S4/2014, B787-8, ET-AOP

3 AAIB Bulletin S4/2014 SPECIAL Farnborough House Berkshire Copse Road Aldershot, Hants GU11 2HH Tel: 01252 510300 Fax: 01252 376999 www.aaib.gov.uk This Special Bulletin contains facts which have been determined up to the time of issue. It is published to inform the aviation industry and the public of the general circumstances of accidents and serious incidents and should be regarded as tentative and subject to alteration or correction if additional evidence becomes available. © Crown copyright 2014 AAIB Air Accidents Investigation Branch SERIOUS INCIDENT Aircraft Type and registration: B787-8, ET-AOP No & Type of Engines: 2 General Electric GEnx-1B turbofan engines Year of Manufacture: 2012 (MSN 34744) Location: London Heathrow Airport Date & Time (UTC): 12 July 2013 at 1534 hrs Type of Flight: Not applicable Persons on Board: None Injuries: Not applicable Nature of Damage: Extensive heat damage in upper portion of rear fuselage Information Source: AAIB Field Investigation Introduction This Special Bulletin contains information on the progress of the investigation into a ground fire on an unoccupied Boeing 787-8, registration ET-AOP, at London Heathrow Airport on 12 July 2013. It follows the publication of Special Bulletin S5/2013 on 18 July 2013. The AAIB are assisted in the investigation by Accredited Representatives from the National Transportation Safety Board (NTSB) (representing the State of Design and Manufacture), the Civil Aviation Authority of Ethiopia (representing the State of Registry and the Operator) and the Transportation Safety Board (TSB) of Canada (representing the State of component manufacture), with technical advisors from the Federal Aviation Administration (FAA), the operator and the aircraft and component manufacturers. 4� Crown copyright 2014 AAIB Bulletin: S4/2014 ET-AOP EW/C2013/07/01 In Special Bulletin S5/2013, the AAIB reported the existence of extensive heat damage in the upper portion of the aircraft’s rear fuselage, particularly in an area coincident with the location of the Emergency Locator Transmitter (ELT). The absence of any other aircraft systems in this area containing stored energy capable of initiating a fire, together with evidence from forensic examination of the ELT, led the investigation to conclude that the fire originated within the ELT battery. Five Safety Recommendations are made. Description of the ELT The ELT is a location device, designed to notify rescue authorities of an aircraft’s location in the event of an emergency. The ELT system installed on the B787-8 is a Honeywell RESCU 406AFN Automatic Fixed ELT, manufactured under contract by Instrumar Limited. The ELT transmitter unit (TU) is located in the aft fuselage crown above the passenger cabin ceiling, to the left of the aircraft centreline, and is mounted on two adjacent fuselage frames, between two thermo-acoustic insulation blankets. The ELT is activated automatically by an internal acceleration sensor, or manually by the flight crew. Unless activated it operates in ‘ armed’ mode, using no power. Examination of the ELT CT 1 scans and a teardown examination of the ELT revealed that the internal battery pack had experienced severe disruption, exhibiting evidence of a very high-energy thermal event, consistent with having experienced a thermal runaway 2 . All five cell cases had been breached and burnt battery material had been ejected into the battery compartment and outside of the ELT case. Significantly, the ELT battery wires were found to be improperly installed, in that they had been crossed and pinched together between the battery cover-plate and the ELT case, adjacent to one of the cover-plate fasteners (Figures 1 and 2). The cover-plate was notably bulged in this location, as the wires prevented it from sitting flush against the ELT case. Due to the fire damage it was not possible to determine the pre-incident condition of the insulation in the area where the wires were pinched, nor the gasket which normally sits between the cover-plate and the ELT case. However, metallurgical analysis revealed strong evidence of metal-to-metal contact between the positive wire and the underside of the battery cover-plate. The surface treatment of the aluminium cover-plate had been worn away in the contact location, indicating that the integrity of the insulation may have become compromised over time, exposing the copper conductor to the cover-plate. The ELT case is electrically grounded to the aircraft Current Return Network. As such, direct Footnote 1 Computed Tomography. 2 In the case of lithium batteries, the term ‘thermal runaway’ refers to a self-sustaining, uncontrollable increase in temperature and pressure. It is an exothermic reaction, releasing more heat energy during the reaction than was absorbed to initiate and maintain the reaction. Thermal runaway can culminate in a cell exhibiting violent venting of toxic or flammable gases or electrolyte, decomposition, fire and explosion. The heat released by the affected cell can also heat adjacent cells, such that the failure propagates to other cells in the battery. 5� Crown copyright 2014 AAIB Bulletin: S4/2014 ET-AOP EW/C2013/07/01 See Figure 2 Battery compartment Footprint of battery cover-plate Positive wire Negative wire Contact point with battery cover-plate Figure 1 ELT case with battery cover-plate removed; tops of battery cells visible Figure 2 Crossed and pinched ELT battery wires 6� Crown copyright 2014 AAIB Bulletin: S4/2014 ET-AOP EW/C2013/07/01 contact between the positive conductor and the cover-plate could have provided a path for current to flow through the battery circuit to electrical ground, essentially creating a ‘short-circuit’ condition. The nature of the battery failure was such that much of the battery material was consumed, and that which remained was extremely fragile. Therefore, despite extensive forensic examination and CT scanning of the battery and the individual cells, it has not been possible to determine with certainty the sequence of cell failures within the battery or the pre-failure state of the safety features in the circuit. Description of the ELT battery The ELT is powered by a five-cell, non-rechargeable ‘lithium-metal’ 3 battery, installed in the transmitter unit (Figure 3) and manufactured by Ultralife Corporation, under contract to Instrumar Limited. Figure 3 ELT battery pack and ELT transmitter unit, showing battery compartment Each of the Lithium-Manganese Dioxide (LiMnO 2 ) ‘D-cells’ 4 has a nominal voltage of 3 V and the cells are connected in series to provide a 15 V battery, with a nominal capacity of 11.1 ampere-hours (Ah) (Figure 4). The battery is rated to a maximum continuous discharge current of 3.3 amps. Two external battery wires connect the positive and negative terminals of the battery to the ELT power supply circuit card. The chemistry in the LiMnO 2 cells is considerably different to that used in the rechargeable large-format lithium-ion cells used in the B787 main and APU batteries. Footnote 3 ‘Lithium-metal’ batteries, also known as ‘lithium primary’ batteries, are high-energy electrical storage devices that rely on electrochemical reactions to deliver energy. They are non-rechargeable batteries in which the anode is made from a layer of metallic lithium. 4 A ‘D-cell’ is a cylindrical cell with a nominal diameter of 33.2 mm and a length of 61.5 mm. Battery pull-tabs Battery wires and connector ELT wires and connector 7� Crown copyright 2014 AAIB Bulletin: S4/2014 ET-AOP EW/C2013/07/01 Figure 4 Battery pack illustration (outer shrink-wrap not shown for clarity) The ELT battery circuit has a polymeric Positive Temperature Coefficient (PTC 5 ) protective device installed on the negative wire. The PTC is an automatically-resettable device designed to sense temperature. It will ‘trip’, or transition from a low-resistance to a high‑resistance state, if the temperature ex

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