A Boeing 787-8 Dreamliner sustained substantial damage in a ground fire while parked at Stand 592 at London-Heathrow Airport (LHR), U.K. The airplane had arrived from Addis Ababa (ADD), Ethiopia as flight ET700 at 06:27 hours local time in the morning and was scheduled to depart as ET701 back to ADD at 21:10. After passenger and crew disembarkation, the aircraft was towed to Stand 592, next to a fire station along taxiway E. Ground power was switched off. Approximately at 16:34 an employee in the air traffic control tower noticed smoke emanating from the aircraft and activated the crash alarm. The Rescue and Fire Fighting Service (RFFS) arrived on scene at 16:35 and discharged water and foam onto the outside of the aircraft. One fire fighter removed the power umbilical cables from the aircraft as a precaution. Fire fighters equipped with breathing apparatus entered the aircraft at 15:37 via the L2 door and encountered thick smoke. As they moved to the rear of the aircraft the smoke became denser so they opened further cabin doors to clear the smoke. At the rear of the passenger cabin they observed indications of fire in a gap between two overhead luggage bins. They were unable to use a hose-reel as the gap was too small and discharged a handheld ‘Halon’ extinguisher through the gap, about 20 minutes after entering the cabin. This was ineffective, so they removed some ceiling panels to expose the area and to get better access. At this point a small amount of flame was visible. This was extinguished with several pulses of water spray from their hose-reel, about 25 minutes after entering the cabin. A thermal-imaging camera was used to identify affected areas requiring further cooling. Investigation showed that the fire was initiated by the uncontrolled release of stored energy from the lithium-metal battery in the ELT. The ELT battery wires, crossed and trapped under the battery compartment cover-plate, probably created a potential short-circuit current path which could allow a rapid discharge of the battery. Neither the cell-level nor battery-level safety features were able to prevent this single-cell failure, which then 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 directly 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 of this a slow-burning fire became established in the fuselage crown, which continued to propagate from the ELT location at a slow-rate, even after the energy from the battery thermal runaway was exhausted. The following causal factors were identified in the ground fire: a) A thermal runaway failure of the lithium manganese dioxide battery in the ELT resulted in the uncontrolled release of stored energy within the battery cells. b) The location and orientation of the ELT, and the compromised seal on the battery cover-plate, allowed the resulting hot gas, flames and battery decomposition products to impinge directly on the aircraft’s composite fuselage structure, providing sufficient thermal energy to initiate a fire in the rear fuselage crown. c) The resin in the composite material provided fuel for the fire, allowing a slow-burning fire to become established in the fuselage crown, which continued to propagate from the ELT location even after the energy from the battery thermal runaway was exhausted. d) The Navigation Radio System safety assessment conducted in support of the ELT certification, did not identify any ELT battery failure modes which could represent a hazard to the aircraft, and therefore these failure modes were not mitigated in the ELT design or the B787 ELT installation. The following factors most likely contributed to the thermal runaway of the ELT battery: a) The trapped ELT battery wires created a short-circuit condition, providing a current path for an unplanned discharge of the ELT battery. b) The ELT battery may have exhibited an unbalanced discharge response, resulting in the early depletion of a single cell which experienced a voltage reversal, leading to a thermal runaway failure. c) The Positive Temperature Coefficient (PTC) protective device in the battery did not provide the level of external short-circuit protection intended in the design. d) There was no evidence that the reset behaviour, and the implications of the variable switching point of the PTC, had been fully taken into account during the design of the ELT battery. e) The absence of cell segregation features in the battery or ELT design meant the single-cell thermal runaway failure was able to propagate rapidly to the remaining cells.