The initial report from British investigators into the 787 Dreamliner fire at Heathrow Airport delivered good news for Boeing’s trouble-plagued jet.

Though how the July 12 fire started and why it was so severe remain mysterious, the investigators definitively fingered a small electronic device that’s found on lots of other airplanes as the source of the fire.

“It doesn’t look like this fire had anything to do with the unique attributes of the 787,” said Hans Weber, a respected technical expert in the aviation world and president of consulting firm Tecops International.

The incident, not long after two battery fires grounded the 787 for three months, appears to be “incredible bad luck” for the Dreamliner program, he said.

The report from the U.K.’s Air Accidents Investigation Branch (AAIB) also revealed that, despite how it looked on TV footage, the fire did not burn through the roof of the Dreamliner.

Still, repairing the damaged Ethiopian Airlines jet will clearly be a major challenge for Boeing’s engineers.

The AAIB interim report, released Thursday, confirmed the fire centered on an electronic device weighing less than 7 pounds, an Emergency Locator Transmitter (ELT), which transmits location data to satellites in the event of a crash.

Investigators recommended the Federal Aviation Administration (FAA) order the devices disabled on all 787s pending further investigation and recommended the U.S. agency also review the devices’ performance in other planes.

Boeing said it supports those recommendations, calling them “reasonable precautionary measures to take as the investigation proceeds.”

The focus on a rare malfunction of this electronic beacon damped fears that the fire was the result of some broader problem with the 787’s electrical systems.

The report points to the ELT, which sits in the fuselage ceiling just in front of the tail fin, as the sole source of the fire, either due to the lithium batteries inside it or from a short in a connecting wire.

It did not explain how such a relatively small energy source — an inactive battery with five 3-volt cells carrying just 55 amp-hours of charge — could severely damage a substantial area of the jet’s carbon-fiber-reinforced plastic skin.

For comparison, the 787’s main batteries that proved so problematic earlier this year each weighed 63 pounds and had eight 32-volt cells. Each of those two batteries generates 150 amps on power-up; the total charge isn’t clear.

In addition, the emergency beacon’s batteries use a lithium-manganese chemistry that is relatively stable compared with the more volatile lithium-cobalt chemistry of the main batteries.

The AAIB report says Honeywell, which supplies the ELT, has produced some 6,000 units of the same design. They are on a wide range of aircraft, and the 787 fire has been “the only significant thermal event” to date.

Yet firefighters initially struggled to put out the fire, according to the report.

After the control tower noticed smoke from the plane, firefighters rushed to the jet, entering through a passenger door, and “encountered thick smoke” that became denser as they moved to the rear, where they saw signs of fire “above the ceiling panels.”

They initially tried to extinguish the fire with a handheld halon fire extinguisher. When this proved ineffective, they ripped out a ceiling panel and doused the fire with water from hoses.

Because airliners have neither fire detection or suppression systems above the cabin ceilings, the report says, “had this event occurred in flight it could pose a significant safety concern and raise challenges for the cabin crew in tackling the resulting fire.”

Disabling the devices isn’t a problem for U.S. carriers, for whom the emergency beacon is optional and not an FAA requirement.

Some foreign regulators do require the ELT, though they will likely waive the rule if the FAA follows the U.K. agency’s recommendation.

Like other airplanes, the 787 also has separate, portable ELTs in the emergency slides that deploy as life rafts if the plane goes down in water. AAIB spokesman Sam Bond said these are not covered by the safety recommendation and won’t have to be disabled.

Honeywell said temporarily disabling the ELTs on the 787 is “prudent,” and called for a review of the safety “of any lithium battery-powered ELTs from the variety of manufacturers who sell them.”

Investigators still can’t explain how the fire started, though they know it centered on the ELT’s small lithium-manganese battery.

The AAIB said investigators have seen “some indications of disruption to the battery cells,” although it is unclear whether the fire “was initiated by a release of energy within the batteries or by an external mechanism such as an electrical short.”

In the latter case, a short could have set off the batteries, providing the energy for the ignition.

The report states that the aircraft was unpowered at the time the fire broke out. Although it was connected to a ground power source, this source was switched off.

Besides the ELT, the report concludes, “there are no other aircraft systems in this vicinity which, with the aircraft unpowered, contain stored energy capable of initiating a fire in the area of heat damage.”

Boeing spokesman Marc Birtel said the ELT is powered entirely by its battery and that the only connection to the airplane is a wire to the flight deck to turn it on or off.

“It does not receive power from the airplane,” Birtel said.

He added that “we have seen failures of other types of ELTs in the past that resulted in varying but lesser degrees of heat damage.”

A check of the FAA database that records in-service incidents affecting planes operated by U.S. airlines found 10 ELT incidents involving Boeing jets and four with Airbus jets.

However, all of those incidents were minor. The ELTs malfunctioned or were broken and had to be replaced, but there was no mention of heat damage.

The report also indicates that, despite what looked like gaping holes in the fuselage crown on TV, the fire did not completely burn through the top of the fuselage.

While it described “blackened and peeling paint and damage to the composite structure,” it made no mention of holes in the skin.

A person familiar with the details behind the report, who asked not to be identified, confirmed the fire did not burn through the plane’s carbon-fiber-reinforced plastic skin.

TV pictures showed clearly the pattern of frames and stiffeners showing beneath the skin, but it seems these were visible only as areas of varying scorch marks rather than through a hole in the skin.

Nevertheless, the composite skin above the fire — from the photos, an area at least 8 feet long by 2 feet wide — is clearly badly damaged and will likely have to be replaced.

Weber said Boeing engineers will have to figure out how hot the fire was and inspect the carbon-fiber material to determine whether it lost some of its structural strength.

“My guess is, it has, at least in some areas,” he said.

How it can be fixed depends on how extensive any weakened area is.

Weber said there is a well-established procedure for applying a composite patch, either bonded or fastened to the existing skin. But he said it’s possible the damaged area is too big to be patched.

In the worst case, he said, Boeing might have to replace the entire aft fuselage section, a single barrel-shaped piece measuring 19 feet in diameter and 23 feet long.

In fact, this option could prove easier than patching. That’s because any fix will entail proving to the FAA that the repair is sound.

Frank Abdi, chief scientist and founder of Long Beach, Calif.-based AlphaSTAR, which specializes in structural modeling and analysis of composite materials, said that to apply a substantial patch Boeing would have to do a computer simulation of the repair then “prove by analysis and by test that such a thing is possible.”

Retired Boeing manager Ed Wilson, who worked with composites on the B-2 and 777 programs, said that replacing the entire back end could well be the quickest fix.

Patching it, he said, “you’ll spend more time screwing around trying to convince regulators that it’s fixed.”

Weber said replacing the entire rear-fuselage section is technically feasible.

A Boeing Dreamlifter cargo plane could deliver a new section to Heathrow, where the plane would be jacked up in a hangar and the damaged section removed.

The most time-consuming part of such an operation would be disconnecting all the systems wiring and ducting that feeds into the rear fuselage, then reconnecting after a new section is in place.

The question is, would such a massive repair make economic sense?

Boeing may choose to spend whatever it takes to avoid a write-off so early in the program and to show its customers that, even in a case of serious damage, the 787’s innovative structure can be repaired.

“All the airlines will be following this very closely,” Weber said.

Meanwhile, the “incredible bad luck” of the 787 continued Thursday as a Japan Airlines (JAL) flight from Boston to Tokyo with 184 passengers had to turn back, returning safely to Boston five hours after takeoff.

JAL spokeswoman Carol Anderson said the action was “a standard precautionary measure” after the pilot received a maintenance message indicating a faulty fuel pump.

Photo Credit: Emergency crew surrounds a Boeing 787 Dreamliner, operated by Ethiopian Airlines, which caught fire at Britain's Heathrow airport. Reuters