Aviation investigators in the United Kingdom released a special bulletin today urging the emergency locator beacons in the Boeing 787 be disabled following a "fire event" at London's Heathrow Airport last week. More than 6,000 of the beacons have been produced by Honeywell International, and this is the first known "significant thermal event," according to the Air Accidents Investigation Branch.

In a statement, the AAIB recommends the Federal Aviation Administration in the United States "initiate action for making inert" the device. It added a further review should be completed.

"It is recommended that the Federal Aviation Administration, in association with other regulatory authorities, conduct a safety review of installations of Lithium-powered Emergency Locator Transmitter systems in other aircraft types and, where appropriate, initiate airworthiness action."

Honeywell's emergency locator beacon (orange) used in the Boeing 787.

The locator beacons are used in all airliners, as well as many other aircraft. The models used in the 787 are roughly the size of a toaster and weigh nearly seven pounds. The ELT sends out signals on multiple frequencies that can be received via satellite, or by other aircraft and search personnel. They are designed to send out the signal after a crash, and can be used to locate the airplane. The ELT used in the 787 has a lithium-manganese dioxide battery as its power source, it is completely independent of the aircraft's own electrical system. The AAIB said examination of the ELT showed indications of disruption to the battery cells. But the agency added it is not yet clear if the combustion in the 787 was started by the batteries, or by something such as an external short, damaging the batteries afterwards.

Both Honeywell and Boeing say they support the recommendations from the AAIB, with the airplane maker calling them "reasonable precautionary measures to take as the investigation proceeds." The recommendation to deactivate the devices comes as investigators continue to try and determine whether the fire originated with the ELT, or if it was started by some other source.

The AAIB says there was significant heat damage to the upper part of the fuselage just in front of the tail. There was damage to the composite structure in the area where the 787's ELT is located. The airplane was parked at Heathrow and nobody was on board the airplane at the time of the fire. A ground crew agent had turned off power to the airplane after being instructed to do so by one of the airplane's crew members. The power was turned off, but the airplane was left plugged into the cord that provides power according to the investigators.

"The engineer visually confirmed on the flight deck that ground power was no longer available," the AAIB says.

Roughly 10 hours later, a person in the air traffic control tower noticed smoke coming from the airplane and fire fighters were sent to the scene. Once in the airplane, the fire fighters opened the doors to clear the smoke, and saw "indications of fire above the ceiling panels," inside the airplane. After attempting to fight the fire with hand-held halon extinguishers, they removed the ceiling panels and used water from hoses to extinguish the flames.

The incident occurred on the same Ethiopian Airlines 787 that was used to return the Dreamliner back to passenger service in April. The 787 had been grounded for about three months by the FAA after a pair of incidents involving the airplane's own lithium-cobalt batteries led to a fire on one airplane in Boston, and a separate thermal event on another 787 in Japan. Boeing designed a fix for the pair of 63-pound batteries including a redesign of the battery cells, and a containment and venting system to secure the batteries in the airplane.

The lithium-manganese batteries used in the ELT are much smaller than the airplane's main batteries.

Both companies are working with investigators to try and determine the source of the fire.