Seattle, Washington - Alaska Airlines gradually resumed flights with its Boeing 737 MAX 9 fleet on Friday, three weeks after a mid-flight blowout of a panel and emergency landing prompted sweeping inspections of the aircraft, the company said.

The first trip was flight 1146 from Seattle to San Diego, which left 90 minutes past its scheduled 2:20 PM departure time and arrived in California at 6:14 PM.

The voyage comes after the Federal Aviation Administration on Wednesday announced a maintenance and inspection program to clear the MAX 9 to resume service.

Alaska Airlines said it expects inspections on its fleet of 65 MAX 9 planes to be completed by the end of next week, allowing for a resumption of its schedule.

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"Each of our 737-9 MAX will return to service only after the rigorous inspections are completed and each plane is deemed airworthy according to FAA requirements," the company said. "The individual inspections are expected to take up to 12 hours per aircraft."

The FAA grounded 171 MAX 9 planes with a similar configuration to the one in the January 5 incident, in which a door plug blew out mid-flight.

While nobody was seriously injured in the incident, inspectors have said the episode could have been catastrophic.

The grounding resulted in 3,000 Alaska Airlines flight cancellations in January. The company said Thursday that it expects a $150 million hit from the grounding.

United Airlines, which has the largest fleet of Boeing models affected by the grounding order, said Thursday that the first flight of one of its aircraft was scheduled for Sunday but did not rule out an earlier return to service.

The US Transportation Safety Board (NTSB), tasked with determining the circumstances surrounding the January 5 incident, told AFP on Friday that one of its investigators was due to return to Boeing's Renton plant in Washington state the same day. The team of investigators will establish a chronology from the production stages to the in-flight accident, the agency said.

A report on the investigation is expected next week.