The door plug blew out while the Alaska Airlines aircraft was at 16,000 feet on 5 January, causing all 737 Max 9s to be grounded.
A report by the US National Transportation Safety Board said the aircraft had only entered service on 11 November and that the part which failed was fitted by a supplier before delivery to Boeing’s final assembly line.
The report said four bolts and a hinge were not recovered from the door plug, which was located in a back garden along the flight route.
Hours before the NTSB report was released, the US Federal Aviation Authority said it would examine “unacceptable” quality issues at Boeing and use the “full extent” of its authority to hold the manufacturer accountable.
The 737 Max-9 has now been returned to service after inspections of each aircraft. None are used by UK carriers.
