The British Air Accidents Investigation Branch issued a safety bulletin (PDF) faulting the aircraft’s Emergency Location Transponder for the fire.
The ELT, which is built by Honeywell, is designed to broadcast an aircraft’s location even in the event of a main power failure, and in the case of the 787 is above the cabin ceiling, close to the exterior skin of the aircraft.
The ELT seems most likely at fault because of its lithium-ion battery, which is designed to give the system power even if the plane’s main power gets cut off. According to the bulletin: “There are no other aircraft systems in this vicinity which, with the aircraft unpowered, contain stored energy capable of initiating a fire in the area of heat damage.”
The report recommends that the ELTs in every 787 be made “inert” until Honeywell can come up with a fix. While the AAIB noted that the ELT hasn’t shown a track record of spontaneously combusting, they said that because a flight crew isn’t well equipped to deal with a fire above the cabin ceiling, the devices should be disabled to avoid that risk.
Honeywell issued a statement stressing that the investigation was not complete, but that it will will offer support to Boeing and other parties.
“As a safety-first focused company, we support the AAIB’s proposal and will offer assistance to Boeing and the airlines if needed. The investigation continues, and it’s premature to jump to conclusions. Temporarily addressing the ELTs on Boeing 787s as a precautionary measure is prudent. The Boeing 787 ELT product action is a straightforward process, and we do not anticipate any material financial impact to Honeywell. We also support conducting safety reviews for installations of any lithium battery-powered ELTs from the variety of manufacturers who sell them.”
Boeing issued a statement calling the actions proposed by the AAIB “reasonable,” and reaffirming its confidence in the 787.
As a party to the investigation, Boeing supports the two recommendations from the UK Air Accidents Investigation Branch (AAIB), which we think are reasonable precautionary measures to take as the investigation proceeds. We are working proactively to support the regulatory authorities in taking appropriate action in response to these recommendations, in coordination with our customers, suppliers, and other commercial airplane manufacturers.
We are confident the 787 is safe and we stand behind its overall integrity.