The mid-cabin exit door (MED) plug that separated from an Alaska Airlines Boeing 737 Max 9 on 5 January appears to not have had retainer bolts reinstalled following work to replace damaged rivets in a nearby area during the aircraft's assembly. The US National Transportation Safety Bureau states in its preliminary investigation report on the accident that the aircraft, registered N704AL (MSN 67501), had its fuselage arrive at Boeing's Renton facility on 31 August. Records show that on 1 September damage to five rivets was noted in an area forward of the left MED plug. Replacing those rivets required the door plug to be removed for access, and this was undertaken by Spirit AeroSystems staff. The NTSB obtained photos shared between Boeing personnel that showed the plug back in place following the repair but noted "evidence of the left-handed MED plus closed with no retention hardware (bolts) in the three visible locations" required to secure the plug to the fuselage. "These Boeing personnel were discussing interior restoration after the rivet rework was completed during second shift operations that day," the report states. "The investigation continues to determine what manufacturing documents were used to authorise the opening and closing of the left MED plug during the rivet rework." Fleets data shows that the aircraft was delivered to Alaska Airlines on 31 October and although the aircraft underwent a wifi installation with AAR in Oklahoma City after delivery, the NTSB found "no evidence that the left plug MED was opened after leaving Boeing's facility." The investigator adds that it will schedule interviews with Boeing and Spirit personnel at a future date and will be examining both companies' safety management systems. It will also "assess the FAA’s involvement in the manufacturers’ development of their respective SMS programs and the level of oversight applied to each". Boeing responded soon after the report was published, issuing a statement in which it restated several actions taken to strengthen its quality control measures. "Whatever final conclusions are reached, Boeing is accountable for what happened," says the company's president and chief executive Dave Calhoun. "An event like this must not happen on an airplane that leaves our factory. We simply must do better for our customers and their passengers. We are implementing a comprehensive plan to strengthen quality and the confidence of our stakeholders. It will take significant, demonstrated action and transparency at every turn – and that is where we are squarely focused.” Spirit issued its own brief statement following the release of the report. "As we review the NTSB’s preliminary report, we remain focused on working closely with Boeing and our regulators on continuous improvement in our processes and meeting the highest standards of safety, quality and reliability," it states.

WestJet will start flights to Seoul, Reykjavik and Edinburgh as part of a wider network expansion announced for its 2024 northern summer schedule. The Canadian carrier says it will begin flights from Edmonton to Atlanta, Nashville and San Francisco; and from Calgary to Seoul, Reykjavik in Iceland and Deer Lake in Newfoundland. The other routes include Vancouver-Detroit, Halifax-Edinburgh, St. John's-London Gatwick and Regina-Minneapolis. Some of the routes that will be reinstated include flights from Halifax to Dublin, London Gatwick and Orlando; and from Toronto to Dublin, Edinburgh, Providenciales, Puerto Plata, and Moncton. The airline adds that there will be more flights between Hawaii and Canada and it will be the only carrier providing year-round service between Canada and Maui-Kahului, from both Calgary and Vancouver. WestJet says it will increase transborder seat capacity by 26% this summer, including 70% growth to its codeshare partner Delta Air Lines hubs. "This summer will see the return of seasonal, leisure-focused flying, to connect Eastern and Western Canada, and substantial growth in service to sun destinations, further cementing our position as Canada's leading leisure airline," says WestJet Group Executive vice-president and chief commercial officer John Weatherill.

Boeing has warned that "some near-term 737 deliveries" could be delayed as a result of 50 undelivered aircraft at its assembly line in Renton requiring rework because of miss-drilled holes in the single-aisles' fuselages. In a letter to employees dated 4 February, Boeing Commercial Airplanes chief executive Stan Deal states that the US airframer will spend "several days in the Renton factory this week" to inspect aircraft and complete rework. Boeing on 1 February was informed by a supplier that "two holes may not have been drilled exactly to our requirements" on "some fuselages", Deal says, without naming the supplier. Spirit AeroSystems – sole fuselage supplier for the 737 programme – confirms that one of its employees identified the issue. "Once notified, we began immediate actions to identify and implement appropriate repair solutions," the aerostructures manufacturer tells Cirium, adding: "We are in close communication with Boeing on this matter." Deal says: "We currently believe we will have to perform rework on about 50 undelivered airplanes." He notes that the condition is "not an immediate flight safety issue" and in-service 737s can continue operating as normal. "While this issue could delay some near-term 737 deliveries, this is the only course of action given our commitment to deliver perfect airplanes every time," he states, adding that time dedicated to address the issue is aimed at demonstrating "the premium we place on quality, safety and, ultimately, stability in our factories". Deal acknowledges that "many employees" during a recent quality stand down "voiced frustration with travelled work and how unfinished jobs – either from our suppliers or within our factories – can ripple through the production line. "These employees are absolutely right. We need to perform jobs at their assigned position." He adds "we have to maintain this discipline within our four walls" and hold suppliers to the same standard, noting that a major supplier was recently instructed to hold its shipments until all jobs have been completed to specification. "While this delay in shipment will affect our production schedule, it will improve overall quality and stability." Deal says that the time allocated to addressing miss-drilled fuselage holes will be used to "catch up on unfinished jobs across all 737 factory positions". Deliveries of unaffected aircraft will meanwhile continue as planned. The fuselage hole rework comes amid increased scrutiny on Boeing after a number of quality issues. Last year, Spirit detected a production issue with attachment fittings for 737 vertical stabilisers and subsequently found miss-drilled holes in aft pressure bulkheads, which required extensive inspections and rework both at the aerostructures manufacturer and Boeing. The inflight loss of a door plug on a brand new 737-9 operated by Alaska Airlines on 5 January – attributed to the failure of fittings in the door plug installation – prompted temporary grounding and inspection of international Max 9 jets with the same door configuration, and an investigation by the US Federal Aviation Administration in manufacturing practices at Boeing. As part of the investigation, the FAA will not allow Boeing to expand Max production rates for the time being.