The FAA Friday issued an airworthiness directive (AD) requiring modifications of the battery systems on Boeing 787-8s before the aircraft can resume operations.

The AD is the first acknowledgment that a battery modification expected to be proposed to the regulator by Boeing could allow the aircraft to return to service. The AD effectively prepares the ground for a modification program and also represents the first official confirmation from any party that such an action on this scale will be required.

The FAA says the AD affects six U.S. registered 787s, all of which are operated by United Airlines. These six aircraft and 44 more were grounded in January when the global fleet was grounded.

Although Boeing tacitly acknowledged it is developing a fire-proof containment for the battery, there has been growing speculation that an associated redesign of the lithium-ion unit is the only way to fully satisfy safety concerns.

The AD does, however, contain the caveat that “other actions” will be considered to meet the airworthiness ruling, opening the possibility that an interim modification to the system as a whole (such as the initial containment modification) may be acceptable to allow operations to restart soon. The FAA acknowledges further rulings will follow. “As the investigation progresses, we might determine that additional action is necessary,” says the agency.

The AD comes as FAA Administrator Michael Huerta hears details of Boeing’s modification proposals from a delegation led by Commercial Airplanes president Ray Conner. The meeting, which occurred Friday, marked the 38th day since the agency grounded the 787, one more day than the McDonnell Douglas DC-10 grounding in 1979.