WASHINGTON — The National Transportation Safety Board (NTSB) Tuesday said the probable cause of last year’s in-flight mid-exit door (MED) plug blowout on an Alaska Airlines (AS) Boeing 737 MAX 9 was Boeing’s failure to “provide adequate training, guidance and oversight” to its factory workers.

The NTSB also found the Federal Aviation Administration (FAA) was ineffective in ensuring Boeing addressed “repetitive and systemic” nonconformance issues associated with its parts removal process.

The NTSB also concluded that in the two years before the accident, Boeing’s voluntary safety management system, or SMS, was inadequate, lacked formal FAA oversight, and did not proactively identify and mitigate risks. The investigation found that accurate and ongoing data about overall safety culture is necessary for an SMS to be successfully integrated into a quality management system.

2024 started downhill for Boeing. On January 5, an Alaska Airlines (AS) Boeing 737-9 experienced a mid-flight blowout of the MED. The incident on flight 1282 occurred on a flight from Portland to Ontario at 16,000 feet, causing an uncontrolled aircraft decompression. Thankfully, no fatalities were registered, and the aircraft landed safely.

The MED plug was found in a Portland neighborhood two days after the accident. When investigators examined the recovered plug, they found evidence that the four bolts needed to secure the plug were missing before the accident occurred. Without the bolts, NTSB investigators found the unsecured plug “had moved incrementally upward during previous flight cycles” until it departed the airplane during the accident flight.

The door plug blowout prompted immediate safety reviews, which uncovered missing maintenance paperwork and saw Boeing divulge secure information, among other things. Consequently, the FAA grounded all Boeing 737-9 jets for three weeks.

The NTSB released a preliminary investigation on February 6, 2024, which stated that the damage patterns on the door plug indicated that the four bolts intended to secure the door plug had been missing when the accident occurred. They also reviewed Boeing records that showed evidence that the plug had been installed with no bolts.

NTSB Investigation Findings

“The safety deficiencies that led to this accident should have been evident to Boeing and to the FAA — should have been preventable,” NTSB Chairwoman Jennifer Homendy said. “This time, it was missing bolts securing the MED plug. But the same safety deficiencies that led to this accident could just as easily have led to other manufacturing quality escapes and, perhaps, other accidents.”

According to the NTSB investigation, the facts that led to the MED blowout are as follows:

1. The airplane had been delivered to AS three months before the day of the MED blowout.
2. The door plug was opened without the required documentation in Boeing’s Renton, Washington, factory on Sept. 18, 2023, to perform rivet repair work on the fuselage.
3. The door plug was closed the following day.
4. While Boeing’s procedures called for specific technicians to open or close MED plugs, none of the specialized workers were working at the time the door plug was closed.
5. The absence of proper documentation of the door plug work meant no quality assurance inspection of the plug closure occurred.
   

The investigation also highlighted the need for additional training on flight crew oxygen masks and their communication systems and the need for greater voluntary use of child restraint systems by caregivers of those under two years of age.

The NTSB issued new safety recommendations to the FAA and Boeing. Previously issued recommendations were reiterated to the FAA, Airlines for America, the National Air Carrier Association and Regional Airline Association.

The executive summary of the report, including the findings, probable cause and safety recommendations, is available online​. Additional material, including the preliminary report, previously issued safety recommendations, news releases, the public docket, investigative updates and links to photos and videos, is available on the accident investigation webpage.

The final report will be published in the coming weeks on NTSB.gov.