Amid mounting signs that Boeing will have to wait longer than it hoped for FAA approval to start test flights of its proposed 787 battery fix, the NTSB has announced plans to hold a forum and investigative hearing in April to review battery technology, safety and the design and certification of the aircraft’s battery system.

News of the April events emerged as the board released a densely packed interim report on its ongoing investigation of the Jan. 7 Japan Airlines 787 battery fire at Boston Logan International Airport. This incident and a subsequent inflight battery issue on an All Nippon Airways aircraft led the FAA to ground the worldwide 787 fleet.

The report does not give a specific cause for the battery failure but details the events of the Boston incident and findings from the examination of the battery. It also cites test results of related components, initial reports on the flight recorder data and a description of the 787 electrical power system certification plan.

A list of ongoing and planned investigative activities also is included.

The report’s findings describe the shortcomings of the power system, which was designed to vent smoke overboard from the aircraft. According to the NTSB, the system failed to function because on the ground, with the aircraft’s auxiliary power unit shut down, it lacked power after the battery caught fire. “As a result, smoke generated by the APU battery could not be redirected effectively outside the cabin and aft [electrical equipment] bay.”

The finding is not surprising since the system is designed to rely primarily on differential pressure in the cabin above the bays, which draws air through the flight deck panels and the forward electrical equipment racks to create a reverse flow of air across the battery. This flow then exits through an override valve to an overboard venturi.

Boeing says that while the override mode supplies adequate cooling during cruise, it acknowledges that the airflow naturally decreases as the cabin pressure differential reduces.

The NTSB report also notes the battery did not behave as either Boeing or system subcontractor Thales indicated. In particular, the battery’s power discharge was “not at the constant rate described by the Boeing or Thales documents and included large changes and reversals of power within short periods of time,” the NTSB says.

The report also includes new details of Boeing’s initial development testing and certification assumptions for the battery. It notes, for example, that pre-production testing of the aircraft’s lithium-ion batteries did not produce a fire after an intentional short-circuit. This led Boeing to conclude that the risks of an event similar to either of the incidents that took place in January were remote.