Alaska Airlines resumes flying Boeing 737 Max 9 plane after door plug blowout

Alaska Airlines resumed flying the Boeing 737 Max 9 following fleet inspections for the first time on Friday after a door plug fell off one of its planes mid-air three weeks ago.

Alaska Flight 1146 -- the airline's first Max 9 flight since the aircraft was grounded -- departed Seattle-Tacoma International Airport Friday afternoon, bound for San Diego.

Two other Max 9 flights are expected to depart Friday afternoon -- one from Las Vegas to Portland, Oregon, and another from Seattle to Ontario, Canada -- according to the airline.

The service return comes two days after the Federal Aviation Administration released final instructions to airlines to begin conducting inspections of their 737 Max 9 planes.

Alaska confirmed in a statement on Wednesday that it planned to return some of its 737 MAX-9 aircraft back to the skies on Friday following a thorough inspection.

"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 airline said Friday. "The individual inspections are expected to take up to 12 hours per aircraft."

The FAA had grounded approximately 171 Max 9s worldwide after the door plug fell off a few minutes after Alaska Flight 1282 took off from Portland International Airport on Jan. 5. Passengers captured footage showing a hole where the door plug came loose. No one was seriously injured and the plane made an emergency landing safely.

The incident remains under investigation by the National Transportation Safety Board.

The FAA also is increasing its oversight over Boeing and began an audit of the company's production and manufacturing.

Boeing said in a statement Wednesday that they would continue to fully cooperate with the FAA and "will continue to cooperate fully and transparently with the FAA and follow their direction as we take action to strengthen safety and quality at Boeing."

"We will also work closely with our airline customers as they complete the required inspection procedures to safely return their 737-9 airplanes to service," the statement continued.

The FAA said it approved its detailed set of inspection and maintenance instructions after a thorough review of data from 40 inspections of grounded planes. The FAA also convened a Corrective Action Review Board made up of safety experts who scrutinized and approved the inspection and maintenance process.

The enhanced maintenance process includes an inspection of specific bolts, guide tracks and fittings, detailed visual inspections of left and right mid-cabin exit door plugs and dozens of associated components, retorquing fasteners and correcting any damage or abnormal conditions.

United COO Toby Enqvist signaled in a note to employees that the company intends to have the 737 Max 9 back in the air on Sunday.

Alaska's fleet is expected to be back to full service in the first week of February, CEO Ben Minicucci said during an earnings call on Thursday. The airline has 65 737 Max 9 planes in its fleet.

ABC News' Amanda Maile and Clara McMichael contributed to this report.