Friday, March 15, 2013

Why the 787 battery fix will work

Why the 787 battery fix will work:
Boeing’s extensive and detailed briefing from Tokyo on the 787 battery redesign finished a few hours ago and, for the first time since this business began with the initial JAL fire in January, I have had a glimpse into why the company has been so quietly confident in its updated system.
There is light at the end of the tunnel for beleaguered 787 operators and, no, I don’t think it is an oncoming train. The reason? The briefing by Vice President and 787 Chief Project Engineer Mike Sinnett answered one nagging, fundamental concern that many have expressed over the little that was ‘known’ about the design. From very scant information, observers had gleaned that Boeing’s redesign philosophy was to essentially mitigate the potential of a lithium ion battery failure – the most obvious expression of this being a large fire proof box in which the battery will be enclosed.
But would the FAA ever contemplate certification of a system which – in a worst case scenario involving a battery fire – ultimately depends on containing a blaze onboard an aircraft? The answer, as we now know, is that the FAA will not have to make that decision because the box, or enclosure, is designed to prevent a fire rather than ‘contain’ it.

The 'enclosure' (Boeing)
The solid enclosure “has a very small amount of air in it” says Sinnett. In lab tests in which individual cells in the eight-cell GS Yuasa 32V battery have been deliberately failed, the vaporized electrolyte that spews out pushes open a pressure disc “in about 1.5 seconds,” he adds. The vapor passes out through vent tubes into the atmosphere, whether at 35,000 ft or at ground level on the ramp.
Even when the electrolytes oxidize “they don’t release sufficient oxygen to materially contribute to combustion,” Sinnett says. A battery failure was induced in an enclosure in the test lab by exposing it to heat transmitted through the casing. “In no cases were we able to ignite electrolytes coming out of the cell,” he adds. The test team then drew oxygen back into the enclosure. “There was a small amount of combustion for 200 milliseconds and it went out again.”

Enclosure and vent line to atmosphere (Boeing)
By sealing off the battery from the outside, the enclosure starves the fire of air before it can even get started. The bottom line is the enclosure appears to provide not only the ultimate safeguard should a failure occur, but also an additional active means of preventing one in the first place.
Other changes, probably more germane to the still unknown original cause of the failures, include revised chargers which stay within a tighter voltage range – neither over or under-charging the battery. The internal configuration of the battery is also revised with spacers and insulation for improved thermal and electrical isolation, as well as with better wire harnesses to reduce the chances of short circuiting.

The revised 'gentler' charger (Boeing)
What seems to speak volumes about Boeing’s confidence in the redesign is the fact that just one test flight is currently deemed sufficient to verify the improvements. Of course there are also hundreds, possibly thousands, of additional ground test hours still lying ahead of the Boeing battery team. There is also the possibility of additional test flights if not everything goes to plan – a situation that the 787 development team has become all-too-familiar with over the years.

The ultimate irony in this elaborate fix is the 150 lb of the hefty enclosure eliminates much of the weight benefit of using lithium ion batteries in the first place. All the more reason then for Boeing to ensure this fix works, so that the other operational and supposed maintenance benefits of this technology can finally be brought to bear. Click HERE to read a report from the press comference.

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