B39M, en-route, Beaverton OR USA, 2024
B39M, en-route, Beaverton OR USA, 2024
On 5 January 2024, a Boeing 737-9 which had just departed Portland was climbing through 14,800 feet when there was a loud bang followed by a rapid decompression. The cause - the loss of a fuselage plug fitted at an unused door location - was immediately obvious. An emergency was declared and a landing back at Portland was made after a total of twenty minutes airborne. The Investigation is continuing but has found that the fuselage plug involved was installed without being properly secured and noted that the aircraft had only recently been delivered new to the operator.
Description
On 5 January 2024, a Boeing 737-9 (N704AL) being operated by Alaska Airlines on a scheduled domestic passenger flight from Portland to Ontario CA as AS1282 was climbing through 14,800 feet in day VMC six minutes after takeoff when there was a loud bang followed by a rapid decompression. The cause of this was immediately obvious as a door-sized opening had appeared in the rear cabin where there was no evidence of any exit. An emergency was declared and the flight landed back at Portland fourteen minutes later. No passengers were immediately adjacent to the opening and the consequent injuries to seven passengers and one of the cabin crew who were in the vicinity of it were all minor. Minor internal damage occurred to the passenger cabin in the immediate vicinity of the opening and the piece of detached fuselage structure was subsequently found in the back yard of a private house in Beaverton and recovered for examination but most of the associated bolts and fittings were not found.
(Left) left side seat rows 25 and 26 following the disappearance of the fuselage plug.
(Right) an external view of the hole in the left side fuselage. [Reproduced from the Official Report]
Investigation
An Accident Investigation is being carried out by the US National Transportation Safety Board (NTSB). Relevant data has already been downloaded from the FDR. The CVR was downloaded successfully, but it was determined that the audio from the accident flight had been overwritten because the CVR circuit breaker had not been deactivated after the airplane landed in time to preserve the recording. Recorded ATC communications during the flight were also available.
The Captain had a total of “about 12,700 hours” flying experience which included “about 6.500 hours” on type. The First Officer had a total of “about 8,300 hours” flying experience which included “about 1,500 hours” on type.
What Happened
The flight took place with the Captain as PF and after departure from runway 28L was cleared to climb to FL 230. Following the loud bang, when the fuselage panel was lost and decompression occurred, both pilots reported that their ears had popped. The aircraft was climbing through 14,800 feet and reached a maximum altitude of 16,320 feet before the Captain initiated a descent. He reported that when the noise and decompression had occurred, he had been pushed into the HUD and his headset had been dislodged to the extent that it had nearly fallen off his head. The First Officer said that her headset had been completely removed by the rapid outflow of air from the flight deck. Both pilots reported having immediately donned their oxygen masks and added that the flight deck door had been blown open and that it had been very noisy and difficult to communicate with each other. They then contacted ATC, declared an emergency and requested a lower altitude. Clearance to 10,000 feet was initially given and the Captain reported that the ‘Rapid Decompression’ Checklist had then been completed. Further descent and positioning for runway 28L proceeded without further event and landing was followed by taxi to the allocated gate for a normal disembarkation. It was found that following decompression, all cabin crew and passenger oxygen masks had deployed immediately as required. The flight crew reported that the locked flight deck door had opened during the depressurisation and it was noted that in an FCOM revision issued ten days after the accident, Boeing confirmed that this was in accordance with the design of the door.
Damage to the aircraft structure was subsequently assessed by the Investigation to have been “substantial” on the basis that it had affected the structural strength of the aircraft. Damage in the passenger cabin was found to have included damage to seat rows 25ABC and 26ABC (seat 25A was rotated out and rearwards by approximately 10°- 20° towards the opening and the back of seat 26A was rotated forward and outboard towards the opening) as well as deformation of the forward toilet doorframe and “buckling and displacement of the sidewall panels and trim” throughout the aircraft.
Why It Happened
It was immediately recognised that the missing fuselage plug was one of two which had been fitted in place of optional “Mid Exit Doors” (MEDs) on each side of the aircraft to the rear of the wing (see the illustration below). These MEDs are required to meet emergency exit requirements when the longest 737 variants are configured with the maximum possible number of seats which was not the case with Alaska Airlines’ fleet.
It was noted that the airline had, like some other operators, decided not to configure its aircraft to the maximum possible passenger capacity for a number of reasons. These included the reduced weight of (28.5kg for the MED plug instead of 68kg for the door), more passenger comfort because of the lower density of passengers, the provision of a full-sized passenger window and because a plug lacks the complexity of a door which has more parts and associated maintenance concerns.
The location of the MEDs. [Reproduced from the Official Report]
It was established that the “accident MED plug was manufactured by Spirit AeroSystems Malaysia on 24 March 2023” and received by Boeing subcontractor Spirit AeroSystems Wichita on 10 May 2023. It was then installed on the fuselage by them before being shipped to Boeing by rail on 20 August 2023 and arriving at Boeing’s facility at Renton eleven days later.
It was noted that the MED plug is installed in the fuselage by means of two upper guide fittings and two lower hinge fittings. The two upper guide fittings engage with two “upper guide rollers” which are fixed to the upper sides of the fuselage opening and the two lower hinge guide fittings engage with two “lower hinge fittings” which are attached to the lower hinge bracket assemblies at the bottom of the plug. Once the plug is in place, it is secured from moving vertically by four bolts, one at each upper guide fitting and one at each lower hinge guide fitting. These four bolts are secured using castellated nuts and cotter pins. Any outboard motion of the plug is prevented by 12 stop fittings (6 along each forward and aft edge) which are installed on the fuselage door frame structure. The door plug is only intended to be partially opened (by means of the hinge at its base) for maintenance and inspection which is only possible once the four previously-mentioned bolts have been removed.
After detailed examination by NTSB structures and materials specialists of the recovered MED plug, it was found that there was evidence of damage consistent with the MED plug having moved upwards, outboard and aft during its separation from the airframe. Overall, it was concluded that “the observed damage patterns and absence of contact damage or deformation around holes associated with the vertical movement arrestor bolts and upper guide track bolts in the upper guide fittings, hinge fittings, and recovered aft lower hinge guide fitting indicated that the four bolts that prevent upward movement of the MED plug were missing before the MED plug moved upward off the stop pads”.
Separately, when the records associated with the fuselage after arrival at Renton were examined, it was found that a Boeing Non-Conformance Record had been raised noting five damaged rivets on the edge frame forward of the subject MED plug. In order to replace the damaged rivets, it would have been necessary to open the MED plug which would have required removal of the four bolts securing the two upper guide fittings and the two lower hinge guide fittings. The damaged rivets were replaced on 19 September by Spirit AeroSystems personnel but a photo taken later the same day after the MED plug had been closed showed that the three retention bolts which should have been visible (one is obscured by insulation, the three locations are circled) were not fitted (see below).
The MED Plug after rivet rectification work. [Reproduced from the Official Report]
The Investigation is satisfied that the MED plug involved was not opened after the rivet rectification work completed on 19 September so the current working position is that it is possible that the four retention bolts which prevent an installed MED plug from moving vertically upwards were either removed and not replaced at Renton during the work on 19 September or were not fitted when the fuselage arrived from Wichita. It is planned to carry out interviews with Boeing and Spirit AeroSystems’ personnel at a future date and the Investigation also intends to look at Boeing’s SMS and Spirit AeroSystems’ ongoing development of its voluntary SMS program and at the FAA’s involvement in the development of these SMS programs and the level of regulatory oversight applied in each case.
Initial Safety Action taken as a result of the findings of the Investigation so far are noted as having included the following:
- Alaska Airlines immediately grounded its fleet of B737-9 aircraft in order to inspect the MED plugs and began these inspections on 6 January 2024.
- The FAA issued an Emergency AD on 6 January 2024 requiring all operators of 737-9 aircraft to conduct specific related inspections before returning them to service.
- The FAA issued SAFO (Safety Alert for Operators) 24001 on 21 January 2024 for operators of the Boeing 737-900ER which have the same MED plug as the accident aircraft but are a different variant of it. This recommends that such operators inspect, as soon as possible, the four locations where the retaining hardware secures the MED plug to the airframe.
- Boeing has issued multiple Multi-Operator Messages (MOMs) the most recent of which at the time of publication of this Preliminary Report was MOM-24-001001B(R4) dated 24 January 2024 which contains revised instructions for inspecting 737-9 aircraft that have the MED plugs.
A Preliminary Report was published on 6 February 2024.