Boeing and supplier Spirit AeroSystems have completed engineering work for a design change to 737 Max mid-cabin door plugs, part of the airframer’s efforts to prevent a repeat of the January 2024 in-flight door-plug failure.
Boeing is expected to start equipping new jets with the revamped design in 2026 and then to issue a service bulletin calling for in-service jets to be equipped with the update, National Transportation Safety Board (NTSB) investigator Clint Crookshanks said on 24 June.
“As of June 2025, the engineering for the design enhancement has been completed, and Boeing and Spirit are continuing certification planning and compliance work that includes notification to the [Federal Aviation Administration],” says Crookshanks.
He spoke during an NTSB hearing into the cause of the January incident, which involved an Alaska Airlines 737 Max 9.
“The design enhancement adds two pieces of structure to the plug that will prevent it from moving, if they’re installed correctly,” Crookshanks says.
After taking off and ascending through about 15,000ft on 5 January 2024, the Alaska 737 Max 9’s left-side mid-cabin door-plug blew out, leaving a gaping hole in the jet. The pilots landed the aircraft and eight people aboard suffered minor injuries.
The NTSB has already attributed the incident to Boeing workers at Renton failing to install four bolts intended to hold the plug in place.
Wichita-based Spirit, which Boeing is working to acquire, produces 737 fuselages and the door-plugs. It ships fuselages, with the plugs already installed, to Boeing.
But in the case of the Alaska jet, Boeing workers had removed and then improperly reinstalled the plug to complete rivet rework.
After the incident, Boeing said it was working on design changes to help prevent a repeat, but few details had previously been revealed.
Crookshanks notes on 24 June that the door-plugs open by moving upward and then rotating out.
Boeing and Spirit’s pending design changes include adding “hammer-shaped secondary retention devices” to the forward and aft sides of the plugs, he says. Those devices “must be rotated into position when completing the installation of the plug”.
The devices engage with fittings on the fuselage to prevent the plugs from moving upward as part of the opening sequence. Also, unless properly installed, they prevent the installation of sidewalls within 737 Max cabins, Crookshanks adds.
The fix also includes adding lanyards atop the door-plug bolts to “permanently secure the bolts to the plug” and “provide a visual indication”, says Crookshanks. “They’ll hang there and be visible to a mechanic that had taken the bolts out.”
Boeing declines to comment about its timeline for rolling out the redesign.
“We at Boeing regret this accident and continue to work on strengthening safety and quality across our operations,” the company says.
