Here's Everything We Know From The Probe Of The Dreamliner Battery Failure

National Transportation Safety Board Chairman Deborah A.P. Hersman provided an update this morning on the investigation into the cause of the fire in a parked Boeing Dreamliner jet in Boston, one month ago today.

The probe began the day after the fire, about a week before the FAA issued an air worthiness directive that led to the worldwide grounding of the 50 787s Boeing has delivered so far.

Hersman noted that the NTSB investigation is looking only at the Boston fire (a similar event occurred on a 787 flight in Japan a week later), and that the goal is to provide facts about what happened, not analysis or fixes.

The decision on when to allow the Dreamliners to return to the air, and what changes may need to be made, rests with the FAA.

Here's what we know:

Hersman said NTSB investigators found that at the time of the battery failure, the voltage of the lithium-ion battery dropped from 32 volts to 28 volts, indicating that one of the eight battery cells had failed.

Based on examinations of the burned battery (including a CT scan), the NTSB believes a short circuit in cell number six cascaded to the seven others , leading to a thermal runaway: uncontrollable heat and the battery's failure.

The probe has ruled out the possibility of an external short circuit and that a mechanical impact triggered the event (all mechanical damage occurred after the short circuit).

Now the investigation will consider the state of charge in the cell, the delivery of the electrical charge, and how the cells are arranged in the battery.

(Tesla and SpaceX founder Elon Musk has argued that the cells in such a battery should be spaced further apart, to prevent thermal runaway, advice Boeing may follow.)

The probe has not yet discovered a probable cause of the short circuit, or where exactly in the cell the problem originated, but Hersman promised a more detailed report within 30 days.

These problems go way back

The NTSB is also questioning the FAA certification process that allowed the Dreamliner to enter commercial service in the first place.

Because the jet uses lithium-ion batteries, a new feature not covered by existing regulations, the FAA provided nine special conditions Boeing would have to meet for certification.

But Boeing's findings from its own tests — it is common practice for aircraft manufacturers to do their own testing — have not lined up with recent tests by the NTSB, or what happened on the 787 in Boston.

According to Hersman, Boeing found that a short circuit in one cell would not lead to thermal runaway, which is exactly what happened.

On top of that, Boeing said a "smoke event" triggered by a battery failure would occur less than once every 10 million flight hours.

To date, the world's fleet of Dreamliners has logged fewer than 100,000 flight hours, and experienced two battery failures, on two separate aircraft, in the space of two weeks.

That means, Hersman said, that the process of evaluating the plane's safety is flawed: "The assumptions used to certify the battery must be reconsidered."

Hersman noted that the NTSB is working closely its French and Japanese counterparts, as well as Boeing, and that the goal of the probe is to get the planes back in the air, safely.

Just when that will happen, and under what conditions, is the purview of the FAA.