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Boeing and government investigators now believe the July 12 fire on a 787 Dreamliner at Heathrow Airport in London was likely caused by the incorrect installation of a small lithium battery inside an electronic device.

If that’s confirmed, the fire was due to human error, not a Boeing design flaw.

U.K. investigators who examined the device, called an Emergency Locator Transmitter (ELT) and made by Honeywell, found that the internal wires connecting the battery to the ELT had been trapped and pinched when the cover was reattached as the batteries were inserted, according to two sources with knowledge of the matter, one inside Boeing and one outside.

In photos of what was left of the device, “You can clearly see the two wires crossed over each other. It’s quite evident the wires show evidence of being smashed,” one source said.

Installing the battery package entails unscrewing the cover of the relatively small device, dropping the battery pack of five cells into a slot and connecting the two wires that protrude from the battery pack to a receptacle in the ELT.

It appears the wires were trapped when the cover was put back on.

The leading theory concerning the cause of the fire at this stage is that the pinching of the wires compromised the insulation, and the crossed wires short-circuited to start the fire, which was then fueled by the lithium ion battery, said the two sources.

Boeing has issued a service bulletin to operators telling them to either remove or inspect the batteries.

That will be followed up later this week by a mandatory Airworthiness Directive from the Federal Aviation Administration telling 787 operators “to inspect for proper wire routing and any signs of wire damage or pinching,” according to an FAA statement last week.

The inspection involves removing the 7-pound ELT from its bracket, unscrewing the cover and examining the wires. It takes less than an hour, one source said.

The ELT, which transmits location data to satellites in the event of a crash, is not unique to the 787. Honeywell has produced about 6,000 of them for many different jets since 2008.

Honeywell could not be reached late Tuesday for comment. Boeing spokesman Marc Birtel said the jet maker “cannot comment due to the ongoing investigation.”

The fire on the Ethiopian Airlines 787 caused intense heat damage above the ceiling at the back of the passenger cabin. It scorched a large area of the carbon-fiber skin on the crown of the fuselage just in front of the vertical tail fin.

The two sources suggested that Honeywell might have replaced the batteries at some stage before delivery of the jet because the devices sat on the shelf during the years-long 787 program delays.

If this were correct, it could explain why the accident happened to the 787 in particular.

Investigators are also looking at whether overheating of the batteries as they sat parked in the sun during the four-month grounding of the 787 fleet earlier this year could have led to internal damage that contributed to the failure.

There’s no evidence of moisture damaging the batteries, which had been another theory put forward in the press.

For now the FAA will require inspections only of the 787 fleet, but it will later have to consider whether to demand closer inspection of ELTs on other jets — especially when the batteries are changed.

Dominic Gates: (206) 464-2963 or