability of failure at any moment in time (which is why it's quoted in a "per flying hour" figure rather than an MTBF). These are very different characteristics and cannot be related to eachother directly.

Secondly two arrisings in 52,000 hours does not mean that the MTBF is 26,000 hours; it means that the MTBF probably lies somewhere in the range 1,200 hours to 10^10 hours. Assuming the batteries had been through burn-in/PRAT programmes (which it's rather difficult to NOT do for aircraft equipment) then they'd be in the "random" or (as often misnamed) "constant failure rate" region of the bathtub curve. In this region it's the spaces between the failures that are random, and you need a decent population of samples to draw any conclusions about failure rates. Two is far too small a number  from which to deduce ANYTHING.

Finally there's comment about the "driving the nail through test" as if it was the only test performed. Again, I don't know about this specific system but I *do* have experience of qualifying equipment onto aeroplanes, and I would be very surprised if the battery system didn't get exactly the same set of basic qualification tests as every other significant part of the avionics - thermal, vibration, shock, bump, humidity, mould etc etc.

All batteries re dangerous because they store energy, and if the energy gets out through an unintended back door that leads to "bad stuff"(tm). Aircraft batteries tend to be big, and so there is a lot of stored energy, that's all. Now the only difference between the LithiumCobalt (as these are) technology and the previous Nickel or lead-based technologies is that the Lithium cells have a self-oxidising electrolyte that will burn in the absence of air, so it's difficult to extinguish. So the trick is to stop them getting hot in the first place. For this reason all non-trivial lithium charging systems use single-cell charging techniques (monitoring the voltage of each cell throughout the charge). This is proven technology, and we really need to wait to find out what the ACTUAL failure mode was, rether than getting hysterical about the subject before we know what really happened.

£0.02 supplied,

PDR

g_ost
User Rank
Gold
Re: Troubling news
g_ost   3/15/2013 4:23:43 AM
NO RATINGS
I'm(was) an aircraft design engineer. In my old times we didn't implement systems not able to work at -50 deg C. the planes are not flying or landing only in California....

Charles Murray
User Rank
Blogger
Re: Troubling news
Charles Murray   3/14/2013 2:37:20 PM
NO RATINGS
You (and the NY Times) are correct, ragnar175. Boeing worked with the FAA on the approval.

Arby Bernt
User Rank
Iron
No Mechanical Containment!
Arby Bernt   3/14/2013 2:20:34 PM
NO RATINGS
I cannot help but wonder how much experience the battery box engineers have with lithium cells. Without mechanical endplates and tiebars, the batteries will physically swell at about 90SOC, gradually fracturing the internal structure.

Eventually they will short, typically during the end of the charge cycle. This feature is well known, yet completely lacking in the sheetmetal Boeing housing. 

Maybe change the name to "Bowing"...

 

Arby

RAGNAR175
User Rank
Iron
Re: Troubling news
RAGNAR175   3/14/2013 1:56:24 PM
NO RATINGS
In that same NYT article it stated that the FAA is working with Boeing on a re-design so maybe they will get it right this time.

loadster
User Rank
Gold
Re: Troubling news
loadster   3/14/2013 1:22:54 PM
NO RATINGS
Easy there, ghost rider. The nature of a blog is armchair quarterbacking. If the premise is that the lithium battery suffers inherent dendrite growth, then you've answered the mail, the chemistry is not suitable for aircraft safety. My premise was something wasn't passively managed properly external to the battery. Charge rate, temperature, discharge, short circuit or load test. The main premise here is not to bang the square error into the historical round past failure mode. It is more about questioning whether the solution is to properly compartmentalize the potential for fire or prevent a runaway condition or detect and countermand a defective battery when bad conditions can be sensed. Even those simple nicad packs on power drills have basic circuits that detect when one cell has reached max charge condition. I think the Boeing/Yuasa course will work in the short run to get dreamliner out of money black-hole limbo. Those batteries will be the best maintained in the fleet, while something else smolders and degrades. And I don't think this is about incompetence or deliberate malfeasance, either. Flying is a balancing act, between money, time and safety. But most birds don't think about that.

Elizabeth M
User Rank
Blogger
Re: Troubling news
Elizabeth M   3/14/2013 12:48:32 PM
NO RATINGS
Ha, well the NY Times usually reports things quite accurately (give or take a few missteps along the way). Well then the FAA was asleep at the wheel, too! But you're right, both Boeing and the FAA must shoulder the blame for this and hopefully won't make the same mistake again.

RAGNAR175
User Rank
Iron
Re: Troubling news
RAGNAR175   3/14/2013 12:37:18 PM
NO RATINGS
From what I've read in the past the FAA approved the design of the batteries before production.  However I read this in the New York Times so might not be totally accurate. If true Boeing can't get all of the blame. But I agree with you I would expect more from both groups.

melllowfelllow
User Rank
Gold
Where's the beef??
melllowfelllow   3/14/2013 12:08:15 PM
I am not a member of the psychic community so I will not pretend to know the cause of the fire.  I am troubled by the information that is missing from reports that I have read.  If I was in the position of determining reliability, I would have decades of historical data that could be used directly, or by extrapolation, for most critical components - except for the batteries.  A casual criticality analysis would also point to the batteries as a potential danger area.  This being the case, I would cover my butt with a barrage of different empirical testing and ensure adequate monitoring was in place.  The bottom line is that there is circumstantial evidence that adequate testing was not performed, internal Boeing review processes did not catch it, and the Feds went along with everything.

The major item of concern for me is that some number of Boeing engineers 'touched' this project without a serious level of concern.  IMO, the "we drove a nail" test should have been plural and emphasized - "We ran 37[?] long term tests involving shorts, vibration, load changes,...".  During my career I have observed that companies in defensive mode after a disaster are usually determined to deflect responsibility by doing a data dump of techno-babble like "we calculated that this would last forever based on manufacturer's data and we confirmed this by running 5,000 different tests that simulated 600 years of operation, blah, blah,...".

The simple question in my mind is "Are these failures due to cover up or incompetence"??

6lz



 

ScotCan
User Rank
Platinum
Re: Troubling news
ScotCan   3/14/2013 11:23:06 AM
Interestingly dendrites have cropped up as the cause of the above system's failure. In the case of Cell#6 in the Boeing cluster an internal short was indicated in earlier posts. If this was caused by dendrite growth inside the cell then any evidence of that would be completely destroyed.

Dendrite growth leads to soft shorts to begin with, then as time progresses the condition worsens and if the cell is in a battery cluster, the other cells overwhelm the faulty cell to an extent where temperatures rise.

In Duracell we could detect zinc dendrite growth by x-raying the cell...it would show as a blurred outline penetrating the separator.

There are several research papers on the subject of dendrite growth in Lithium systems

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