US launches Boeing investigation after blowout

Airline regulators in the US have formally launched an investigation of Boeing's processes, after a door plug blew off one of its planes.

The Federal Aviation Administration (FAA) said it would examine whether Boeing failed to ensure its completed jets matched their approved design.

The FAA had already grounded most of the 737 Max 9 fleet for inspection.

The reviews after the emergency on the Alaska Airlines flight have uncovered issues such as loose bolts.

"This incident should have never happened and it cannot happen again," the FAA said. "Boeing's manufacturing practices need to comply with the high safety standards they're legally accountable to meet."

Boeing said it would "cooperate full and transparently" with the investigation.

Company boss, Dave Calhoun, had previously described the problem as a "quality escape". It means the incident was caused by some failure in quality control in the plane, which had been in service for just eight weeks before the blowout.

He told CNBC there were still questions that needed to be answered about how the incident was allowed to happen.

"What broke down in our gauntlet of inspections? What broke down in the original work that allowed for that escape to happen," he said.

The door plug is a piece of fuselage, with a window, that fills the space where an emergency exit would be in certain configurations.

The part broke off a Boeing 737 Max 9 jet that was operated by Alaska Airlines minutes after its take-off from Portland, Oregon on Friday.

No serious injuries were reported after an emergency return to the airport but the FAA on Saturday grounded 171 planes that had the same door plug installed.

It is unclear when the planes will be allowed to fly again.

Transportation Secretary Pete Buttigieg has said the government would not be rushed into clearing the grounded planes, despite hundreds of flight cancellations.

He said the aircraft "need to be 100% safe".

"The only consideration on the timeline is safety," he said. "Until it is ready, it is not ready. Nobody can or should be rushed in that process."

Alaska Airlines has cancelled about 20% of its flights after 65 of its Max 9s were grounded. United Airlines, the other major US 737 MAX 9 operator, has 79 of the planes in its fleet out of action.

It said it expects "significant" cancellations on Thursday after 167 flights did not go ahead on Wednesday.

Alaska Airlines said it still needed revised inspection and maintenance instructions from Boeing, which must be approved by the FAA before it can begin flying the planes again.

"We will only return these aircraft to service when all findings have been fully resolved and meet all FAA and Alaska's stringent standards," the airline said.

Both Alaska and United said on Monday they had found loose parts on a number of the grounded aircraft.

The part which fell off was eventually found in a teacher's back garden without its four bolts.

Jennifer Homendy, the chair of the National Transportation Safety Board, which is investigating what happened on the flight, said on Monday it was possible the bolts were missing from the start but they might have come off in the descent.

The incident has revived scrutiny of Boeing, which has been trying to repair its reputation after crashes in 2018 and 2019 involving a different version of the 737 Max plane killed 346 people.

Its popular 737 Max planes were subsequently grounded globally for more than 18 months. It has reported a string of smaller issues as production resumed.