British investigators confirmed in an interim report Thursday that the 787 fire at Heathrow on July 12 centered on a small electronic device, an Emergency Locator Transmitter (ELT), 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, damping fears that the fire was the result of some broader problem with the plane’s electrical systems.
The ELT, which sits in the fuselage crown just in front of the tail fin, transmits location data to satellites in the event of a crash.
The report from the U.K.’s Air Accidents Investigation Branch (AAIB) says 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.
After the control tower noticed smoke emanating from the plane, firefighters rushed to the aircraft and, entering through a passenger door, “encountered thick smoke” that became denser as they moved to the rear of the aircraft.
“At the rear of the passenger cabin, they observed indications of fire above the ceiling panels,” the report states.
They initially tried to extinguish the fire with a handheld halon fire extinguisher. When this proved ineffective, the firefighters ripped out a ceiling panel and doused the fire with water from hoses.
The AAIB 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 AAIB recommended that the U.S. Federal Aviation Administration (FAA) instruct airlines operating the 787 to disable the ELT boxes “until appropriate airworthiness actions can be completed.”
There are also separate, portable ELTs in the emergency slides that deploy as life rafts if the plane goes down in water.
AAIB spokesman Sam Bond said these are not covered by the safety recommendation and won’t have to be disabled.
The UK agency also suggested the FAA review the safety of the ELT devices in other aircraft types.
Boeing said it supports the AAIB’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 the aircraft was unpowered at the time the fire broke out. Although it was connected to a ground power source, this source was switched off.
Besides the ELT, the report concludes, “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.”
Boeing spokesman Marc Birtel said the ELT is powered entirely by its battery and that the only connection to the airplane is a wire to the flight deck to turn it on or off.
“It does not receive power from the airplane,” Birtel said.
Birtel also said that ”we have seen failures of other types of ELTs in the past that resulted in varying but lesser degrees of heat damage.”
A check of the FAA database that records in-service incidents affecting planes operated by U.S. airlines found 10 ELT incidents involving Boeing jets and four involving Airbus jets.
However, all of those incidents were minor. The ELTs malfunctioned or were broken and had to be replaced, but there was no mention of heat damage.
The report also seems to indicate that, despite what looked like gaping holes in the fuselage crown on TV pictures of the event, the fire did not in fact completely burn through the top of the airplane fuselage.
There is no mention in the description of damage to the aircraft of any holes. Instead, the report refers only to “blackened and peeling paint and damage to the composite structure.”
TV 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 — from the photos, an area approximately 8 feet long by 2 feet wide — is clearly badly damaged and will have to somehow be replaced.
Dominic Gates: (206) 464-2963 or firstname.lastname@example.org