The investigation into the midair blowout of a fuselage door plug on Alaska Airlines Flight 1282 on Jan. 5 has confirmed that four bolts that should have kept the door plug in place were not installed when Boeing delivered the aircraft, the National Transportation Safety Board said Tuesday in a preliminary report on the incident.
"Four bolts that prevent upward movement of the mid exit door plug were missing," the report states.
The NTSB said the door plug was opened at Boeing's Renton factory so a team from supplier Spirit AeroSystems of Wichita, Kan., could repair damaged rivets adjacent to the door plug on the 737 MAX 9 jet.
The fix required removal of insulation and interior panels at that location and the opening of the door plug. After the rivets were repaired, a Boeing team worked to restore the interior.
Federal regulations require that every manufacturing job that goes into assembly of an airplane be documented. And critical tasks have to be signed off by quality inspectors.
A month after the blowout, though, Boeing has not provided the NTSB with documentation about who opened and re-closed the door plug, how exactly it was done and with what authorization.
"The investigation continues to determine what manufacturing documents were used to authorize the opening and closing of the left mid exit door plug during the rivet rework," the NTSB report states.
The sequence of a fateful repair
The NTSB said the door plug was manufactured by Spirit in Malaysia and shipped to Wichita in May 2023. There it was installed on the MAX 9 fuselage destined for Alaska Airlines.
One minor quality issue was noted in Spirit's records, but did not require rework.
Otherwise, there were no non-conformances noted before the fuselage arrived by rail on Aug. 31 at Boeing's Renton final assembly plant from Spirit AeroSystems in Wichita, with the door plug fully installed.
The following day, damage to five rivets on the frame around the door plug was recorded.
The work to repair those rivets wasn't done until mid-September. By then the fuselage interior insulation and sidewalls had been installed. To do the repair on the rivets, the insulation and sidewalls covering the door plug were removed and the door plug opened.
"To open the mid exit door plug, the two vertical movement arrestor bolts and two upper guide track bolts had to be removed," the NTSB report states.
A team of Spirit mechanics completed repair of the rivets on Sept. 19.
After the rivet rework was completed by Spirit during second shift that day, Boeing personnel discussed restoring the interior.
A photo taken by a Boeing mechanic was texted to another at about 6:40 p.m. that day, apparently to provide the recipient a progress report on the restoration work. It shows the door plug back in place and closed, but with no retention bolts in place.
"Photo documentation obtained from Boeing shows evidence of the left-hand mid air door plug closed with no retention hardware (bolts) in the three visible locations," the report states. (Some insulation hung down and obscured the view of the location of the fourth bolt in the photo.)
"Boeing has serious work to do to rebuild its safety culture and the confidence of its customers and the flying public," U.S. Rep. Rick Larsen of Everett, the leading Democrat on the House Transportation Committee, said in a statement Tuesday. "Failure to re-install bolts on a safety-critical component of this 737 MAX 9 aircraft is a serious error that signals larger quality control lapses that must be corrected."
After noting that it had received no documentation on how this work was done, the NTSB states that interviews of Boeing and Spirit AeroSystems personnel "will be scheduled at a future date."
Citing a person familiar with how the work was done, The Seattle Times reported last month that it was Boeing mechanics who improperly reinstalled the door plug.
After the NTSB report was posted online, Boeing CEO Dave Calhoun said in a statement that "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," Calhoun added. "We are implementing a comprehensive plan to strengthen quality."
Spirit AeroSystems spokesperson Joe Buccino said the 737 MAX fuselage supplier is "working closely with Boeing and our regulators on continuous improvement in our processes."
Terrifying in-flight incident
The explosive opening of a door-size hole in the passenger cabin of a 737 MAX 9 at 16,000 feet over Portland terrified passengers. One mother held tight to her teenage son, seated just in front of the hole, as the clothing was ripped from his upper body and blown out into the void.
The NTSB report says the boy's seat was pulled backward and out "approximately 10 degrees — 20 degrees toward the opening."
The incident, which could have been catastrophic if it had occurred higher up, stirred outrage toward Boeing and brought the FAA's oversight of airplane production under scrutiny.
The door plug that blew out is a panel used to seal a fuselage cutout for an optional emergency exit door halfway between the wings and the rear passenger exit door.
An actual door is installed only by a few low-cost carriers with high-density seating; most airlines, including Alaska and United, instead have this permanent plug. To a passenger seated at that location, it looks like just another cabin window.
The door plug is held in place while pressurized by 12 metal stop pads. The only way it could have blown out is if the plug had shifted upward so that the stop fittings on the plug had moved above the stop pads on the door frame. That's how it is opened for maintenance.
The four bolts the NTSB says were missing should have been installed to ensure it couldn't physically move upward.
Another photo in the NTSB preliminary report shows deformation on the lower edge of a stop fitting on the door plug and a matching deformation on the upper edge of the corresponding stop pad.
It illustrates clearly how the two were misaligned and barely still in contact when the explosive blowout occurred and finally ripped the fitting over and away from the stop pad.
Yet another photo in the report shows one of the four critical bolt holes, this one on the aft upper side. The paint in the hole was intact and pristine with no evidence of any contact damage. Likewise for the other holes.
NTSB investigators concluded from the "absence of contact damage or deformation around holes" that the bolts simply were not in place when the door blew violently out.
The plane was delivered to Alaska on Oct. 31 and flew for just over two months before the incident.
With the four retaining bolts missing, vibration during landing and takeoff over those two months must have gradually shifted the plug upward, until on Jan. 5, the edges of the 12 stop fittings and corresponding stop pads were barely in contact and the plug was primed to blow out.
Scrambling to ensure it cannot happen again
Since the in-flight incident, Boeing has scrambled to shore up production quality both inside its MAX assembly plant in Renton and at the Wichita factory where its supplier Spirit AeroSystems builds the entire fuselage frame, including initial installation of the door plug.
Boeing added quality inspections in both factories, invited airlines to send teams to inspect their planes while being built, and promised an independent external audit of its quality management.
Last month, Boeing held a "Quality Stand Down" in Renton, pausing all work in the factory for a day so employees could join discussion sessions on how to enhance quality and safety.
And the FAA has moved aggressively to show it is strengthening oversight.
The day after the incident, the safety agency grounded all MAX 9s in the U.S. pending inspections of the door plugs. (More than 90% have completed those inspections and are back flying for United Airlines and Alaska, the two U.S.-based carriers operating that MAX model.)
The FAA then told Boeing it won't allow any increase in the current MAX production rate of 38 jets per month until it's satisfied the quality control is improved.
It has launched an investigation into whether Boeing failed to maintain its quality system in accordance with federal regulations. And the FAA has increased its on-site presence at both the Renton and Wichita assembly plants.
Sen. Maria Cantwell, D-Wash., chair of the Senate Commerce Committee, said the NTSB finding that the four critical bolts were missing "underscores how important quality assurance is from manufacturers and how important quality control inspections from both manufacturers and the FAA are to the safety process."
Based on similar NTSB investigations, a final report establishing the probable cause of the Alaska Flight 1282 incident and issuing safety recommendations may take another year or so.
