Investigators looking into the emergency beacon fire which damaged an Ethiopian Airlines Boeing 787 on July 12 are focusing on whether a pinched wire in the unit could have sparked a short circuit, igniting the battery.

The new suspect in the search for a cause of the fire, which erupted in one of the 787’s emergency locator transmitters (ELT) while the aircraft was parked at London Heathrow Airport, emerged from forensic analysis of the Honeywell-built unit by the UK Air Accident Investigation Branch (AAIB). According to sources close to the investigation the remnants of the RESCU 406AFN showed evidence of a kink in the wiring which was likely related to its original assembly rather than something that could have occurred during its installation in the airframe.

However other potential causes, including moisture build up, have not been ruled out. The FAA, which is preparing to issue an airworthiness directive calling for mandatory inspections of the ELT, says “these inspections would ask operators to inspect for proper wire routing and any signs of wire damage or pinching, as well as inspect the battery compartment for unusual signs of heating or moisture.”

Although the AAIB recommended temporary de-activation of ELTS, the FAA says it will only require mandatory inspections. As of July 20, Boeing was not certain whether the European Aviation Safety Agency would follow the suggestion of the AAIB or adopt the same line as the FAA.

On July 18, the AAIB said the ELT should be made inert “until appropriate airworthiness actions can be completed.” The AAIB added that it was 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. In the case of an electrical short, the same batteries could provide the energy for an ignition and suffer damage in the subsequent fire.”

The 787 is fitted with two 6.6 lb. Honeywell RESCU 406AFN 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 battery cells, a different chemistry from the lithium-cobalt batteries used in the main and auxiliary power unit (APU) batteries that caused a fleet-wide grounding earlier this year.