A raft of issues at Boeing, including those related to inexperienced workers and documentation failures, led to manufacturing oversights that resulted in the January 2024 in-flight failure of a 737 Max 9’s mid-exit door (MED) plug.
Regulatory lapses by the Federal Aviation Administration also played a role, according to the National Transportation Safety Board (NTSB), which on 24 June disclosed its conclusions.
NTSB officials still cannot say which Boeing employee or employees, during assembly, improperly closed the door plug that later blew out at 15,000ft during an Alaska Airlines flight. No records seem to exist.
The plug had been closed by Boeing workers. But the four bolts intended to secure it were not installed prior to the jet being delivered.
“Based on the written statements and [interviews], no one had knowledge [of] who opened or closed the MED plug because… there was no removal paperwork,” says NTSB investigator Pocholo Cruz.
“The safety deficiencies that led to this accident should have been evident to Boeing and the FAA,” says NTSB chair Jennifer Homendy. “An accident like this only happens when there are multiple system failures.”
“The same safety deficiencies could just have easily led to other quality escapes and other accidents,” she adds.
Probable cause, according to the NTSB:
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“Boeing’s failure to provide adequate training, guidance and oversight necessary to ensure that manufacturing personnel could consistently and correctly comply with its parts removal process.”
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“FAA’s ineffective compliance enforcement surveillance and audit planning activities, which failed to adequately identify and ensure that Boeing addressed the repetitive and systemic non-conformance issues.”
The NTSB’s board held the 24 June meeting to review the door-plug incident and to approve both its conclusions and recommendations.
“We at Boeing regret this accident and continue to work on strengthening safety and quality across our operations,” the company says. “We will review the final report and recommendations as we continue to implement improvements.”
The FAA says it “takes NTSB recommendations seriously and will carefully evaluate those issued today. The FAA has fundamentally changed how it oversees Boeing since the Alaska Airlines door-plug accident and we will continue this aggressive oversight to ensure Boeing fixes its systemic production-quality issues”.
NTSB’s recommendations
To Boeing: Finish certifying a door-plug design change to prevent future oversights; Revise processes to clearly specify when parts removal records are required; Improve on-the-job training programme to specify when workers are considered “fully qualified”; Revise safety risk management process to identify “root causes of compliance issues”.
To the FAA: Improve systems and processes so FAA inspectors can better “identify, record, track and effectively address repetitive and system discrepancies and nonconformance issues”; Convene a third party to review Boeing’s safety culture.
The Alaska 737 Max 9’s left-side door plug blew out at nearly 15,000ft on 5 January last year, causing rapid depressurisation. The pilots managed to land back in Portland, with seven passengers and one flight attendant suffering minor injuries.
Investigators quickly determined the plug failed because the securing bolts had not been installed. The incident shone a spotlight on Boeing’s production system, revealing that major quality and safety issues remained despite Boeing’s assurances.
Spirit AeroSystems – which Boeing is now preparing to acquire, as soon as next month – produces 737 fuselages and the mid-cabin door plugs. It delivers them as a single unit with the plugs installed in the fuselages.
The plugs – one on each side of some 737 Max 9s – cover what would otherwise be a pair of emergency exits. Boeing offers the plugs as an option because Max 9s with 179 or fewer seats do not, according to regulations, need those exits.
The NTSB says the Alaska jet’s door plugs had been properly installed when delivered by Spirit to Boeing’s 737 final assembly site in Renton, Washington. But Boeing workers discovered that the jet required rework to five rivets. They removed the plug to access the rivets but failed to install the bolts when replacing the plug.
“We do not know where those bolts are… Those bolts may have been discarded,” says investigator Cruz.
The plug removal should have been associated with Boeing creating a “removal document” – a record providing workers with instructions and flagging the need for a quality inspection, he adds. That document was never created.
When the Alaska jet was being assembled in 2023, Boeing had 24 employees in Renton assigned to “door teams”, which were charged with handling door-related work.
But only one of those workers had previously opened a mid-cabin door plug – and that person was on vacation when the Alaska jet’s plug was removed and then reinstalled, says NTSB investigator Nils Johnson. “Every other person we interviewed… or got a written statement from, denied having any knowledge of… or any previous training on how to open the MED plug.”
NTSB human performance investigator Sabrina Woods notes Boeing lost thousands of experienced workers as it downsized following two 737 Max crashes in 2018 and 2019 and then early during the Covid-19 pandemic.
As demand returned, Boeing rapidly ramped up hiring, taking on thousands of new and inexperienced workers.
“Many of the personnel that were hired came from non-manufacturing sources,” Woods says. “Boeing did not conduct a change management or risk assessment on the changes in the workforce.”
She says Boeing’s on-the-job training did not prepare door-team members adequately to deal with door plugs because the plugs typically do not need to be removed.
