Boeing is stretching out the timeline for patching anti-stall software under scrutiny in two crashes of its best-selling 737 MAX jetliners, promising to take as much time as necessary to get it right.

The Federal Aviation Administration now says it expects to receive the final draft of Chicago-based Boeing’s software fix “over the coming weeks.” The U.S. agency joined its international peers in grounding the aircraft, the latest version of one of the world’s most widely flown planes, in mid-March after flight data from a downed Ethiopian Airlines plane showed choppy ascents and descents during takeoff, mirroring those before a Lion Air crash in Indonesia on Oct. 29.