Airlines should “adopt the design changes which will be prepared by Boeing and implement them as soon as possible”, says the JCAB report, dated 19 December.
The JCAB called for the battery redesign despite determining that two of the three protective layers from a May 2013 battery redesign worked as intended, and the safety of the aircraft was never at risk during the incident on 14 January 2014.
However, public concerns may still linger about the two cell venting incidents the year before that triggered a four-month grounding of the 787 fleet, an English translation of the JCAB report says.
“It is considered that, for keeping peace of mind for the passengers and the public people [sic] on the safety of B787 fleet, further improvements of the reliability for cells and battery system are necessary,” the report says.
The JCAB wants Boeing to assess all of the potential causes identified in three 787s and then “accelerate” making possible design changes, followed by an “early” certification process.
The JCAB’s call for a second battery redesign goes beyond the recommendations from two previous investigations about the 2013 battery incidents by the Japan Transportation Safety Board (JTSB) and the US National Transportation Safety Board (NTSB).
The report also pushes for more action than Boeing has so far acknowledged is necessary.
“We remain confident in the comprehensive improvements made to the 787 battery system and in the overall performance of the battery system and the safety of the airplane,” Boeing says in an emailed reply to questions. “We continue to work reliability improvements on the 787 and will do so throughout the span of the program.”
The 787 is designed with two lithium ion batteries made by Japan-based GS Yuasa. One battery in the aft electronic equipment (E/E) bay supplies power to start the auxiliary power unit. The other battery is located in the forward E/E bay and supports the avionics systems in case all six onboard power generators fail. Both batteries are composed of eight cells that deliver 32V nominally.
Despite having similar functions, the Boeing design contrasts significantly with the lithium ion batteries installed in the Airbus A350-900. The Airbus supplier, Saft, designed a system with four batteries, each composed of 14 cells delivering 25V nominally combined. Thus, the A350-900 uses more batteries, with less power demanded from each cell than the 787 system.
All three major investigations into the 787 battery failures have deduced that an internal short circuit occurred within a cell, but have not been able to isolate the cause of the short circuit. The reports have suggested several possible causes, including cold soaking, the formation of dendrites and manufacturing errors.
The first two battery failures on the 787 posed serious safety concerns. In both incidents, a single overheating cell vented, causing adjacent cells in the tightly packed battery box to fail and partly melting the aluminum enclosure.
In response, Boeing redesigned how the batteries were installed. More spacing and thermal shielding was added between each cell. The eight-cell pack was enclosed in a stainless steel case. If one or more cells overheated anyway, new ducts would vent the heat and smoke directly off board.
According to the JCAB, two of the three layers of protection – preventing a thermal runaway within the battery and damage to the rest of the aircraft – worked as Boeing intended. But the third cell venting incident showed that the first layer of protection – preventing a cell from overheating in the first place – failed, the JCAB report says.
(Stephen Trimble - FlightGlobal News)