Regulators in the US have launched an official investigation into the Boeing 737 MAX 9 jet after a window and chunk of fuselage blew out of one in mid-air last week.
The Federal Aviation Administration (FAA) is seeing whether Boeing neglected to check if the panel that flew off the Alaska Airlines passenger plane was safe and manufactured to meet regulatory standards.
Its investigation centres on the plugs used to cover emergency exits that the airline doesn’t use.
The FAA grounded 171 Boeing aircraft, most of which are operated by US carriers Alaska Airlines and United Airlines, until inspections of the jets were completed.
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It remains unclear when the planes will get the green light to fly again, in what has been a shaky start to the year for Boeing, the world’s largest aerospace company.
Both Alaska and United said on Monday they had found loose bolts on multiple grounded aircraft during preliminary checks.
They have cancelled hundreds of flights so far, with Alaska saying this equates to 110 to 150 flights per day.
A door plug – which the National Transportation Safety Board (NTSB) says has been recovered – tore off after an Alaska Airlines plane took off from Portland, Oregon, causing depressurisation and forcing pilots to turn back.
The plane, which was flying at 16,000 feet (4,900m) with 171 passengers and six crew on board, landed safely.
The fuselage plug area of the affected Alaska Airlines flight
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Flight suffers mid-air blowout
While only minor injuries were reported from the blowout, the situation could have been “very dangerous,” according to David Learmount, consulting editor at Flightglobal.
The FAA said the incident “should have never happened and it cannot happen again”.
It asked Boeing to respond promptly and report the “root cause” of the problem with the door plug and the steps the company would take to prevent a recurrence.
Boeing said in a statement it would “cooperate fully and transparently with the FAA and the NTSB on their investigations”.
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Spirit AeroSystems, a subcontractor which installed the door plug that blew out of the Alaska Airlines plane, said in a statement that “quality and product integrity” are a priority.
It is the latest issue for Boeing after its 737 MAX aircraft were grounded for a year and a half following two crashes in 2018 and 2019 which killed everyone on board.
Meanwhile, rival manufacturer Airbus is closely monitoring the investigation, as the company reported record annual jet orders for 2023.
One of its United Airlines flights landed prematurely in Florida after pilots were alerted about a possible problem with the door on an Airbus A319. The plane took off “after the issue was addressed,” the airline confirmed.
