Alaska Airlines Boeing 737-9 MAX Incident: Emergency Exit Panel Separates Mid-Flight Leading to Rapid Depressurisation

On January 6th, 2024, an Alaska Airlines Boeing 737-9 MAX, registration N704AL, performing flight AS-1282 from Portland, Oregon, to Ontario, California, experienced a serious in-flight incident shortly after takeoff. The aircraft, carrying 171 passengers and 6 crew members, encountered a rapid depressurisation when a mid-cabin emergency exit panel and its surrounding components separated from the aircraft at approximately 16,000 feet. Despite the alarming situation, the flight crew successfully handled the emergency, returning to Portland for a safe landing around 20 minutes after departure. No injuries were reported.

Incident Overview:

• Initial Climb and Panel Separation: The aircraft was climbing out of Portland’s runway 28L when a loud bang was heard inside the cabin, indicating that one of the mid-cabin emergency exit panels and its holding components had separated from the fuselage. This occurred near seat row 26, an area not typically used as an emergency exit by Alaska Airlines but covered by an interior panel. Passengers seated near the affected area reported a rapid outflow of air, which caused oxygen masks to deploy as the cabin experienced sudden depressurisation. Phones and other loose items were sucked out of the cabin, and a young boy had his t-shirt pulled off by the force of the decompression while his mother held onto him to prevent him from being sucked out as well.
• Emergency Descent: Upon the failure of the panel, the crew immediately donned their oxygen masks and declared an emergency. The captain reported the rapid depressurisation in a noisy transmission, indicating the seriousness of the situation. The aircraft performed an emergency descent to 10,000 feet, where the air is breathable without supplemental oxygen, and began a return to Portland. The aircraft landed safely on runway 28L, approximately 20 minutes after the incident occurred.
• Aircraft Damage: Inspection of the aircraft post-landing revealed that the entire mid-cabin exit panel on the left-hand side had been blown out, leading to significant internal damage. Seat rows 26 and 25 were particularly affected, with seats torn and headrests missing. The cockpit door also flew open during the rapid decompression, as designed, but fortunately, no one was trapped in the lavatory. Additionally, portions of the fuselage and internal cabin panelling were damaged in the immediate vicinity of the missing panel.

Investigation and Grounding:

• NTSB Involvement: The National Transportation Safety Board (NTSB) immediately launched an investigation into the occurrence. John Lovell, an investigator who had previously worked on the Ethiopian Airlines 737 MAX crash in 2019, was appointed to lead the inquiry. Preliminary findings confirmed that a mid-cabin door plug, which is not operational as an exit but is installed for high-density configurations, had separated from the aircraft. The NTSB sought public assistance in locating the missing door plug, and radar data suggested that it might have fallen somewhere between Barnes Road and I-217 in Portland’s Cedar Hills neighbourhood. The missing panel was later recovered and sent to Washington for further analysis.
• FAA Emergency Airworthiness Directive: On January 7th, 2024, the FAA issued an Emergency Airworthiness Directive (EAD 2024-02-51) requiring immediate inspection of all Boeing 737-9 MAX aircraft equipped with mid-cabin door plugs. The directive grounded these aircraft until thorough inspections were completed. This action affected 171 aircraft globally, including Alaska Airlines’ fleet of 65 Boeing 737-9 MAX jets. The inspections were estimated to take between 4 and 8 hours per aircraft. Alaska Airlines reported that 25% of its grounded fleet had already undergone inspections without any findings, and the airline expected to resume normal operations soon.
• Manufacturer and Maintenance Scrutiny: As part of the investigation, the NTSB and Boeing teams scrutinised the manufacturing records and maintenance history of the incident aircraft. The aircraft had entered service with Alaska Airlines on October 31st, 2023, and had been delivered by Boeing on November 11th, 2023. Investigators found no immediate indication that the emergency exit panel had been tampered with during post-manufacturing maintenance. However, records showed that during the manufacturing process, rivets near the mid-cabin door plug had been replaced, requiring the panel to be opened and reinstalled. The investigation will continue to focus on whether this reinstallation was properly completed, as it was found that the bolts intended to secure the panel in place were missing.
• Panel Manufacturing Defect: The NTSB’s investigation revealed that the “stop portions” designed to keep the door plug secure were intact, but the four bolts used to anchor the panel were missing. Evidence indicated that these bolts may have been absent during the aircraft’s manufacturing process, leading to the panel’s separation mid-flight. The investigation also noted damage to the 12 stop pins and fittings that were supposed to prevent vertical movement of the panel, further suggesting a manufacturing defect.

Subsequent Developments:

• Further Findings by Airlines: Following the incident, United Airlines reported finding loose bolts in the door plugs of several of its own Boeing 737-9 MAX aircraft, suggesting that the issue may not have been isolated to Alaska Airlines’ fleet. This prompted additional scrutiny of the manufacturing process for these aircraft.
• Inspection Outcomes: By February 6th, 2024, the NTSB had released a preliminary report summarising the sequence of events. The report confirmed that the cabin depressurisation occurred as the aircraft climbed through 16,000 feet, with the door plug separating due to missing bolts. The flight crew, with over 20,000 hours of combined experience, acted swiftly to manage the situation and return the aircraft to Portland safely. The captain and first officer both reported hearing a loud bang, followed by a rush of air that nearly dislodged their headsets. The crew immediately followed emergency procedures, including descending to a lower altitude and executing the rapid decompression checklist from their Quick Reference Handbook (QRH).
• Impact on Fleet: Alaska Airlines grounded its fleet of 65 Boeing 737-9 MAX aircraft pending inspection and necessary corrective actions. The inspections, alongside those mandated by the FAA for all similarly configured aircraft, ensured that no further incidents of this kind would occur until each aircraft was thoroughly examined.

Conclusion:

The Alaska Airlines Boeing 737-9 MAX incident serves as a stark reminder of the importance of thorough manufacturing and maintenance processes. The separation of the mid-cabin emergency exit panel and the resulting rapid depressurisation posed a serious risk to the safety of passengers and crew. Thanks to the quick response of the flight crew, a potential disaster was averted, and all passengers returned safely to Portland. The investigation continues, with the NTSB and Boeing working to determine the full scope of the issue and prevent similar occurrences in the future.