The US National Transportation Safety Board (NTSB) has rebuked Boeing for making public comments about the ongoing 787 battery investigation in a sign of escalating tensions.

The letter sent to Boeing's lawyers on 21 March comes a week after 787 chief engineer Mike Sinnett, speaking at a press conference in Tokyo, quibbled with the NTSB's previous descriptions of the 7 January incident in Boston.

NTSB general counsel David Tochen notes in his letter that Boeing is "obviously familiar" with the agency's investigative process, which requires prior notification to the agency about the technical content.

Boeing's failure to notify the NTSB "is inconsistent with our expectations", Tochen writes.

Boeing was not immediately available to comment on the NTSB letter.

In his Tokyo briefing, Sinnett disputed the NTSB's initial description of fire and an "explosion" inside the lithium-ion battery of the 787 on 7 January in Boston. He said there were only two, 7.62cm (3in) spurts of flame, and both were caused by arcing of electrical wires outside the battery box.

Sinnett also argued that Boeing has found no evidence of "thermal runaway" at the aircraft level, which the company considers the only meaningful safety risk. The NTSB has determined that a thermal runaway chain reaction occurred within the battery cells, and has not ruled out the possibility that a fire erupted inside the battery box.

The dispute between Boeing and the NTSB has escalated at a key time in the two-month-old 787 grounding. The FAA has accepted Boeing's plan to test what it considers a "permanent fix" for the battery problem, that would allow the 787 to return to commercial service even as investigators continue to search for a root cause of the battery failures.

Source: Air Transport Intelligence news