Officials investigating the fire on an Ethiopian Airlines 787 in London last week are focused on how condensation in the plane and a possible pinched wire in an emergency beacon may have sparked the blaze, according to people familiar with the probe.
The US Federal Aviation Administration said on Friday it will call for inspections of the beacons made by Honeywell on Boeing Co 787 jetliners, but stopped short of requiring airlines to disable or remove the devices, as British authorities investigating the fire had recommended.
The FAA said inspections should ensure wires are properly routed, and should look for pinched wires or signs of unusual moisture or heat. It gave no further details on how those factors may have contributed to the fire.
But one source close to the inquiry told Reuters that investigators had found a pinched wire in the casing of the emergency locator transmitter (ELT) aboard the aircraft.
The news comes after the British Air Accidents Investigation Branch (AAIB) on Thursday said the Honeywell beacon was the likely source of the fire, but said it was still trying to understand what ignited the plane.
The July 12 fire rekindled concern in the industry about Boeing's advanced carbon-composite Dreamliner, which was grounded for more three months this year after two incidents involving overheated lithium-ion batteries. The AAIB said the London fire was not related to those batteries.
The Honeywell ELT is delivered fully assembled and is installed by Boeing. The unit that was involved in the fire had not been opened, suggesting the pinched wire originated at the Honeywell plant, according to one person familiar with the investigation.
Honeywell declined to comment. Boeing declined to comment on the investigation but said it is working with airlines to either inspect or remove the beacons to meet regulatory guidelines.
Investigators also are trying to determine if condensation on the plane seeped into the ELT, triggering a short circuit in the unit's lithium-manganese battery, which is made by Ultralife Corp, according to people familiar with the investigation. The sources were not authorised to speak publicly because the probe is still going on.
Condensation is normal on all big airliners, but the 787 has a higher level of humidity for longer periods to make passengers more comfortable, about 15 per cent for the 787 compared with 4-5 per cent for conventional metal aircraft, Boeing said. The humidity can be much higher when any plane is on the ground with doors open, perhaps 95 per cent, because it matches the ambient air. At cruising altitude, however the air is dry and moisture comes mostly from passengers.
Water conducts electricity, and high moisture levels could raise the likelihood of short circuits. Long-term exposure to moisture can cause corrosion on electrical wires and batteries.
Boeing's chief 787 engineer, Mike Sinnett, told Reuters that the humidity controller on the 787, made by CTT Systems AB of Sweden, is designed to "dry out the crown" or upper fuselage, of the aircraft, and prevent moisture from accumulating. Without the system "we would wind up having that water stay in the insulation."
Sinnett said the aircraft is designed to combat corrosion, and the main purpose of the CTT system is to eliminate water because the water's weight cut's fuel efficiency.
He declined to comment on the ongoing AAIB probe, but said any investigation is "going to look at all potential avenues."
The AAIB also is looking at the placement of the ELT, which is bolted onto a bracket attached to the frame of the plane - exactly where condensation builds up, one of the sources said.
"Condensation, humidity and installation - that's the focal point of the investigation," the source said.
The ELT must pass a test to prove it can be submerged under one meter of water for one hour. It is unclear what effect any longer-term condensation buildup inside the plane would have on the device, which is encased in aluminium, and its battery, the source added.
The ELT is not required by US aviation regulations but is mandated by some countries.
Boeing spokesman Marc Birtel said all 68 787s in operation have the beacons because all its airline customers chose that option. Boeing installs the beacons in different areas of the plane, depending on an airline's preference and its home country's regulations.
Birtel this week said Boeing had no plans to switch suppliers and that the company will "continue to work with the investigators and regulators to devise acceptable mitigating action if required."
The non-rechargeable, lithium-manganese batteries in the ELT have been used for decades in products like digital cameras and pacemakers, because of their long life.
Last week's fire came less than three months after Ethiopian Airlines and others resumed flying the brand-new all composite plane, following the FAA grounding.
Investigators are also looking at what effect the long grounding may have had on the batteries and electrical systems used on the plane, said one of the sources.
The Ethiopian Airlines plane sat outside in Africa for months, raising questions about whether that could have affected the battery in the locator beacon, said the source.
Two of the sources emphasised that investigators were continuing a comprehensive review of a variety of components and issues. But they said the complex interaction of humidity and wiring on the plane was a clear focus.
One source, who is close to Boeing, said the 787 may need better isolation of electrical components.
Investigators have yet to determine what prompted the lithium-ion batteries involved in the earlier fire to overheat. Boeing resolved the issue by redesigning those batteries to better guard against heat buildup, encasing them in fireproof steel boxes and cutting a vent in the plane to dump smoke and heat away from passengers.