A top Boeing executive has apologized for the problems highlighted by the mid-flight blowout of a door plug on an Alaska Airlines flight, as two airlines began returning the troubled 737 MAX 9 planes to service. The comments Friday from Stan Deal, CEO of Boeing’s commercial plane unit, came three weeks after a door plug on an Alaska Air flight blew out in mid-air, focusing intense scrutiny on the huge aviation manufacturer and forcing the grounding of 171 planes for safety checks. Alaska Air, a major user of the model, began returning its MAX 9 planes to service Friday, and United Airlines followed on Saturday. “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’s Deal wrote in a note to staff. “Frankly, we have disappointed and let them down. We are deeply sorry.” United Airlines, whose fleet of 79 Max 9 planes is the world’s largest, used the model for a flight Saturday morning from Newark, New Jersey, to Las Vegas, Nevada, with 181 passengers and crew on board. It said other flights would follow. The MAX 9 accounted for around 8 percent of United’s capacity in the first trimester, said CEO Scott Kirby. Analysts say MAX 9-related disruptions could cost airlines millions of dollars. Boeing’s Deal said that in the weeks since the Alaska Airlines incident, several steps had been taken “to strengthen quality assurance and controls.” They included adding new levels of quality checks for affected models and appointing a retired navy admiral, Kirkland Donald, to oversee an independent review of Boeing’s safety and quality practices. This week, 10,000 Boeing employees working on 737 MAX production paused their work for a day to discuss ways of improving safety practices, “a quality stand-down at a scale we have not done before,” according to Deal. He added that along with Alaska Airlines and United, Aeromexico and Turkish Airlines are set to return their 737 MAX 9’s to service “in the coming days.” The US Federal Aviation Administration grounded 171 MAX 9 planes after the January 5 incident, in which a door plug blew out mid-flight. While nobody was seriously injured, inspectors have said the episode could have been catastrophic. The US Transportation Safety Board (NTSB) has been investigating the incident and is expected to report on its findings next week.