Chastised by the top US aviation regulator, Boeing on Thursday at last acknowledged that its 727 MAX aircraft will not return to the skies until next year.
Steve Dickson, head of the Federal Aviation Administration, met Thursday with Boeing chief Dennis Muilenburg to express concerns the company is rushing to get the jets back in the air, the agency said.
The FAA has stressed it wants a thorough review of the 737 MAX, which has been grounded worldwide since March following two fatal crashes.
Boeing had repeatedly said it expected to win the green light for the MAX to return to service before 2020.
But after Thursday's meeting it issued a statement saying, it will "work with the FAA to support their requirements and their timeline as we work to safely return the Max to service in 2020."
In an email to congressional oversight committees on Capitol Hill earlier Thursday, the FAA said Dickson was "concerned that Boeing continues to pursue a return-to-service schedule that is not realistic due to delays that have accumulated for a variety of reasons."
"More concerning, the administrator wants to directly address the perception that some of Boeing's public statements have been designed to force FAA into taking quicker action," the message said.
"The administrator wants to make clear that both FAA and Boeing must take the time to get this process right."
That underscores Dickson's statements on Wednesday saying the FAA will not be able to certify the 737 MAX to return to service this year given the number of steps left to complete.
Boeing said Muilenburg had a productive meeting and "reaffirmed with the FAA that safety is our top shared priority and we committed to addressing all of the FAA's questions as they assess MAX certification and training requirements."
Major carriers with 737 MAX jets in their fleets have signaled skepticism about Boeing's more optimistic timeline and forecast return-to-service dates in the first quarter of 2020.
Boeing and the FAA have been under intense scrutiny for their responses to issues with the aircraft, including the flight-handling system involved in both accidents, the Maneuvering Characteristics Augmentation System, or MCAS.
The FAA letter to lawmakers comes a day after Dickson faced another grilling before a congressional committee.
Meanwhile, American Airlines said Thursday it had pushed back its return-to-service date for the MAX to April 7 from March 4, adding that passengers booked on MAX flights between those dates would see their reservations rescheduled.
Earlier on Thursday, Southwest said it had reached a confidential agreement with Boeing partially compensating the airline for costs related to the grounding of the jets.