British investigators confirmed in an interim report Thursday that the Boeing 787 fire at Heathrow on July 12 centered on a small electronic device, an emergency locator transmitter, and have ordered the devices disabled on all 787s pending further investigation.
The focus on a rare malfunction of this device appears to be good news for Boeing Co., damping fears that the fire was the result of some broader problem with the plane’s electrical systems.
The emergency locator transmitter, or ELT, which sits in the fuselage crown just in front of the tailfin, transmits location data to satellites in the event of a crash.
The report from the U.K.’s Air Accidents Investigation Branch states that Honeywell, which supplies the ELT, has produced some 6,000 units of the same design. They are fitted to a wide range of aircraft, and the 787 fire has been “the only significant thermal event” to date.
Even so, the report notes that because commercial airliners “do not typically carry the means of fire detection or suppression in the space above the cabin ceilings … had this event occurred in flight, it could pose a significant safety concern and raise challenges for the cabin crew in tackling the resulting fire.”
The Air Accidents Investigation Branch has therefore issued an official safety recommendation asking the U.S. Federal Aviation Administration to instruct airlines operating the 787 to “initiate action for making inert the ELT boxes … until appropriate airworthiness actions can be completed.”
The FAA is also asked to conduct a “safety review” of the ELT devices in other aircraft types besides the 787.
Boeing said it supports the British agency’s recommendations, calling them “reasonable precautionary measures to take as the investigation proceeds.”
The ELT boxes contain a set of small lithium-manganese batteries.
“Detailed examination of the ELT has shown some indications of disruption to the battery cells,” the report states. “It is not clear, however, whether the combustion in the area of the ELT was initiated by a release of energy within the batteries or by an external mechanism such as an electrical short.
“In the case of an electrical short, the same batteries could provide the energy for an ignition and suffer damage in the subsequent fire.”
The report states that besides the ELT, “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 also seems to indicate that, despite what looked like gaping holes in the fuselage crown in pictures of the event, the fire did not completely burn through the top of the airplane fuselage.
There is no mention of holes in the description of damage to the aircraft. Instead, the report refers only to “blackened and peeling paint and damage to the composite structure.”
Media pictures showed clearly the pattern of frames and stiffeners showing beneath the skin, but it may be that these were visible only as areas of varying scorch marks rather than through a hole in the skin.
Nevertheless, the composite skin above the fire is clearly badly damaged, and will have to be replaced.