United and Alaska find loose bolts on Boeing 737 Max 9s grounded after midair blowout

United Airlines and Alaska Airlines found loose bolts and other hardware in their preliminary inspections of grounded 737 Max 9 airliners, the airlines said late Monday. The Federal Aviation Administration had ordered all 171 U.S.-based Max 9s taken out of service for inspection on Saturday, following Friday evening's emergency landing of Alaska Flight 1282 after an emergency door panel popped out from the fuselage shortly after takeoff from Portland International Airport, leaving an open hole in the side of the plane.

United and Alaska are the only U.S. airlines with Max 9s, one of the larger versions of Boeing's popular 737 aircraft family. (Alaska used 65 Max 9s, while United operated 79.) The FAA and Boeing on Monday sent the airlines detailed procedures for inspecting the grounded planes, though the airlines are awaiting FAA approval to begin the inspections.

In preliminary inspection work, however, United technicians "found instances that appear to relate to installation issues in the door plug — for example, bolts that needed additional tightening," the airline said. The loose bolts were discovered in at least five United Max 9s, in different parts of the door plug, The Air Current reported. Alaska later said its crew discovered "loose hardware" around some Max 9 plug areas.

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Flight 1282 safely returned to Portland with no serious injuries reported, though the National Transportation Safety Board said if the door plug had broken from the Max 9 at cruising altitude of 36,000 feet, not 16,000 feet, it likely would have been a catastrophe.

Alaska had recently placed restrictions on the same brand-new Max 9, barring it from flying to Hawaii, after onboard instruments warned about cabin pressure problems, the NTSB said Monday. Inspectors will look at whether those warnings were related to the failure of the door plug, but they are expected to focus on the manufacturing processes at Boeing and Spirit AeroSystems. Spirit makes the body for the 737 Max, including installing the door plugs on models configured not to use the optional emergency exits, at its plant in Wichita, Kansas.