July 18, 2013
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.