The U.K. Air Accidents Investigation Branch (AAIB) is asking the FAA to turn off the Honeywell emergency locator transmitters (ELTs) in all Boeing 787s “until appropriate airworthiness actions can be completed.”

The action comes as the AAIB continues to investigate a July 12 “fire event” onboard an Ethiopian Airlines 787-8 that was unoccupied and unpowered on Stand 592 at London Heathrow Airport.

“The initial technical investigation confirmed extensive heat damage in the upper portion of the rear fuselage, with significant thermal effects on aircraft insulation and structure,” says the AAIB in a special bulletin released today.

Investigators determined that the most severe heat damage and highest temperatures were close to the crown of the fuselage on the left side of the aircraft, which coincides with the location of the aircraft’s ELT and its wiring that is mounted internally on structure close to the aircraft skin.

“There are no other aircraft systems in the vicinity which, with the aircraft unpowered, contain the stored energy capable of initiating a fire in the area of heat damage,” says the AAIB.

The 787 has two 6.6 lb. Honeywell RESCU 406 AFN ELTs located in the main passenger cabin, one forward by the forward lavatory, aft of the flight deck bulkhead, the second in the aft cabin, above the galley outboard storage area.

The units contain an internal electronics box and external antenna and are powered by five lithium-manganese dioxide cells, a different chemistry from the lithium-cobalt batteries used in the main and auxiliary power unit (APU) batteries that caused a fleet grounding earlier this year.

An ELT automatically transmits distress signals on 121.5 MHz, 243MHz and 406MHz if switched on or if triggered by deceleration forces indicative of a crash. The ELT also has an option for including GPS position information in the output data packet.

The AAIB says a detailed examination of the ELT revealed “some indications of disruption to the battery cells,” but adds that it is not clear 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,” which could have provided a source of ignition for the batteries.

Honeywell, a member of the investigation team, says the RESCU 406 was certified in 2005 and is flying on “a number of” other aircraft models in addition to the 787. “We’ve not seen nor experienced a single reported issue on this product-line,” the company has previously said. The AAIB says Honeywell has produced “some 6,000 units of this design” for a variety of aircraft types, but that the July 12 incident is “the only significant thermal event” experienced.

The AAIB is also calling on the FAA and other regulators to conduct safety reviews on installations of lithium-powered ELTs on other aircraft.

“Large transport aircraft do not typically carry the means of fire detection and suppression in the space above 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,” the AAIB says.

The AAIB notes that turning off the aircraft’s ELT “does not mean that a plane is ‘invisible’ as planes also have portable ELTs (for example on dinghies if they crash at sea).”

Honeywell in a statement, says, “As a safety-first focused company, we support the AAIB’s proposal and will offer assistance to Boeing and the airlines if needed. The investigation continues, and it’s premature to jump to conclusions. Temporarily addressing the ELTs on Boeing 787s as a precautionary measure is prudent. The Boeing 787 ELT product action is a straightforward process, and we do not anticipate any material financial impact to Honeywell. We also support conducting safety reviews for installations of any lithium battery-powered ELTs from the variety of manufacturers who sell them.”

In a separate statement, Boeing notes, “As a party to the investigation, Boeing supports the two recommendations from the AAIB, which we think are reasonable precautionary measures to take as the investigation proceeds. We are working proactively to support the regulatory authorities in taking appropriate action in response to these recommendations, in coordination with our customers, suppliers, and other commercial airplane manufacturers.

“We are confident the 787 is safe and we stand behind its overall integrity,” Boeing adds.

[Ed note: Story updated to include Honeywell’s comment]