The door panel that blew off an Alaska Airlines Boeing 737 Max 9 in mid-flight appeared to be missing four key bolts, an initial investigation has concluded.
Photographic evidence showed that the bolts were missing from the door plug, which had been removed to fix rivets that were damaged in the production process, according to independent US National Transportation Safety Board (NTSB).
“The investigation continues to determine what manufacturing documents were used to authorise the opening and closing of the left plug during the rivet rework,” the report said.
The door plug was manufactured by Boeing’s supplier Spirit AeroSystems, and originally installed in the fuselage before being delivered to the manufacturer.
Seven passengers and one flight attendant received minor injuries in the January 5 incident which led to a “rapid decompression” of the cabin as the panel fell off, forcing the aircraft on a US domestic flight to return to Portland in Oregon.
The accident led to 171 of the B737 Max 9s being grounded by the US Federal Aviation Administration (FAA). Aircraft were cleared to return to service late last month after inspections.
Boeing chief executive Dave Calhoun said in response to the NTSB initial findings: “Whatever final conclusions are reached, Boeing is accountable for what happened.
“An event like this must not happen on an airplane that leaves our factory. We simply must do better for our customers and their passengers.
“We are implementing a comprehensive plan to strengthen quality and the confidence of our stakeholders.
“It will take significant, demonstrated action and transparency at every turn – and that is where we are squarely focused.”
The company has implemented a control plan to ensure all 737-9 mid-exit door plugs are installed according to specifications, according to a statement.
Plans to improve overall quality and stability across the 737 production system are also being introduced.
Boeing is opening its factory to 737 customers to conduct their own additional reviews, and pledged to “fully and transparently support the FAA’s investigation, audit and oversight actions”.
Calhoun added: “This added scrutiny – from ourselves, from our regulator and from our customers – will make us better. It’s that simple.”
FAA head Mike Whitaker told the US Congress that inspections of 737 Max aircraft had shown that “the quality system issues at Boeing were unacceptable and require further scrutiny”.
He promised to put more “boots on the ground” in Boeing factories in order to increase scrutiny of the manufacturer.