Japanese aerospace union officials have cast new light on problems with the Boeing 787's power distribution panels, saying one malfunction last year caused a burnt circuit board, and disclosing two other previously unpublicised incidents.
Power-panel faults, while unrelated to the battery problems that have grounded the 787 since January, are another nagging issue with the plane's electrical system.
On flights in March, April and June of last year, faults in power-panel circuit boards on Dreamliners operated by Japanese carrier All Nippon Airways resulted in error messages in the cockpit, said airline spokeswoman Nao Gunji. Each time, the panels were inspected after landing.
In the case of the fault on an April 7 flight, a circuit board was found to have shorted, causing "slight discolouration" from burning, Ms Gunji said.
ANA replaced all three circuit boards, she said.
The Japan Federation of Aviation Workers' Unions, which represents ANA pilots, on Wednesday highlighted the incidents at a news conference in Tokyo.
Shozo Tsue, the federation's secretary-general, said the April 7 incident "was serious and caused damage to the surrounding area".
Ms Gunji said the purpose of the news conference was to ask the government "to ensure the safety of the aircraft" and take the time to find out what happened.
The union's account reveals the April 7 incident as one of the four instances of power-panel short circuits cited in January by Boeing vice-president Mike Sinnett.
Mr Sinnett, the 787's chief project engineer, said in each of the power-panel incidents electrical arcing inside a circuit board - "a low-energy arc that lasted milliseconds, very small" - had damaged the board and shut down some of the plane's electrical power. He said the small spark inside the circuit boards produced no safety hazard, only a loss of function that was handled by the plane's multiple, redundant power systems.
In January, Boeing had not yet found the root cause but, Mr Sinnett said, the problems had been traced to a batch of faulty circuit boards inside the power-distribution panels, which are located, like the batteries, inside an electronics bay.
Boeing said the investigation of the power-panel incidents was still ongoing.
Following the ANA power-panel faults, on December 4, another power-panel short circuit occurred on a United flight out of Houston, Texas, forcing the pilot to divert to New Orleans.
A few days later, a similar fault occurred on the delivery flight of a Qatar Airways 787 from Washington state to Doha. And later in December, a second United jet was grounded after another power-panel malfunction.
Kazuo Harigai, assistant secretary of the union, said "there have been lots of problems with the  electrical system".
Boeing said that because the 787 had more electrical systems than other aeroplanes, "it stands to reason" that component malfunctions encountered in service had been "mostly electrical in nature".
Tests may begin
The US Federal Aviation Administration is close to approving tests of Boeing's approach to fixing the batteries on its 787 jets - and the tests could begin next week. The FAA could still demand changes to Boeing's proposed battery design if problems develop in the laboratory and flight tests, which will take several weeks. But the decision to start the tests will be a big step in Boeing's efforts to get the innovative jets, which have been grounded since mid-January, back in the air.