U.S. aviation regulators intend to retain full control over all aspects of Boeing Co.’s grounded 737 Max as the planemaker prepares to finalize fixes and restart delivering its best-selling model.

Even relatively routine approvals of new planes coming off the assembly line, which would normally be done by Boeing employees, will be done by the Federal Aviation Administration, the agency said Tuesday.

The Max has been grounded since March after two deadly crashes that killed 346 people.

The FAA is emphasizing its authority after Administrator Steve Dickson sent a video to employees earlier this month saying they shouldn’t be pressured to return the plane to service.

Boeing had said it might be able to begin delivering the first of hundreds of idled planes before the end of the year, but the FAA hasn’t said whether it would allow that to happen before new pilot training standards are adopted early next year.

“The FAA notified Boeing today that the agency will retain authority over the issuance of Airworthiness Certificates for all newly manufactured 737 Max aircraft,” the agency said in the statement.

”We continue to follow the lead of the FAA on the safe return to service of the Max,” Chaz Bickers, a Boeing spokesman, said in a telephone interview.

The FAA decision adds to the difficult logistics that Boeing faces once the Max is cleared to fly. At its current production pace, Boeing could have 386 newly built Max in storage by the close of 2019, Cai von Rumohr, an analyst with Cowen, said in a report last month.