Spirit AeroSystems, an aerospace supplier, was responsible for the production and initial installation of a fuselage part on a new Boeing 737 Max 9 aircraft that experienced a blowout recently. However, Boeing also plays a significant role in the standard completion process, according to sources. Due to the intricate two-stage installation process, it is anticipated that investigators will scrutinize whether any defects occurred at Spirit's large fuselage factory in Wichita, Kansas, or at the Boeing facility near Seattle.
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Following the incident where an eight-week-old Alaska Airlines plane, carrying 171 passengers and six crew members, lost a door-replacement panel during a flight in the U.S., regulators grounded most Boeing 737 MAX 9s for safety inspections. The plane landed safely despite having a large hole in its side. The 737 MAX 9, currently Boeing's largest single-aisle aircraft with a seating capacity of up to 220 people, includes an optional extra door to provide the required number of evacuation routes when airlines choose to install the maximum number of seats. However, most airlines operating the jet have opted for a more relaxed layout with fewer seats and do not require the additional door, which adds weight and reduces cabin flexibility. Therefore, the door is typically deactivated before delivery using a "plug." Similar optional doors or fill-in replacement structures were also available on a previous model, the 737-900ER.
As part of the manufacturing process, Spirit constructs fuselages for 737s and transports them by train with the special door assembly partially installed. At Boeing's Renton, Washington, plant, the non-functioning door is typically removed, and the gap is used to install interiors. The part is then reinstalled, and the installation is completed. Finally, the hull is pressurized to 150% to ensure everything is functioning correctly.
This process implies that determining where any defect was introduced during assembly may not be straightforward. Door plugs have been utilized for years across the industry to modify aircraft and provide flexible layouts. The investigation will involve structural experts to determine whether design or manufacturing contributed to the accident. The sources, who requested anonymity due to the confidential nature of the investigation, suggested that the part may have been installed or rigged incorrectly. Both Boeing and Spirit declined to comment on the industrial processes.