Alaska Airlines passengers had a bizarre fright not long after leaving Portland, Ore., this month. A door-size portion of their jet popped off at 16,000 feet, activating overhead oxygen masks and forcing a hasty return to the airport. No one was seriously hurt. The next day, aviation regulators grounded 171 other 737 Max 9 jets made by Boeing for inspection of bolts securing a panel that seals an unneeded emergency exit opening – the part that blew out suddenly, apparently because of loose bolts. The plane involved went into service eight weeks earlier.

The Federal Aviation Administration (FAA) is examining whether the Seattle company didn't ensure that completed jets matched their approved design. Loose parts were found on some grounded planes by United Airlines and Alaska Airlines, the only U.S. carriers flying the Max 9. The FAA also is scrutinizing the plane's production line because the agency believes there were "significant problems" with the Max 9, as well as "other manufacturing problems." Jennifer Homendy, chair of the National Transportation Safety Board, says it’s pursuing a theory that bolts supposed to hold the door plug in place were never installed.

Poor design of flight controls in an earlier 737 Max model was found to have a role in two crashes in 2018 and 2019 that killed 346 people, so the new incident renews concern among travelers and safety officials. It also disrupts flight schedules. Alaska Airlines, with 65 grounded planes, cancelled about one-fifth of its flights. United has 79 planes out of action. It's unclear when the 737 Max will be allowed to fly again. "The only consideration on the timeline is safety," says Transportation Secretary Pete Buttigieg.

Agency head says: "We are working to make sure nothing like this happens again. Our only concern is the safety of American travelers, and the Boeing 737-9 Max will not return to the skies until we are entirely satisfied it is safe." – Mike Whitaker, Federal Aviation Agency administrator

Manufacturer says: "We are committed to ensuring every Boeing airplane meets design specifications and the highest safety and quality standards." – Recent company statement

Boeing's image setback: Last decade's crashes grounded the 737 Max globally for over 18 months. The company fired its chief executive and agreed to a $2.5-billion settlement with the Justice Department.