Officials investigating the fire on a parked Ethiopian Airlines Boeing 787 are focused on how condensation and increased humidity could have sparked the blaze at London's Heathrow Airport last week, three sources familiar with the probe said.
British authorities said on Thursday an emergency beacon made by Honeywell International Inc was the likely source of the fire, and called for the device to be turned off. But the UK Air Accidents Investigation Branch (AAIB) said it was still trying to understand what sparked the fire.
The U.S. Federal Aviation Administration is close to taking steps to follow the AAIB recommendations.
The July 12 fire rekindled concerns in the industry about Boeing Co's advanced carbon-composite Dreamliner, which was grounded for more three months earlier this year after two incidents involving overheated lithium-ion batteries. The AAIB said the London fire is not related to those batteries.
Investigators are still trying to determine if condensation on the plane may have seeped into the Honeywell emergency locator transmitter (ELT), triggering a short circuit in the unit's battery, which is made by Ultralife Corp, according to the sources, who were not authorized to speak publicly because the probe is still ongoing.
Condensation is normal on big airliners, but the 787 has a higher level of humidity to make passengers more comfortable. The Dreamliner is pressurized at about 6,000 feet, compared to 8,000 for most other airliners.
Water conducts electricity, so high moisture levels could raise the likelihood of short circuits. Long term exposure to moisture can cause corrosion on electrical wires and batteries.
The 787 carries humidity controls made by CTT Systems AB of Sweden. The company had no immediate comment. CTT's website said its system controls condensation on aircraft, including the 787, and is in use on other aircraft.
Officials are also looking at the placement of the emergency locator transmitter, which is bolted onto a bracket attached to the frame of the airplane - this is exactly where condensation tends to build up, one of the sources said.
"Condensation, humidity and installation - that's the focal point of the investigation," the source said.
The emergency beacon must pass a test to prove it can be submerged under one meter of water for one hour. It is unclear what effect any condensation buildup inside the plane would have on the device, which is encased in aluminum, and its battery, the source added.
Honeywell makes the beacon, which is not required by U.S. aviation regulations but 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 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 emergency beacons are powered by non-rechargeable, lithium-manganese batteries 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 the hot African sun for months, raising questions about whether that could have affected the battery in the locator beacon, said the source.
Thursday's AAIB report did not mention the condensation issue but said a detailed examination of the burned ELT and "the possible mechanisms for the initiation and sustaining of the fire" were continuing. It also noted that there was no equipment to detect or suppress fire in the area where the blaze broke out.
Two of the sources emphasized 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 question that investigators are wrestling with is whether there is enough insulation to prevent moisture from condensing and short circuiting systems such as the beacon, one of the sources said.
Another 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 power batteries involved in the earlier fire and overheating to melt down. 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.