NTSB releases preliminary report into Alaska mid-flight blowout incident
The NTSB (National Transportation Safety Board) have released their preliminary report of the Alaska Airlines 1282 mid-flight blowout incident. This incident saw a Boeing 737 MAX 9, operated by Alaska Airlines, suffer a fuselage blowout mid flight on January 5th 2024. Read more about it here.
Throughout this article, we’ll take you through the timeline of events and what the NTSB have concluded thus far. Scroll down for the ATC audio of the incident flight. Please note, this is a preliminary report and the full final report will be published by the NTSB at a later date.
Preface
An aircraft’s interior cabin is pressurised to an equivalent altitude of around 8,000ft. This way, passengers can breathe easy and feel comfortable when flying. A safe breathing altitude is deemed to be a maximum of 10,000ft. A flight’s altitude when cruising can vary, however the upper limit of long haul flights is 42,000ft, although typically this will be lower. The pressure inside the cabin relies on the exterior walls and doors to maintain a tight seal, ensuring the cabin reaches no higher than the set cabin altitude (8,000ft). In this flight, a door plug blew out, causing the cabin pressure to decrease and the cabin altitude to rise rapidly to meet the outside altitude. This event is known as a “Rapid Depressurisation”. It is an emergency event and usually requires a ‘mayday’ call to ATC as it poses a danger to life.
MED Plug
So, what actually happened on this flight? Put simply, an aircraft can be customised to an operators liking, just the same as you can choose to have heated seats in your new car. One of the options was to have a mid exit door, or MED, for short. A mid exit door is a door in the centre of the aircraft that can be used, but depending on seat configuration inside the cabin, some operators may choose to have a “plug” installed instead. It’s essentially a blanking plate (it’s a little more complicated but good enough for now) - you won’t be able to tell from the inside, but this is where a door could go if the operator chose one. On this flight, this plug blew out and the findings from the report below will help you to understand why.
Terminology
The terminology used can be confusing. We have explained it below:
Altitude - The height above the ground measured in feet (ft) (it gets more complicated than this but this explanation is good enough for this article)
Cabin Pressure - The pressure inside of the aircraft, measured in pounds per square inch, or, PSI
Heading - Direction of aircraft measured in the 360 degrees of a compass
Differential Pressure - The difference in pressure between the outside atmosphere and the inside of the cabin
Kts - Knots. A measurement of speed used for flight
Master Caution - A warning sound & light installed in Boeing aircraft to alert the flight crew of an issue requiring urgent attention.
PDX - Portland International Airport (PDX is the IATA code)
ATC - Air Traffic Control
NTSB - National Transportation Safety Board, the American body responsible for investigation air incidents
FAA - The Federal Aviation Administration, the American authority responsible for aviation safety
Pilot Flying/Monitoring - Throughout a flight, roles are equally divided between the captain and first officer to manage workload. Pilot flying is responsible for ensuring the aircraft is flying and safe, whilst pilot monitoring communicates with ATC, runs checklists and monitors instruments.
Timeline
On January 5th 2024, around 17:06 local time Alaska Airlines 1282, a Boeing 737 MAX-9, took off from Portland International Airport (PDX). At around 17:12, the recorded cabin pressure dropped rapidly at an altitude of a flight level just below 15,000ft. This is the timeline of events:
17:06:47 The airplane departed PDX runway 28L.
17:12:33 The recorded cabin pressure dropped from 14.09 PSI to 11.64 PSI, at an outside flight altitude of 14,830ft. Speed was 271kts. The altitude warning activated and sounded. Differential pressure was at 5.7 PSI and rapidly decreased to 0 PSI over the next few seconds.
17:12:34 Master caution activated. Cabin pressure dropped to 9.08 PSI at an outside flight altitude of 14,850ft. Speed was 271kts. Aircraft heading was 123°.
17:12:52 Master caution was deactivated [most likely by one of the flight crew].
17:13:41 Aircraft continued to climb, reaching a maximum altitude of 16,320ft. Airspeed was 276kts. Heading 120°.
17:13:56 Selected altitude changed from 23,000ft (cleared altitude level from ATC), to 10,000ft (the safe breathing altitude).
17:14:35 Master caution activated for 3 seconds.
17:16:56 Aircraft began a left turn from a heading of 121°. Altitude was now recorded at 10,120ft.
17:17:00 Aircraft descended below 10,000ft.
17:18:05 Aircraft altitude now at 9,050ft. Speed at 271kts. Altitude warning deactivated, now that the aircraft descended below 10,000ft. Cabin pressure was at 10.48 PSI.
17:26:46 Aircraft landed safely on runway 28L at PDX.
Key information from the report
Accident aircraft registration N704AL delivered to Alaska Airlines October 31, 2023.
Aircraft put into service by Alaska on November 11, 2023. Aircraft had accumulated 510 hours and 154 cycles at time of incident.
The 3 previous flights prior to the accident had noted that the pressure controller light had illuminated, suggesting an issue with the ability to keep the cabin pressurised.
Main findings of report
The NTSB has been looking extensively into this incident. When it occured, many operators of the 737 MAX-9 chose voluntarily to ground their 737 MAX aircraft (Alaska Airlines being one of them), however it wasn’t long before the FAA and other authorities chose to mandate a grounding until adequate safety inspections could be made. They were cleared for flight last week.
Both of the pilots held current Airline Transport Pilot Licenses (ATPL) - the captain accumulated 12,700 hours of flight experience with 6,500 of these were in the 737 MAX. The first officer (FO) accumulated 8,300 hours with 1,500 in the 737 MAX. The captain was pilot flying and FO was pilot monitoring. They both stated that all procedures running up to the incident were '“unremarkable”. Upon reaching 16,000ft, the crew heard a loud bang, the cockpit door swung open and their ears popped. They immediately put on their oxygen masks, as procedures require.
They then proceeded to declare a mayday (VIDEO BELOW), requesting a lower altitude. They were cleared to 10,000ft.
The NTSB has concluded that the system responsible for the pressurisation of the aircraft was working normally, aside from the light mentioned in ‘Key information from the report’ section above. NTSB testing was extensive and found that all other systems functioned as intended. The cockpit door opening was a function by design.
There was damage to the passenger cabin but no indications of any other failures.
The NTSB recovered the MED plug that failed, causing the incident. They noted damage from the event, although it all seemed to be manufactured in accordance with the engineering drawings.
Following a thorough investigation of the parts and aircraft, the NTSB have indicated that certain locking bolts were missing from the door prior to the incident. These bolts are responsible for keeping the MED plug down and in a locked position. The NTSB has conducted further investigation to determine how these bolts were missing. It was found that upon the aircraft arriving at Boeing’s facility in Renton, Washington, a ‘Non-Conformance Record’ (NCR) was generated. These records are generated when employees find something that does not conform to standards and must be addressed.
The NCR
The NCR was generated due to issues with damaged rivets just forward of the MED plug area. In order to fix these rivets, the MED plug had to be removed, along with the locking bolts responsible for the incident.
See below photo showing the damaged rivets that had to be fixed, prior to the incident.
The rivets were repaired successfully, however on replacement of the door the locking bolts were not replaced. This is evidenced from a post repair photo taken by engineers - seen below. This photo was taken just prior to interior restoration on September 19th 2023. The blue circles show missing locking bolts.
The NTSB followed up on their investigations by tracing where the aircraft went after Boeing’s facility, and they were satisfied that the MED plug was not removed again, after leaving Boeing’s facility.
The NTSBs investigation is ongoing and is subject to change. It’s important to remember that this can be a great lesson learned and it is thankful no one was seriously injured during this event. Boeing have accepted responsibility and are working to improve their safety processes throughout the company.
See below video of ATC communications of the event. Some audio is inaudible. Please lower your volume.
See below a great video from Petter over at Mentour Pilot, on what happens in the cockpit during a rapid depressurisation.