United Airlines Holdings Inc. is poised to resume service of some Boeing Co. Max 9s starting on Sunday, weeks after the jets were grounded following a mid-air blowout of a door plug during an Alaska Airlines flight. 

In a letter to employees, United said it had received final approval from the Federal Aviation Administration to complete the process of returning its Max 9 fleet to service following a “thorough inspection process.” United is one of Boeing’s biggest customers, and the airline has been frustrated with the planemaker’s inability to overcome quality lapses, Bloomberg earlier reported.

The FAA this week approved the inspection procedures airlines must carry out to resume flights using Max 9s. The jets were grounded after the Jan. 5 incident in which the panel blew out of a jet in the middle of a flight, leaving a gaping hole in the fuselage. Regulators have also stepped up scrutiny of the company’s manufacturing.

Alaska Airlines has restarted flying the Max 9s, Boeing told its employees on Friday, adding that Aeromexico and Turkish Airlines, as well as United, will bring theirs back in the coming days. 

“Our long-term focus is on improving our quality so that we can regain the confidence of our customers, our regulator and the flying public,” Boeing said in the message to all its commercial airplane employees.