The door cover used to replace the emergency exit on the Boeing 737 MAX 9 that encountered an incident may have a design or installation flaw.
The oxygen masks were deployed when Alaska Airlines flight 1282 had its door burst open on January 5th. (Video: CBS)
The Boeing 737 MAX 9 of Alaska Airlines took off from Portland International Airport in Oregon on January 5th, heading to California, USA. However, 20 minutes later, the aircraft carrying 177 passengers had to make an emergency landing when the door cover on the fuselage burst open, creating a large opening equivalent to an emergency exit.
No passengers were seriously injured in the incident. The U.S. National Transportation Safety Board (NTSB) has launched an investigation into the incident involving the Boeing 737 MAX 9 at an altitude of over 4,800 meters. Jennifer Homendy, the chair of the board, stated that the situation could have been much worse if it had occurred at a higher altitude. If the fuselage failure had happened at a cruising altitude of about 10,000 meters, the cabin would have immediately lost pressurization, all oxygen would escape, and passengers inside would quickly lose consciousness and suffer from hypothermia. Those who unbuckled their seatbelts to move around the cabin could also be sucked out through the hole.
The accident raised many questions. One of the most concerning issues is why such a fuselage failure could occur on a commercial aircraft. For the past half-century, cabin decompression incidents have primarily resulted from problems with luggage doors and metal fatigue in the fuselage, not from the failure of the main cabin door.
The door cover can be observed from outside the aircraft. (Photo: Alaska Airlines).
What is a Door Cover?
The part of the fuselage that burst open on January 5th is called the door cover. The door cover of the 737 MAX 9 weighs approximately 28 kg, is 1.2 meters tall, and 0.6 meters wide. This component is not exclusive to the 737 MAX 9 or Boeing aircraft. Airlines often use door covers to seal unnecessary emergency exits, effectively turning unused emergency doors into regular windows. Passengers inside usually do not realize there is a door there, but those observing the aircraft from outside can see the outline of the door cover.
The number of emergency exits required for each aircraft can vary based on passenger capacity. In addition to the exits on the wings, at the rear, and near the cockpit, the original design of the 737 MAX 9 includes two additional emergency exits at the tail section. Some airlines operate this aircraft model at maximum passenger capacity and therefore must retain all original emergency exits.
According to Boeing, the 737 MAX 9 was certified by the Federal Aviation Administration (FAA) in 2018. It is Boeing’s largest single-aisle aircraft and can carry up to 220 passengers with a range of about 5,300 km. Alaska Airlines chose to operate this model with fewer passengers, meaning there was no need to keep the two additional emergency exits.
“People often think ‘the more, the better’ when it comes to emergency exits, but exits with slides or life rafts increase the aircraft’s weight and incur additional maintenance costs throughout the aircraft’s lifecycle,” aviation expert Robert Ditchey explained about the reasoning behind replacing emergency exits with door covers in the Los Angeles Times.
After Alaska Airlines ordered the modified 737 MAX 9, Spirit AeroSystems – the company involved in manufacturing the fuselage – removed the original door and installed the door cover. The modified fuselage was then transferred to Boeing in a state of “installed but not completed,” allowing Boeing to remove the door cover and use that passage to access the cabin and install additional internal components, according to Reuters. Ultimately, Boeing sealed the door cover and subsequently delivered the aircraft to Alaska Airlines.
The door cover that burst from the 737 MAX 9 found in Portland, Oregon, on January 8. (Photo: NTSB/X).
Why Did the Door Cover Burst Open?
The NTSB has not yet determined the exact cause of the incident, but one possibility is a flaw in the bolts used to attach the door cover to the rest of the aircraft frame. On January 8th, United Airlines reported discovering loose bolts and other installation issues on the door covers of Boeing 737 MAX 9 aircraft.
“We do not yet know what went wrong,” Ditchey said, referring to the possibility that some bolts were missing, incorrectly sized, improperly tightened, or due to mechanical flaws in the metal. He added that there could be a structural flaw in the aircraft frame, but assessed that this possibility is lower.
According to Ditchey, the incident indicates that there may have been a design flaw in the door cover. Doors on commercial aircraft are specifically designed to not be able to open while the cabin is pressurized. They are also designed to withstand the cabin pressure. Even without bolts or fixed latches, passengers would not be able to open the emergency door on a fully pressurized aircraft. To ensure they do not burst open, door covers are designed like wedges: the inside is larger than the outside, allowing cabin pressure to help keep them securely in place.
“In my opinion, the door cover was poorly designed, as it could burst open and indeed did in this case,” Ditchey said, explaining that it seems to have been screwed in from the outside rather than being fitted into the aircraft frame from the inside.
