(Reuters) – A fire that scorched the top of a Boeing Co 787 Dreamliner at London Heathrow airport last summer was likely caused by faulty wiring in an emergency rescue beacon that led to “an uncontrolled discharge” from a lithium-ion battery, the UK aviation safety agency said on Wednesday.

The battery likely discharged unevenly, causing one of its cells to deplete more than the other four, then reverse polarity and absorb energy from the others, the UK Air Accidents Investigation Branch (AAIB) said in a special report.

“Several tests demonstrated that when a cell failed in this manner, the heat released caused the failure to cascade to the remaining four cells,” the AAIB said.

Honeywell said it appreciated the AAIB’s throughness and noted it had worked with the Federal Aviation Administration and Transport Canada on an airworthiness directive requiring “that all applicable ELT units are inspected to verify that the error is not present.” It added, “Honeywell is committed to ensuring the safety of all its products and has implemented a redesign and amended assembly/installation guidelines for this product.”

The July 12, 2013, fire in the emergency locator transmitter (ELT), made by Honeywell International Inc, burned the top of the fuselage of the Ethiopian Airlines jet, taking it out of service for an extended period and renewing concern about use of lithium-based batteries on aircraft. No one was injured in the incident and the jet was parked at the time.

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