Four bolts were missing in the door plug that broke off an Alaska Airlines Boeing 737-9 MAX airplane on Jan. 5, according to a National Transportation Safety Board preliminary investigation released Tuesday. The bolts were intended to prevent the door plug from moving but had been removed by Boeing’s fuselage supplier to replace a different part of the plane.
Manufacturing records for the aircraft noted five damaged rivets on an area near the door plug on Sept. 1, 2023, that required replacement. Opening the door plug was required to access the rivets, which resulted in the removal of the four bolts.
Records show the rivets were successfully replaced per engineering requirements on Sept. 19 at Boeing’s fuselage supplier, Spirit AeroSystems, in Kansas. Boeing’s photo documentation showed the door plug was closed without the bolts in three visible locations.
The door plug on the Alaska Airlines Boeing 737-9 MAX was manufactured by Spirit Aerosystems Malaysia on March 24, 2023 and received at Spirit’s Wichita, Kansas, plant on May 10, 2023.
The NTSB found the door plug was manufactured in accordance with the engineering drawings. Damage to the door plug following the accident indicated it had traveled upward, outboard and aft when it broke free of the plane. There was no evidence of pre-existing cracks or damage to the areas where the door plug attached with bolts.
During its build process, a Spirit AeroSystems quality notice indicated the flushness of the seal was off by 0.01 inches, but Spirit determined that no manufacturing rework was required because it was deemed structurally and functionally acceptable without adversely affecting the form, fit or function of the installation, according to the NTSB.
The plug was installed into the fuselage prior to being delivered to Boeing on August 20, 2023, and was delivered to Alaska Airlines October 31. The NTSB investigation found no evidence that the door plug had been opened after leaving Boeing’s facility.
Alaska put the plane into service on November 11 and had flown 510 total hours before the incident on Jan. 5.
A pressure controller light had illuminated on three previous flights of the aircraft, according to maintenance logs the NTSB reviewed. Pressurization on the 737-9 has three redundancies built into it. Before the blowout, the cabin pressure/cabin altitude data control system had functioned per design, as did the oxygen masks for passengers and the cockpit cabin door, the NTSB found.
While the crew cabin door opened after the door plug blew off the plane and depressurized the cabin, it functioned as designed, the NTSB said.
The door plug blowout damaged seat rows 25 A, B and C and 26 A, B and C. Seat 25 A was rotated out toward the opening by 10 to 20 inches, the NTSB investigation found, while the seatback of seat 26 A was rotated forward and outward.
The report comes the same day Federal Aviation Administrator Michael Whitaker testified before a House committee that the agency is “aggressively expanding oversight of new aircraft with increased floor presence at all Boeing facilities” and is “launching an analysis of potential safety-focused reforms around quality control and delegation.”
The FAA and Boeing have both launched investigations into the company’s production lines to determine if the planes are being built to conform to their approved design.
On Sunday, Boeing reported it would delay some of its 737 plane deliveries after an employee with a supplier said holes on the planes’ fuselages had not been drilled to specification. The improperly drilled holes will delay the deliveries of about 50 Boeing planes.
The NTSB said Tuesday its investigation is ongoing.