With airlines beginning revenue passenger flights with modified Boeing 787s through May and June, the U.S. National Transportation Safety Board appears to be accelerating its search for the root cause of the lithium-ion battery problems that grounded the fleet for nearly five months.

What the agency does know is that one cell in the auxiliary power unit (APU) battery of a Japan Airlines 787 on the tarmac in Boston in January experienced multiple short circuits, leading to a thermal runaway in that cell, which then cascaded to other cells in the battery. What neither the NTSB nor Boeing knows is what caused the short in the first place.

The NTSB is hurrying to find out.

The agency on May 3 issued an “urgent” procurement request to have a Maryland company perform non-destructive computed tomography (CT) scans on as many as 48 Boeing 787 lithium-ion battery cells (there are eight cells in each of two 787 batteries) before and after certain types of testing to try to understand the internal anatomy of the failure. The work was to start on May 6 and extend for one week.

The Washington-based NTSB needed a local company to do the work because lithium-ion battery cells cannot be shipped by air.

Wording in the procurement suggests concern that the fleet is returning to the air without the root cause identified.

“[The tests] must also be completed within the shortest timeframe possible to provide the fastest possible receipt of this information to avoid potential future accidents involving this type of aircraft battery,” says the agency in the documentation.

“Since the FAA has recently approved a plan intended to result in the Boeing 787 being approved for a return to service, the information from these tests (and the CT scans required to support these tests) is needed as soon as possible,” says the NTSB.

The Board voiced similar concerns during a two-day investigative hearing on April 23 and 24 as part of its data gathering on the Boston incident.

“At this point in time, the lab has not come to an understanding of what the root cause is, so how do you go about designing a fix when you don’t know what the root cause is?” NTSB member Earl Weener asked of Boeing during the hearing.

Jerry Hulm, a Boeing systems engineer and associate technical fellow, answered that the company had developed a two-pronged approach focusing on protecting the battery and the aircraft. The FAA approved the fix on April 19.

“Not knowing the exact root cause, our detailed analysis highlighted some improvements that could be made that would improve the capability of the battery to avoid these types of events,” Hulm said of the additional insulation now added between the eight battery cells. “Not knowing the root cause, we had to preclude the smoke from getting into the aircraft,” he said of the new 1/8th-in.-thick stainless steel battery enclosure with a vent line assembly that dumps smoke overboard if a thermal runaway in one cell does occur.

In its continued search for the root cause, the NTSB says it is planning to “conduct teardown examinations as soon as possible” of several aircraft batteries similar to “one involved in an aircraft incident”, which is where the CT scans come in.

“To facilitate those examinations, CT scans of these batteries and their subcomponents are required to non-destructively determine as much information as possible about those components,” the NTSB says.