The Federal Aviation Administration (FAA) has issued its first airworthiness certificate for a Boeing 737 MAX built since March 2019, the agency said on Tuesday.

The FAA on Nov. 18 lifted a 20-month-old grounding order on the MAX after two fatal crashes in five months killed 346 people. The FAA is requiring a series of software changes and new pilot training requirements before planes can return to service.

Boeing has about 450 737 MAX airplanes that have been built since 2019 and are awaiting approval by the FAA before they can be delivered to airlines. Boeing declined to comment on the FAA approval.

"We expect to have sufficient number of inspectors on hand to meet Boeing’s planned delivery schedule for the foreseeable future. We’ll defer to Boeing to discuss the company’s manufacturing and delivery plans," FAA spokesman Lynn Lunsford said.

The FAA separately last week approved an American Airlines training plan for pilots to resume 737 MAX flights, the agency and airline confirmed. That approval clears the way for American to resume MAX flights starting Dec. 29 once it completes required tests and software upgrades to parked planes.

American plans to begin with a single daily MAX flight from Miami to New York's LaGuardia airport. That will mark the return of the MAX to U.S. commercial service.

Boeing's (BA) backlog of planes is worth about $16 billion, investment firm Jefferies estimates.