A report released Wednesday by a U.K. safety agency confirms the cause of the fire inside an Ethiopian Airlines 787 Dreamliner parked at Heathrow last July.
A battery inside a small locator beacon was installed incorrectly, with crossed wires pinched between the device case and the cover plate, causing a short circuit.
The update to the ongoing investigation shows that the supplier of the beacon, Honeywell International Inc., identified this precise installation problem and the potential for crossed wires six months earlier and modified the assembly instructions to prevent it from happening in the future.
Yet Honeywell did not inspect the devices already in service or even communicate the issue to aircraft operators and manufacturers.
The report suggests that the fact that it was a Dreamliner that caught fire was bad luck for the troubled program, not the fault of Boeing Co.
At the time of the incident, there were approximately 3,650 identical batteries in service, “fitted to numerous aircraft types.” There were a further 2,900 very similar batteries in service on other aircraft.
No previous in-service overheating incidents had occurred.
After the Heathrow fire, mandatory inspections of the locator beacon device on 787s and other aircraft worldwide revealed a total of 28 with battery wires similarly pinched between case and coverplate.
Of those, only nine had exposed the wire and six of those were fully charged – indicating that the wire had not made contact with the case to create a short circuit.
Of the remaining three devices with trapped and exposed wires, two failed benignly, protected by built-in safety features designed to kick in when overheating occurs.
The one battery that failed spectacularly could have been on any jet – but it happened to be on a 787 Dreamliner.
The investigation update by the U.K.’s Air Accident Investigations Branch, or AAIB, also points out that the battery involved was a non-rechargeable lithium metal battery, not the lithium-ion type used in the 787’s two main batteries.
The internal chemistry of the small lithium metal batteries in the locator beacon is “considerably different to that used in the rechargeable large-format lithium-ion cells” used in the 787’s main batteries, the report states.
It was the failure of the main lithium-ion batteries early in 2013 that grounded the Dreamliner fleet worldwide for more than three months before the Ethiopian incident.
The fire at Heathrow further damaged the 787’s reputation, creating another link in people’s minds between the new jet and overheated batteries.
But the AAIB update makes clear the Ethiopian jet fire was unrelated to the previous main battery incidents.
Aviation-accident investigations progress carefully and slowly, but the likely cause of the Ethiopian fire had been reported by The Seattle Times just a couple of weeks after it happened.
The device that failed was a seven-pound emergency locator transmitter, or ELT, designed to transmit location data to satellites in the event of a crash.
Supplied by Honeywell, it was manufactured by subcontractor Instrumar, based in St. John’s, Newfoundland. The battery was manufactured by Ultralife, a subcontractor to Instrumar based in Newark, N.Y.
The ELT was installed in the crown of the fuselage above the ceiling in the rear of the passenger cabin.
The fire caused intense heat damage inside the passenger cabin and scorched a large area of the carbon-fiber composite skin on the crown of the fuselage just in front of the vertical tail fin.
In a major repair challenge, Boeing cut out the damaged section and replaced it with a large patch. The plane was returned to service just before the end of last year.
The AAIB update faults the protection mechanisms designed into the ELTs, as well as the certification testing that was done on the devices.
The agency recommends substantial changes to the way that testing is done in the future.
The features designed to prevent overheating of the ELT included safety vents in the base of each of the battery’s five cells and a separator between the cells designed to become less porous when heated.
In addition, the ELT featured a protective element called a positive temperature coefficient, or PTC, device that was supposed to “trip” into high resistance and stop current flowing if the temperature rose above a certain level.
When an ELT was returned in February 2013 due to a discharged battery, Honeywell discovered that it had been incorrectly wired, with the wires pinched between the case and the coverplate.
But with no sign of thermal damage, Honeywell concluded that the PTC had worked as designed.
In April 2013, Honeywell revised the method of assembling the device at Instrumar to avoid the pinched wires on newly manufactured ELTs.
But neither Boeing nor the airlines were informed of the problem. Inspections of in-service ELTs were ordered only after the Ethiopian Airlines fire in July 2013.
The AAIB concluded that “the PTC did not adequately protect the battery in this event.”
In May of this year, Honeywell introduced a redesign to the device that routes the wires underneath the battery so that they cannot be trapped by the coverplate.
Tests conducted by the AAIB looked at worst-case scenarios and concluded that the overheat protection design elements in the ELT need to be strengthened.
The agency issued five new recommendations for action by the Federal Aviation Administration, one of which calls for “enhanced certification requirements for the use of lithium-metal batteries in aviation equipment.”
It recommends that device manufacturers must be required to demonstrate that “the battery and equipment mitigates all hazardous effects” of cell overheating.
In addition, the AAIB asks the FAA to require more rigorous testing to certify such equipment.
The existing requirement is for lab tests of stand-alone batteries. The AAIB says tests should be done with the batteries installed in the devices. And it asks the FAA to work with the industry to devise the best real-world tests.
The report concludes that “better coordination is required” between aviation-safety regulators and the battery, equipment and aircraft manufacturers.