The Boeing 787 may get its wings back before safety investigators in the U.S. and Japan pinpoint the precise cause of lithium-ion battery failures on two separate aircraft earlier this year. Indeed, the precise cause may never be found, Boeing and safety regulators acknowledge, though the search for it continues.
The FAA on Jan. 16 grounded the 787 after the second of two incidents involving Li-ion battery fire and smoke. Neither incident—one on the ground at Boston’s Logan International Airport and the other in Japan while in flight—resulted in physical injury to passengers or crew. Financial damage of significant size is presumed for Boeing and its 787 airline customers.
Although no root cause of the incidents has been established, the FAA on April 19 approved the company’s battery design modifications (view an animation here), paving the way for a return to flight soon. The redesigned batteries must be retrofitted to all existing 787s and incorporated into new ones as they are assembled.
The FAA said it will stage teams of inspectors at modification locations to sign off on the retrofit installations. Although it did not say exactly when the 787 will be allowed to return to flight, the agency said it will issue, this week, instructions to operators for making the changes. A directive granting approval to put the jets back in the air will come in a Federal Register notice, the timing of which the FAA did not provide.
The fix assumes that the battery will not catch fire or otherwise fail but includes safeguards that would ensure 787 safety even if it does.
The same type of Li-ion cells (employing lithium cobalt oxide chemistry) that are believed to have caused the problems will continue to be used; their production at maker GS Yuasa’s plant in Japan has been under heavy scrutiny by investigators, and measures to enhance cell quality have been incorporated into the production process (it has been determined that the cells in both event planes came from the same batch). Improvements to other parts of the battery system are also part of the fix, certification testing on which was completed April 5.
Each 787 has two primary rechargeable Li-ion batteries. They are of the same design with identical part numbers and specifications (65 A·h at 32.2 V), but they serve different purposes. The “main” battery is located in the forward electronics equipment (EE) bay under the main floor near the front of the plane. It is used primarily to “power up” airplane systems before the engines have started and to perform other functions during towing and ground operations.
The APU (auxiliary power unit) battery is located in the aft EE bay under the main cabin floor just behind the wings. Its main function is to start the APU, which is a small turbine engine in the tail that supplies power for ground operations. The APU also starts four generators that in turn are used to start the plane’s two propulsion engines. It's the APU battery that failed.
In flight, the two Li-ion batteries serve mainly a backup role.
It's the APU battery that failed in the Boston incident. The main battery failed in the other incident.
Unresolved root causes
The case of the Boeing 787 would not be the first in which an aircraft is released for re-entry into service after a mishap of undetermined root cause, according to FAA spokesperson Laura Brown. As one example of precedence, she cited for SAE International’s Aerospace Engineering magazine the 1996 case of TWA flight 800, a Boeing 747 that exploded in mid-air off Long Island en route from JFK airport in New York to Rome. It was determined by the FAA’s investigative arm, the National Transportation Safety Board, that the center wing fuel tank exploded, causing the plane to plunge into the Atlantic Ocean and kill all aboard.
Brown said the ensuing investigation resulted in a long list of possible ways the explosion might have been ignited, and the FAA issued more than 100 airworthiness directives to address them. Not being 100% sure that those measures would be fullproof, however, the FAA issued another directive aimed at removing the combustible material in the fuel tank. The overarching solution—invented by an FAA engineer, Brown noted—involves an inerting system in which oxygen in the tank is replaced by nitrogen gas so that even if there is a spark in the tank, combustion cannot take place.
A similar scenario is seen in Boeing’s battery solution. Engineers reviewed a large number of potential battery faults in the original design, and in the aftermath of the January incidents they addressed an even larger field of possible faults. Every source is mitigated in the original design or in the proposed fix, the company says. But, realizing that it cannot claim 100% certainty that the battery will never fail, Boeing developed an overarching solution that involved development of a stainless steel battery enclosure that it says is strong and robust enough to contain the worst that could occur in a battery pack failure; safe flight and landing are ensured.
Asked at a March 15 Boeing technical briefing about his comfort level at allowing the 787 to return to flight without possibly ever knowing the root cause of the battery fire, Mike Sinnett, Boeing Vice President of Engineering and Chief Technology Officer, 787 Program, said: “There are many, many cases when we have a part fail on airplane and we don’t understand what the specific root cause is, and we make appropriate corrections to improve the part. This has served us well in the past. In the case of Logan and Takamatso [the airport where the Japan problem-flight landed], we may never get to the single root cause, but the process that we’ve applied to understanding what improvements can be made is the most robust process we’ve ever followed.”
A preliminary investigation by the NTSB found that the first incident, on the tarmac in Boston Jan. 7 after passengers and crew had deplaned Japan Airlines flight 008, had as its general cause a short circuit in one battery cell that propagated to the other seven. What has not yet been determined is the cause of the short within the first cell.
The agency said in a press release that “charred battery components indicated that the temperature inside the battery case exceeded 500°F.” Boeing claims there was no fire inside the battery case, describing what happened inside the battery box as “cell venting.” Citing the NTSB report, Sinnett noted that a ground service employee upon opening the aft electronics bay saw only two three-inch flames on the outside of the battery box. NTSB spokesman Peter Knudson told Aerospace Engineering that the agency has not ruled out the possibility that the inside of the battery case (which houses the cells, battery monitoring unit, and other components) caught fire.
The agency is not commenting on the investigation by the Japan Transport Safety Board into the All Nippon Airlines Jan. 15 incident. Sinnett said at the March 15 briefing that there was no fire (only smoke) in the ANA incident.
At Boeing’s March 15 briefing, Sinnett said more than 200,000 engineering-hours had been spent in addressing the battery problems. It was decided the same basic battery would continue to be used—albeit with improvements and safeguards.
For the 787 battery, four new or revised tests have been added to the cell-production process to more closely limit variability from one to the next. The production process now includes 10 distinct tests. Plus, each cell will go through more rigorous testing in the month following its manufacture, including a 14-day test during which readings of discharge rates are taken every hour. This new procedure started in early February. More than a dozen production acceptance tests must be completed for each battery.
Boeing, GS Yuasa, and Thales (supplier of the integrated power conversion system of which the battery is a part) have also decided to narrow the acceptable level of charge for the battery, both by lowering the highest charge allowed and raising the lower level allowed for discharge. Two pieces of equipment in the battery system—the battery monitoring unit and the charger—are being redesigned to the narrower definition. The battery charger will also be adapted to soften the charging cycle to put less stress on the battery during charging.
Changes inside the battery will help to reduce the chances of a battery fault developing and help to further isolate any fault that does occur so that it won’t cause issues with other parts of the battery. To better insulate the cells from one another and from the battery box, two kinds of insulation will be added. An electrical insulator is being wrapped around each battery cell to electrically isolate cells from each other and from the battery case, even in the event of a failure. Electrical and thermal insulation installed above, below, and between the cells will help keep the heat of the cells from impacting each other.
Wire sleeving and the wiring inside the battery are being upgraded to be more resistant to heat and chafing, and new fasteners will attach the metallic bars that connect the eight cells of the battery. These fasteners include a locking mechanism.
Finally, a set of changes is being made to the aluminum battery case that contains the battery cells and the battery management unit. Small holes at the bottom will allow moisture to drain away from the battery and larger holes on the sides will allow a failed battery to vent with less impact to other parts of the battery.
The battery case will sit in a new enclosure made of quarter-inch-thick stainless steel, isolating it from the rest of the equipment in the electrical equipment bays. It also will ensure there can be no fire inside the enclosure, thus adding another layer of protection to the battery system. The enclosure features a venting system with titanium tubing to carry vaporized electrolyte directly outside the airplane in case one or more of the cells do fail. The venting electrolyte entrains what little air is inside the enclosure to begin with, Sinnett noted.
New titanium fixtures are being installed in the electronics equipment bays to ensure the housing is properly supported.
“Our first lines of improvements, the manufacturing tests and operations improvements, significantly reduce the likelihood of a battery failure,” Sinnett said. “The second line of improvements, changes to the battery, helps stop an event and minimize the effect of a failure within the battery if it does occur. And the third line of improvements, the addition of the new enclosure, isolates the battery so that even if all the cells vent, there is no fire in the enclosure and there is no significant impact to the airplane.”