A door panel that flew off a Boeing 737 MAX 9 jet mid-flight on January 5 appeared to be missing four key bolts, according to a preliminary report from US investigators that provided the first official look into how the frightening mishap took shape.
Lawmakers and the flying public are desperate for answers to what caused the panel to rip off a brand-new Alaska Airlines-operated jet, in what has turned into a full-blown safety and reputational crisis for Boeing.
Photo evidence released on Tuesday 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 US National Transportation Safety Board report.
"The investigation continues to determine what manufacturing documents were used to authorise the opening and closing of the plug during the rivet rework," the report said.
The US Federal Aviation Administration grounded 171 of the Boeing 737 MAX 9 planes after the incident, most operated by US carriers United Airlines and Alaska Airlines, for inspections. Those planes were cleared to return to service in late January.
Boeing President and CEO Dave Calhoun said on Tuesday: “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 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.
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 in January.
The plug was manufactured by Spirit AeroSystems, the one-time subsidiary of Boeing that separated from its parent in 2005. The part was produced at its facilities in Malaysia and delivered to Spirit's Wichita, Kansas, facility in May 2023.
The door plug in question arrived in Boeing's Renton, Washington, plant on August 31. A record created on September 1 showed damage to the rivets on the plug's frame, the NTSB report says. 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 plug closed with no retention hardware (bolts) in the three visible locations," the report says. MED is short for "mid exit door."
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 FAA has taken a harder line than in the past on Boeing. In late January, it barred Boeing from expanding production of its 737 MAX planes due to quality issues. That 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 lawmakers on Tuesday. "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 were up 1.3 per cent on Tuesday afternoon. The stock has lost more than 20 per cent of its value since the beginning of the year.