The FAA has approved Boeing’s plan for certifying modifications to the troubled lithium-ion battery system for the 787.

“The certification plan is the first step in the process to evaluate the 787’s return to flight and requires Boeing to conduct extensive testing and analysis to demonstrate compliance with the applicable safety regulations and special conditions,” the regulator says in a statement.

Improvements to the battery system include a redesign to minimize initiation of a short circuit within the battery, better insulation of the cells and the addition of a new containment and venting system for the identical primary and backup lithium-ion batteries.

The FAA grounded the U.S. fleet of six 787s belonging to United Airlines on Jan. 16 following a main battery failure on an All Nippon Airlines 787 in flight over Japan earlier that day. On Jan. 10, the backup battery in a Japan Airlines 787 caught fire on the ramp at the Boston Logan International Airport, prompting the NTSB to launch an investigation into the problem.

Early results from the NTSB investigation revealed thermal runaway in both incident batteries.

Operators of the other 44 787s that had been delivered voluntarily grounded their aircraft soon after the FAA order.

Following the Boston incident, the FAA launched a comprehensive review of the 787 design, manufacture and assembly, focusing in part on whether the lithium-ion batteries and the battery system were in compliance with the special conditions the agency issued as part of the aircraft’s certification.

“The FAA will approve the redesign only if the company successfully completes all required tests and analysis to demonstrate the new design complies with FAA requirements,” the agency says of its approval, noting that the airworthiness directive that grounded the widebody remains in effect and that the comprehensive review continues.