Boeing 787 fleet grounded indefinitely as investigators stumped
Short circuit may be culprit for flammable flights
Boeing's flagship fleet of 787 Dreamliner aircraft will be grounded for the foreseeable future after a preliminary report from the National Transportation Safety Board (NTSB) said the cause of two battery fires had yet to be found, and that Japanese investigators are similarly baffled.
"We have not ruled anything out as a potential factor in the battery fire; there are still many questions to be answered," NTSB chairman Deborah Hersman in a statement.
"One of these events alone is serious; two of them in close proximity, especially in an airplane model with only about 100,000 flight hours, underscores the importance of getting to the root cause of these incidents."
That's very bad news for Boeing, which needs to get the issue sorted out before buyers start cancelling orders. Boeing has been in crisis mode since an in-air fire on an All Nippon Airways flight in Japan and a subsequent ground fire in Boston caused the FAA to revoke the aircraft's flight certification in the US. All other 787 operators have grounded their aircraft.
"In order to ensure the integrity of the process and in adherence to international protocols that govern safety investigations, we are not permitted to comment directly on the ongoing investigations," said the company in a statement. "Boeing is eager to see both investigative groups continue their work and determine the cause of these events, and we support their thorough resolution."
The preliminary findings from the NTSB on the Boston fire report that the plane engineers noticed smoke and flames coming from the auxiliary power unit located under wings. Fire crews were on the scene in five minutes, but it took nearly two hours before the fire was reported as "controlled."
The aircraft's 63lb batteries under investigation by the NTSB consist of eight 3.7V cells, all of which were damaged in the incident. All have now been CT scanned for anomalies, and six so far have been forensically taken apart in search of the cause of the problem.
The batteries in the Boston flight have showed some evidence of a short circuit and indications of "thermal runaway" leading to a destructive chain reaction, the NTSB said. Initial tests at a facility in Arizona show no problems, while Japanese investigators report that their APU circuit boards are too badly burnt to provide meaningful data at this time.
The 787's batteries are a critical component in the aircraft's design. Boeing got rid of a lot of internal mechanics on the aircraft to save weight and boost fuel efficiency, and shifted to electronically controlled systems. In order to supply enough power, the aircraft depends on its battery packs located under the wings and cockpit.
To make matters worse, the 787 doesn't have fire-suppression systems in its electronics bays; just smoke detectors. The aircraft has an inert gas system in the cargo hold to counter fire, but the FAA decided similar systems weren't necessary for other areas of the aircraft.
Finding another battery supplier, or a different power source, would be a massive problem since the 787's batteries are a custom design Boeing outsourced to Japanese manufacturer GS Yuasa. Much of the 787's systems were provided by outsourcing partners, a decision Boeing said at the time would result in considerable costs savings.
The old adage of being penny-wise but pound foolish comes to mind. ®
Re: I'd look at the Battery Power Conditioning Circuits
I agree, mostly because I find it hard to believe a japanese company like GS Yuasa would make substandard battery cells.
Li-Ion at too low voltage or too high voltage will start producing metallic lithium (highly volatile), or starts dissolving the copper current collectors into the electrolyte. This dissolved copper then becomes metallic again when battery voltage is restored to safe levels, and the copper might have formed thin strands acting as shunts. Current across those can locally heat up the cell sufficiently to set off thermal runaway (iirc the required temperature is slightly above 100C). Think of it as chemically "growing" an ignition wire into the insides...
When having multiple cells, each cell must be individually monitored to stay within prescribed voltage limits. In order to not get the entire pack limited by the lowest voltage cell and highest voltage cell, you need some sort of system that either adds more charge to the lowest cell, or removes power from the highest cell. Removing from the highest cell is the most common practice. Then you need to design this balancing system so that it fails in a safe way. Many electrical vehicle hobbyists have been bitten by this, their electronics have failed and a single battery cell has been drained totally dead. Trying to either charge or discharge that pack will then most likely cause a fire.
The charging system needs feedback from this balancing system, so that the charging system doesn't charge faster than the balancing system can remove charge from the highest cell(s). Without that feedback, there'll be atleast one cell that gets overvolted for a brief period of time. The damage accumulates (you can't just say "oops, but it didnt blow up this time, so it's ok")
Same applies to discharge portion, must cease discharging when the lowest cell reaches lowest permissible voltage. Using the pack voltage for this purpose will lead to problems.
I've seen a surprising amount of highly skilled electrical and electronics engineers that have absolutely no clue about batteries, they would blow up a Li-Ion pack quite quickly with the battery management they'd design. Alot of EEs seem to treat batteries as some sort of black boxes that work as electronic fuel tanks and manage themselves. I guess it makes sense, they're electronics people not chemistry people.. Wouldn't surprise me if the problem can be traced back to the battery management system...
To my knowledge, Inert gas extinguishers, or anything else, will not help much in a lithium ion battery fire. As they burn they produce their own oxygen. With this kind of fire, you just try to keep anything else from igniting and wait for the fire to burn itself out. Note that it took two hours after the firefighters arrived until the fire was out.
I'd look at the Battery Power Conditioning Circuits
I'm an ex-fuel cell guy, but fuel cells and batteries are two sides of the same coin.Li ion battery's that have been through as much testing as these have, if they start thermally running away when put into service, almost always do so due to an interaction between the charge circuit and the battery resulting in the battery being charged/discharged too fast or overcharged. The cure might be as simple as lowering the maximum charge & discharge current density the power conditioning circuitry allows. Whether that is too low to power all of the entertainment systems, well...