The door seal used to replace the emergency exit door on the Boeing 737 MAX 9 that recently crashed may have a design or installation defect.
Oxygen masks are released when Alaska Airlines Flight 1282's door bursts open on January 5. Video: CBS
On January 5, an Alaska Airlines Boeing 737 MAX 9 took off from Portland International Airport in Oregon bound for California, USA. However, 20 minutes later, the plane carrying 177 people had to make an emergency landing when a door panel in the fuselage broke off, creating a hole as large as an emergency exit.
No passengers were seriously injured in the incident. The US National Transportation Safety Board (NTSB) has begun an investigation into the incident in which the Boeing 737 MAX 9 suffered a blowout at an altitude of more than 15,000 feet. Jennifer Homendy, chair of the board, said the incident could have been much worse at a higher altitude. If the fuselage ruptured at a cruising altitude of about 33,000 feet, the cabin would immediately lose its ability to pressurize, all the oxygen would escape, and passengers inside would quickly become unconscious and freeze. People who unbuckled their seat belts to walk around the cabin could also be sucked out through the hole.
The accident raises many questions. One of the most intriguing is how such a rupture could have occurred on a commercial airliner. Over the past half-century, depressurization incidents have been attributed primarily to malfunctioning baggage doors and metal fatigue in the fuselage, not to major cabin door failures.
The door seal can be seen from outside the plane. Photo: Alaska Airlines
What is a door seal?
The part of the fuselage that came loose on January 5 is called a door seal. The door seal on the 737 MAX 9 weighs about 60 pounds, is 4 feet tall, and is 2 feet wide. It’s not unique to the 737 MAX 9 or Boeing planes. Airlines often use door seals to block unnecessary exits, essentially turning an unused exit into a regular window. Passengers inside will hardly notice that a door is there, but someone watching the plane from the outside can see the outline of the door seal.
The number of emergency exits required on each aircraft can vary depending on the number of passengers. In addition to the exits in the wings, at the rear, and near the cockpit, the original 737 MAX 9 design included two additional exits in the tail section. Some airlines operate the aircraft at full capacity, so all of the original exits must be retained.
The 737 MAX 9 was certified by the US 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 the aircraft with fewer passengers, meaning it did not need to keep two additional exits.
“People often think ‘more is better’ when it comes to emergency exits, but exits with slides or life rafts add weight to the plane and cost more to maintain over the life of the plane,” aviation expert Robert Ditchey explained in the Los Angeles Times about why exits are being replaced with door seals.
After Alaska Airlines ordered a modified 737 MAX 9, Spirit AeroSystems, which built the fuselage, removed the original door and added a door seal. The modified fuselage was then shipped to Boeing in an “installed but unfinished” state, allowing Boeing to remove the door seal and use the passageway to enter the cabin and install additional interior components, according to Reuters . Boeing then resealed the door seal and delivered the plane to Alaska Airlines.
A broken door seal from a 737 MAX 9 aircraft was found in Portland, Oregon, on January 8. Photo: NTSB/X
Why is the door seal coming off?
The NTSB has not determined the exact cause of the crash, but one possibility is that the bolts used to attach the door seal to the rest of the airframe are faulty. On January 8, United Airlines announced that it had discovered loose bolts and other installation issues on the door seals on its Boeing 737 MAX 9 aircraft.
“We don’t know what went wrong yet,” Ditchey said, citing the possibility that some bolts were missing, the wrong size, improperly tightened, or a mechanical defect in the metal. He added that there could be a structural defect in the airframe, but considered that less likely.
The incident suggests that there may be a flaw in the design of the door seal, Ditchey said. Commercial aircraft doors are specifically designed to not open while the cabin is pressurized. They are also designed to withstand cabin pressure. Even without bolts or latches holding the door in place, passengers would not be able to open an emergency exit on a fully pressurized plane. To ensure that they do not come loose, the door seal is designed like a wedge: larger on the inside than the outside, so cabin pressure helps hold it in place.
"In my opinion, the door seal was poorly designed, because it could come off and did in this case," Ditchey said, explaining that it appeared to have been bolted from the outside rather than inserted into the airframe from the inside.
Thu Thao ( Synthesis )
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