A door panel that detached from a Boeing 737 MAX 9 plane mid-flight on 5 January appeared to be missing four key bolts, according to a preliminary report from US investigators.
Photo evidence shows bolts were missing from the door plug, which had been removed to fix rivets that were damaged in the production process, according to the independent National Transportation Safety Board (NTSB).
“The investigation continues to determine what manufacturing documents were used to authorize the opening and closing” of the plug during the rivet rework,” the report said.
The US Federal Aviation Administration (FAA) grounded 171 Boeing 737 MAX 9 planes for inspections after the incident involving a new aircraft operated by Alaska Airlines.
Most of the grounded planes were operated by Alaska and United Airlines and were cleared to return to service late last month.
Boeing President and Chief Executive Dave Calhoun said: “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.”
Boeing added it has “implemented a control plan to ensure all 737-9 mid-exit door plugs are installed according to specifications.”
The NTSB has been focused on how the panel – fitted in 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.
The NTSB did not recover the bolts at the scene and conducted extensive testing and analysis to determine if they had been present before the crash or had come undone during the incident.
All 12 stop fittings became disengaged during the event, the NTSB said last month.
The plug was manufactured by Spirit AeroSystems, the onetime subsidiary of Boeing that separated from its parent in 2005.
The part was produced at its facilities in Malaysia and delivered to Spirit’s facility in Kansas last May.
The door plug in question arrived in Boeing’s Washington plant on 31 August.
A record created on 1 September showed damage to the rivets on the plug’s frame, the NTSB report said.
Four bolts had to be removed to repair the damage, and a photo in the report shows three visible locations where bolts are missing, with the fourth location covered by insulation.
“Photo documentation obtained from Boeing shows evidence of the left-hand MED [mid exit door] plug closed with no retention hardware (bolts) in the three visible locations,” the report said.
Both United Airlines and Alaska Airlines said in the days after the blowout that they had found loose parts on multiple grounded MAX 9 aircraft.
The FAA has taken a harder line than in the past on Boeing.
Last month, the regulator stopped the company from expanding production of its 737 MAX planes due to the quality issues.
This means it can continue producing MAX jets at its current rate, but it cannot increase that rate.
“I certainly agree that the current system is not working, because it’s not delivering safe aircraft,” FAA Administrator Mike Whitaker told politicians. “So we have to make changes to that.”
The FAA is conducting an audit of 737 MAX manufacturing, which is looking at all elements of production at Boeing and fuselage production at its supplier Spirit.
Boeing shares have lost more than 20% of their value since the beginning of the year.