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The NTSB released its prelim on the Jan. 5 depressurization incident in a Boeing 737 MAX 9 where the door plug was blown off at 16,000 feet, reporting that the bolts were missing before the door blew off.
Alaska Airlines flight 1282 in a 737 MAX-9 (N704AL) returned to the Portland International Airport (PDX) after the left mid-exit door (MED) departed the plane, leading to a rapid decompression event. The report said the captain was flying and the first officer was pilot monitoring and the flight crew told investigators the preflight inspection, engine start, taxi, takeoff and departure climb were unremarkable. The crew was cleared to flight level 230 (23,000 feet) and while climbing through about 16,000, the captain said there was a loud bang. The crew said their ears popped and the captain's head was pushed into the heads-up display, pushing his headset up and nearly off of his head. The FO said her headset was completely removed with the rapid outflow of air from the flight deck. Both of the pilots quickly put on their oxygen masks, telling investigators the flight deck door was blown open and it was noisy and hard to communicate.RELATED STORIES:Boeing withdraws 737 MAX 7 FAA exemption requestBoeing CEO admits 'mistake' as FAA launches safety probeLatest on 737 MAX 9 Alaska Airlines emergency - aircraft had prior issues before panel ripped off mid-flightThe flight 1282 crew contacted ATC and declared an emergency, requesting a lower altitude. The flight was assigned to 10,000 feet and the captain requested the rapid decompression checklist, which the FO then received from the Quick Reference Handbook. The FO completed the checklist and the captain flew to PDX, coordinating with ATC. The plane landed on runway 28L and taxied to the gate without further incident.
Investigators removed the cockpit voice recorder and flight data recorder to download the data. The CVR on the plane would retain at least two hours of audio information and the audio from flight 1282 had been overwritten. The prelim noted that this was overwritten because the CVR circuit breaker had not been manually deactivated after landing in time to preserve the flight recording. The FDR data was successfully downloaded with about 1,800 parameters and 68 hours in length, containing 16.5 flights, the oldest of which was from Dec. 30, 2023. According to the FDR data, just over five minutes after takeoff the recorded cabin pressure dropped from 14.09 to 11.64 pounds per square inch when the plane was at 14,830 feet and a speed of 271 knots. The "Cabin Altitude Greater than (andgt;) 10k ft Warning" was activated. The differential pressure was at 5.7 psi and rapidly decreased to 0 over the next few seconds.
The Master Caution activated and the cabin pressure dropped to 9.08 psi at 14,850 ft and 271 knots. Within about 20 seconds, the Master Caution was deactivated. The plane continued climbing, reaching a maximum altitude of 16,320 feet when it began to descend, with an airspeed of 276 kts. The Selected Altitude then changed from 23,000 ft to 10,000 ft, followed by activation of the Master Caution for three seconds. The plane continued to descend and at about 9,050 ft and 271 kts the "Cabin Altitude Greater than (andgt;) 10k ft Warning" was deactivated and the cabin pressure was 10.48 psi.
The report stated that the plane was delivered to Alaska Airlines on Oct. 31, 2023 and put into service on Nov. 11, 2023, with 510 total hours and 154 cycles. The plane was equipped with 178 seats and the MED plugs, each with a standard window, were at row 26 on both sides of the plane. The MED plug was manufactured by Spirit AeroSystems Malaysia on March 24, 2023 and received by Spirit AeroSystems Wichita on May 10, 2023. The MED plug was installed and rigged on the fuselage and then shipped to Boeing on Aug. 20, arriving at the facility on Aug. 31. When the plug is in place, it is secured from moving vertically by four bolts, installed through each upper guide fitting and lower hinge guide fitting. The door plug is only intended to be opened for maintenance and inspection.
The cabin pressurization system, under normal operations, maintains a safe and comfortable cabin pressure altitude at a maximum of around 8,000 feet. The postaccident review of the cabin pressure/cabin altitude data before the decompression event revealed the system functioned per design without cabin altitude or cabin rate exceedances. However, the examination of the plane's maintenance logs found entries indicating the pressure controller light illuminated on three previous flights. The NTSB also examined the two cabin pressure controllers and outflow valve, the oxygen masks, cabin communications and lighting systems, emergency equipment, passenger safety information, flight deck door, aircraft exits and the condition of the cabin interior including all seats, seat tracks, windows, doors and interior paneling.
The damage noted in the cabin included seat rows 25ABC and 27ABC. There was deformation of the doorframe of the outward lavatory and buckling and displacement of the sidewall panels and trim throughout the plane. Investigators did not find any indications of other failures or malfunctions of the plane or any of its systems. The MED plug was found in the backyard of a private residence and shipped to the NTSB's Materials Laboratory for further examination. The plug was mostly intact with some damage from the incident but appeared to be manufactured in accordance with engineering drawings. The two vertical movement arrestor bolts, two upper guide track bolts, forward lower hinge guide fitting and forward lift assist spring were missing and have not been recovered.
The damage noted on the plug was consistent with the MED plug moving upward, outboard and aft during the separation. The report said there was contact damage on the inboard side of the hinge fitting shaft intersecting the vertical movement arrestor bolt hole that was consistent with contact with the MED plug but the hole bore was otherwise undamaged. Investigators also found that the outboard side of the vertical movement arrestor bolt hole was elongated and showed small tensile cracks at the top of the bend, consistent with plastic deformation of the hinge fitting shaft, but the hole bore was otherwise undamaged. The prelim states that the observed damage patterns and the absence of contact damage or deformation around the holes for the vertical movement arrestor bolts and upper guide track bolts in the upper guide fittings, hinge fittings and the recovered aft lower hinge guide fitting indicate that these four bolts were missing before the MED plug was moved vertically and off of the stop pads.
The Manufacturing Records Group traveled to Boeing's Renton, Washington facility to review records for the plane. If defects or discrepancies are found, a Non-Conformance Record or disposition required NCR is created. On Sept. 1, 2023, one day after arriving at the facility, an NCR was created for the plane, noting there were five damaged rivets on the edge frame forward of the left MED plug. The documents and photos show that to access the damaged rivets, the left MED plug needs to be opened, which requires the removal of the four bolts. The rivets were replaced and the work was completed on Sept. 19 by Spirit AeroSystems personnel. Photo documentation from Boeing shows the left MED plug closed without the bolts in three visible locations (one was covered with insulation and not shown in the photo). A Human Performance Investigator joined the group traveling to Spirit AeroSystems to review important documents and observe a door plug installation. The group found no evidence that the left MED plug was opened after leaving Boeing's facility.
After the depressurization incident, Alaska Airlines grounded its fleet of 737-9 aircraft to inspect the MED plugs. Inspections began the next day as the FAA issued an emergency airworthiness directive to require all operators to conduct specific inspections before returning the plane to service. The FAA notified Boeing it was conducting an investigation to determine whether the company failed to ensure that completed products conformed to the approved design, were safe for operation and were in compliance with FAA regulations. Both Alaska Airlines and United, two airlines with a high number of affected aircraft, reported finding loose parts on grounded aircraft. Boeing's CEO Dave Calhoun describes it as a "quality escape." After launching an investigation, the FAA reported it was increasing oversight of Boeing's production and manufacturing.
"It is time to re-examine the delegation of authority and assess any associated safety risks," FAA Administrator Mike Whitaker said. "The grounding of the 737-9 and the multiple production-related issues identified in recent years require us to look at every option to reduce risk. The FAA is exploring the use of an independent third party to oversee Boeing's inspections and its quality system."
On Jan. 21 the FAA published a Safety Alert for Operators for those with the same MED plug as the accident plane, but on a different model. The SAFO was for 737-900ER planes with a similar configuration, requesting operators perform the same inspections. Within days, Boeing announced it was holding a company-wide quality stand down to allow employees to partake in work sessions, giving employees a chance to focus on hands-on learning, reflection and collaboration to identify where quality and compliance need improvement. The company also withdrew its request for an exemption for the 737 MAX 7 to allow the plane to fly passengers while it addressed a design flaw. Boeing opted to withdraw its request and instead focus on finding a solution.
The investigation is ongoing and has been categorized as Class 2. The NTSB has four investigation classes, each involving different levels of severity and determining the resources used and time frame. Class 2 investigations have a broad scope and involve a significant effort to collect evidence in numerous areas and use a serious amount of resources. These types of investigations involve very complex systems, multiple organizations or poor risk controls implemented by the manufacturer, operator, maintainer and regulator. Class 2 investigations have a similar response to Class 1 investigations, which are reserved for very significant accidents or events, but the safety issues may be more limited to a specific plane type or operation.
The investigation will take one to two years to complete and the NTSB will include additional findings and probable cause(s) in the final report.