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Boeing is to consider changes to the design of certain systems on its flagship 787 Dreamliner after the publication of an accident report into a battery fire on board one of the planes at Heathrow airport in 2013.
The Air Accidents Investigation Branch report found that the fire was caused by the plane’s lithium battery-powered emergency locator transmitter (ELT), the homing beacon used to find missing aircraft. The report made 14 recommendations, including action for “making inert the Honeywell International RESCU 406AFN fixed emergency locator transmitter system in Boeing 787 aircraft until appropriate airworthiness actions can be completed”.
Boeing said the report was “extensive and thorough”, and that it would carefully review the recommendations. “It is important that any potential changes to the airplane’s design be reviewed with great care, and with due consideration for any potential unintended consequences. We will work closely with regulators, operators and industry partners to ensure that the safety of commercial jetliners continues at the highest levels.”
The fire occurred on an unoccupied Boeing 787-8 on stand at Heathrow in July 2013. The aircraft suffered extensive heat damage in the upper portion of the rear fuselage, in an area coincident with the location of the ELT.
The fire was found to have been initiated by the uncontrolled release of stored energy from the lithium-metal battery in the ELT. It was identified early in the investigation that ELT battery wires, crossed and trapped under the battery compartment cover plate, probably created a short-circuit current path which could allow a rapid, uncontrolled discharge of the battery. Root cause testing confirmed this latent fault as the most likely cause of the ELT battery fire, most probably in combination with the early depletion of a single cell.
Neither the cell-level nor battery-level safety features prevented this single-cell failure, which propagated to adjacent cells, resulting in a cascading thermal runaway, rupture of the cells and consequent release of smoke, fire and flammable electrolyte.
The trapped battery wires compromised the environmental seal between the battery cover plate and the ELT, providing a path for flames and battery decomposition products to escape from the ELT. The flames impinged on the surrounding thermo-acoustic insulation blankets and on the composite aircraft structure in the immediate vicinity of the ELT. This elevated the temperature in the fuselage crown to the point where the resin in the composite material began to decompose, providing further fuel for the fire.
As a result, a slow-burning fire became established in the fuselage crown and this fire continued to propagate from the ELT location, even after the energy from the battery thermal event was exhausted.
Honeywell, the ELT manufacturer, has carried out several safety actions and is redesigning the ELT unit, taking into account the findings of the investigation.
Boeing and the US Federal Aviation Authority have also undertaken other safety actions around system design and certification.