Last week’s fire in an unoccupied Boeing Co. (NYSE: BA) 787 Dreamliner at London’s Heathrow airport apparently has been traced to a malfunction in the aircraft’s emergency locator transmitter. The U.K.’s Air Accidents Investigation Branch (AAIB) is recommending that all 787 Dreamliners have the transmitters disabled by removing the lithium manganese dioxide battery that power the devices.

The AAIB does not know whether the fire was caused by combustion in the device or an external event such as a short circuit, according to a report in the Financial Times. The AAIB did say that a combustion event in this type of product is “extremely rare.” The emergency locator transmitter is made for Boeing by Honeywell International Inc. (NYSE: HON).

The AAIB also noted that the location of the device in the upper portion of the fuselage at the rear of the plane may not have been as benign had it happened while the plane was in the air:

However, large transport aircraft do not typically carry the means of fire detection or suppression in the space above the cabin ceilings and 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.

Honeywell has manufactured some 6,000 of these transmitters, and this has been the only time one has suffered what the AAIB called “a significant thermal event.” The same device is used by Airbus and Bombardier, although it is not known for certain whether all use lithium batteries.

The U.K. investigators said there is no evidence that the fire was caused by the lithium-ion batteries that Boeing recently replaced on the Dreamliners after fires earlier this year.

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