British air safety officials said emergency transmitters on Boeing 787s should be disabled after finding that one of the squat orange boxes was the only thing with enough power to start a fire in the scorched tail section of a 787 parked at Heathrow airport last week.
The Air Accidents Investigation Branch also recommended that the U.S. Federal Aviation Administration and other regulators carry out a safety review of similar systems in other types of aircraft.
The recommendations in the report issued Thursday aren't binding. The FAA said it will take them under advisement but didn't take any other immediate action.
Boeing supported the recommendations. Its shares rose nearly 3 percent in regular trading. That's about where they were Friday, when investors saw images of a smoking 787 and feared a repeat of the battery problems that led to the planes being grounded earlier in the year.
But in a reminder of how sensitive the market remains to 787 issues, the shares fell 2 percent in after-hours trading after a Japan Airlines 787 flight returned to Boston's Logan Airport because of a possible fuel pump issue. There have been a number of similar minor incidents since the planes resumed flying in late April.
Investigators said Thursday that it was not clear if the fire was caused by the transmitter's lithium-manganese dioxide batteries or a short near or around the transmitter, but recommended that the FAA switch off the Honeywell transmitter in all Boeing 787s "until appropriate airworthiness actions" can be carried out.
A spokeswoman for the investigative branch said the easiest way to make the transmitter systems "inert" — as set out in their recommendations — would be to take out their batteries.
The FAA said it is reviewing the AAIB's recommendation "to determine the appropriate action."
Boeing called the recommendations "reasonable precautionary measures" and said it was working with regulators to take appropriate action in response.
Honeywell said "Temporarily addressing the (transmitters) on Boeing 787s as a precautionary measure is prudent." Honeywell said it doesn't expect any noteworthy financial impact as a result. Sahres of Honeywell International Inc. rose 53 cents to close at $82.97.
Honeywell has made 6,000 of these transmitters and they're used in a wide range of planes, the report said. All 68 of Boeing's 787s have them. So far, the July 12 fire is the "only significant thermal event" for those transmitters, the U.K. report said.
However, the investigators noted that large passenger planes don't usually have fire detection or suppression gear in the space above cabin ceilings, "and 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," the report said.
Boeing spokesman Marc Birtel said that the transmitter isn't required by U.S. federal aviation laws, but is required by some foreign regulators for their airlines or their airspace.
Birtel said the transmitter, which helps with search and rescue operations, takes about an hour to remove from a 787.
The locators are activated in a crash and send a signal that satellites use to calculate the location of the plane.
The locaters are helpful in certain types of crashes, but of little value in others. They're most helpful in a crash in a remote area on land, where rescuers might have a hard time finding the wreckage. They're irrelevant for crashes where someone can see the plane, like the Asiana crash in San Francisco earlier this month. And they're of little help if a plane crashes in water, because it will "sink with the aircraft and be rendered useless," according to a 2005 report on emergency transmitters by a committee of the International Civil Aviation Organization.
They're not the only emergency transmitters on most planes. Some life rafts have their own, which are activated by salt water. And "pingers" attached to the flight data recorder and cockpit voice recorder also send out a signal that searchers can use.
Honeywell's Rescu406 AFN is designed to be used for planes flying over land, according to the company's sales brochure. Its battery is designed to last about 50 hours once it's activated. The orange, aluminum transmitter is the size of a squat loaf of Italian bread.
The Ethiopian Airlines plane had been parked for about 10 hours when a worker in the control tower saw smoke coming out, and activated a crash alarm, the report said. Firefighters were there within a minute. They sprayed water and foam onto the plane. Inside the plane, they found thick smoke, which became denser as they moved toward the back of the plane. Firefighters saw "indications of fire" near the back of the passenger cabin. Halon fire extinguishers didn't put it out, so they pried off a ceiling panel. The fire was put out with water from hoses.
The 787 is Boeing's newest and highest-profile plane. Boeing took a big risk with the 787, using a high-tech plastic for the fuselage instead of the usual aluminum, and using much more electricity to power systems that on other planes would have used air from the outside. The payoff is a much lighter, more fuel-efficient plane.
But those new features have also brought new scrutiny. When lithium-ion batteries on two 787s smoldered in January, regulators grounded the plane for more than three months. Boeing redesigned the battery system and got approval for the planes to return to service.
Boeing shares fell 5 percent the day of the Heathrow fire. They've recovered since as investigators ruled out a connection to the plane's large lithium-ion batteries. The transmitter battery is also lithium-ion, but it's smaller and has a different chemistry that is considered more stable than the batteries that were an issue earlier this year.
Boeing Co. shares rose $2.84, or 2.7 percent, to close at $107.63.
Investors and others are "keenly aware of any gremlins that have disrupted the plane's operations," Jefferies analyst Howard Rubel wrote in a note Thursday. He said Boeing has brought 787 owners together in conferences to compare notes to improve the plane's reliability.
"We are confident the 787 is safe and we stand behind its overall integrity," Boeing said on Thursday.
Freed reported from Minneapolis. AP Business Writer Sarah Skidmore in Portland, Ore. contributed to this report.