Mid-air blowout jetliner had been restricted over warning light concerns

Alaska Airlines decided to restrict the aircraft from long flights over water so that the plane “could return very quickly to an airport” if the warning light reappeared, said Jennifer Homendy, chair of the US National Transportation Safety Board (NTSB).

Ms Homendy cautioned that the pressurisation light might be unrelated to Friday’s incident in which a plug covering an unused exit door blew off the Boeing 737 Max 9 as it cruised about three miles over Oregon.

The warning light came on during three previous flights: on December 7, January 3 and finally on January 4, the day before the door plug broke off.

Ms Homendy said she did not have all the details regarding the December 7 incident but specified the light came on during a flight on January 3 and on January 4 after the plane had landed.

The NTSB said the lost door plug was found on Sunday near Portland, Oregon, by a school teacher who discovered it in his back yard and sent two photos to the safety board.

Investigators will examine the plug, which is 26 by 48 inches and weighs 63 pounds, for signs of how it broke free.

Investigators will not have the benefit of hearing what went on in the cockpit during the flight. The cockpit voice recorder – one of two so-called black boxes – recorded over the flight’s sounds after two hours, Ms Homendy said.

At a news conference on Sunday night, Ms Homendy provided new details about the chaotic scenes that unfolded on the plane as it travelled from Portland to Ontario, California.

The explosive rush of air damaged several rows of seats and pulled insulation from the walls. The cockpit door flew open and banged into a lavatory door.

The force ripped the headset off the co-pilot and the captain lost part of her headset. A quick reference checklist kept within easy reach of the pilots flew out of the open cockpit.

The plane made it back to Portland, however, and none of the 171 passengers and six crew members was seriously injured.

Hours after the incident, the US Federal Aviation Administration (FAA) ordered the grounding of 171 of the 218 Max 9s in operation, including all those used by Alaska Airlines and United Airlines, until they can be inspected.

Alaska Airlines, which has 65 Max 9s, and United, with 79, are the only US airlines to fly that particular model of Boeing’s workhorse 737.

United said it was waiting for Boeing to issue a “multi-operator message”, which is a service bulletin used when multiple airlines need to perform similar work on a particular type of plane.

Boeing is working on the bulletin but had not yet submitted it to the FAA for review and approval, according to a source. Producing a detailed, technical bulletin can take a couple of days.

Without some of their planes, cancellations began to mount at the two carriers. Alaska Airlines said it cancelled 170 flights – more than one-fifth of its schedule – by mid-afternoon on the West Coast because of the groundings, while United had scrapped about 180 flights while salvaging others by finding different planes.

The Max is the newest version of Boeing’s venerable 737, a twin-engine, single-aisle plane frequently used on US domestic flights. The plane went into service in May 2017.

Two Max 8 jets crashed in 2018 and 2019, killing 346 people. All Max 8 and Max 9 planes were grounded worldwide for nearly two years until Boeing made changes to an automated flight control system implicated in the crashes.

The Max has been plagued by other issues, including manufacturing flaws, concern about overheating that led the FAA to tell pilots to limit use of an anti-ice system, and a possible loose bolt in the rudder system.