U.S. safety regulator to release initial report on Boeing 737 MAX cabin blowout

The Jan. 5 incident has become a full-blown safety and reputational crisis for Boeing that will slow plane production and risks it ceding further market share to rival Airbus . Until now, the independent U.S. National Transportation Safety Board has not said what caused the panel to rip off an Alaska Airlines-operated jet as the plane climbed to 16,000 feet after taking off from Portland, Oregon.

The U.S. National Transportation Safety Board will release its preliminary report on the Jan. 5 Boeing 737 MAX 9 door panel blowout on Tuesday, the agency said. The Jan. 5 incident has become a full-blown safety and reputational crisis for Boeing that will slow plane production and risks it ceding further market share to rival Airbus .

Until now, the independent U.S. National Transportation Safety Board has not said what caused the panel to rip off an Alaska Airlines-operated jet as the plane climbed to 16,000 feet after taking off from Portland, Oregon. The U.S. Federal Aviation Administration grounded 171 of the Boeing 737 MAX 9 planes after the incident, most operated by U.S. carriers United and Alaska Airlines, for inspections.

The NTSB has been focused on how the panel - fitted into this MAX 9 model in place of an optional exit - detached from the plane. The plug is held down by four bolts and then secured by "stop fittings" at 12 different locations along the side of the plug and the door frame. All 12 stop fittings became disengaged during the event, the NTSB said in January. The NTSB has not said if the bolts had been installed or not.

Both United Airlines and Alaska Air said in the days after the blowout that they had found loose parts on multiple grounded MAX 9 aircraft. The plug was manufactured by Spirit AeroSystems, the onetime subsidiary of Boeing that separated from its parent in 2005. The production process involves work at both its Wichita, Kansas facility before plane bodies are railed to Boeing's Renton, Washington, plant.

Boeing shares were down fractionally in morning trading Tuesday; the stock has lost more than 20% of its value since the beginning of the year. (Reporting By David Shepardson)

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