What happens when an airplane cabin suddenly depressurizes?

By Jacopo Prisco, CNN

(CNN) — A loud bang, a jolt, and cold air whooshing suddenly through the cabin: these were the immediate signs that something was very wrong aboard Alaska Airlines flight 1282, according to one passenger report.

An “explosive decompression” had occurred, the result of a fuselage plug — which in different configurations of the same aircraft can be an exit door — being violently blown out, leaving a gaping hole in the left side of the plane.

As investigators work to determine exactly what caused the incident, we look at what happens when an aircraft experiences a sudden loss of cabin pressure and the risks for those on board.

Why we pressurize

“The reason aircraft are pressurized is the comfort of the passengers,” says Graham Braithwaite, a professor of aviation safety and accident investigation at Cranfield University in the UK. “As the aircraft climbs, the cabin pressure will eventually settle to about 8,000 feet. So if you’re flying at 35,000 feet, the cabin will feel like you’re at 8,000 feet of altitude.”

This is necessary because as altitude increases, air becomes less dense and — as any mountaineer will know — breathing becomes more difficult as fewer oxygen molecules are taken in due to “thin air.” This effect can be clearly felt in the real world past about 10,000 feet, which is why passenger cabins are set at a pressure of around 8,000 feet, where everyone will feel comfortable without putting too much stress on the plane’s systems.

However, this means that the inside of the plane is at a much higher pressure than the outside, working against a key tenet of physics: Air always moves from high pressure to low pressure areas, and it does so faster if the difference in pressure is high. The wind is a common manifestation of this phenomenon.

If at any point during the flight there is a fuselage breach, physics will prevail and the air from the cabin will rush to the outside: “Just because of that pressure difference,” explains Braithwaite.

In the case of a rapid or explosive decompression, like the one during the Alaska Airlines flight, oxygen masks will automatically drop down: “The instruction in every safety briefing is to put them on rapidly, because the higher you are, the less time you will have into what we call ‘useful consciousness.’ Since the air is thinner, it’s hard to breathe at altitude. So that becomes a really important thing,” Braithwaite adds.

The flight crew will immediately start working to get the aircraft down to about 10,000 feet, where the air will be breathable. “If there’s been a very sudden depressurization, the crew will also try to weigh up whether the aircraft is structurally damaged, versus how quickly they can descend,” Braithwaite says. “If they descend too rapidly, they might be putting that aircraft under a lot more stress.”

Once safely on the ground, the aircraft will be met by rescue teams, and the crew will decide whether passengers are safer on board or if an evacuation is needed. “It’s an incredibly unusual type of event,” says Braithwaite, “and even though people can point to a couple of examples where an aircraft lost a panel or a door in flight, If you compare that to the number of flights that operate every day, these are still incredibly rare events, and they all tend to have unique qualities.”

Previous accidents

The deadliest decompression accident in aviation history happened in 1985, when Japan Airlines Flight 123 suffered severe structural damage due to a faulty repair of the fuselage following a hard landing years earlier. The Boeing 747 lost a large portion of the tail mid-flight and crashed into a mountainous area near Tokyo, killing all but four of the 524 people on board. It is the deadliest ever aviation accident involving a single aircraft.

The most recent death following a rapid decompression dates back to 2018, when a woman died aboard Southwest Airlines Flight 1380, after being partially sucked out of the window of a Boeing 737-700. The window had been blown out by shrapnel from an engine that exploded due to improper maintenance, and the woman was left hanging out of the hole.

That aircraft was at 32,000 feet when the blowout occurred, which made things far worse than the Alaska Airlines flight, whose altitude was just 16,000 feet.

“They were really lucky it was low altitude,” says Jonathan Clark, a professor of aerospace medicine at the Baylor College of Medicine. “From a standpoint of hypoxia, which is always the thing that the pilots worry about, that’s not very high. If that had happened at a high altitude, above 30,000 feet, it could have been far more problematic.”

Hypoxia occurs when the body is deprived of oxygen, and among other things it can severely impair cognitive function, leading to poor decision-making — a big issue if you’re a pilot.

“Visual function can be affected, fine movements impaired and mental processes slowed,” says David Gradwell, an emeritus professor of aerospace medicine at King’s College London.

“These symptoms, although reversible, would progress — unless action is taken — until loss of consciousness occurs. How long it takes for the signs and symptoms of hypoxia to arise varies with altitude and individual factors. Up to 12,000 feet, healthy individuals would notice few hypoxic effects. However, if exposed to an altitude of 35,000 feet, hypoxia would follow perhaps within less than a minute.”

The oxygen masks prevent hypoxia until the plane descends to a lower altitude, but immediately following an explosive decompression, everyone on board will experience some other effects on their bodies.

“You’ll feel your ears pop, and that can be very disorienting,” says Clark. “You can potentially experience barotrauma — an injury from pressure change. It can occur in your sinuses, in your ears. If you have a filling that’s got a hole in it, it can blow the tooth off. If you have a belly full of gas, that gas will expand, and your belly will swell up. The water vapor that is in the air can turn into a cloud and you can’t see anything. It will get very cold, very quickly. There will also be a massive wind blast as all that pressure in the cabin goes out the hole. Anything that’s not tightly secured will fly out. So there will be a lot of disorientation inside the aircraft.”

Decompression is not always explosive; it can also be gradual, either occurring very slowly, or as the result of the cabin failing to pressurize altogether. In both instances it can go unnoticed and hypoxia can kick in before any action is taken, incapacitating passengers and crew.

One such case in 2005 was Helios Airways Flight 522, which did not pressurize at all due to human error and became a ghost flight once everyone on board the Boeing 737-300 lost consciousness at altitude, pilots included. It crashed into a mountain in southern Greece once fuel ran out, killing all 121 on board.

A similar accident happened in 2023, when a small business jet with four people on board lost pressurization and even flew over Washington, DC — prompting the US Air Force to intercept it with fighter jets. The Cessna Citation eventually crashed in rural Virginia.

What you can do

If you’re looking for ways to minimize risk in the unlikely event of a violent decompression, you should start with your seatbelt, Clark suggests.

“I always wear my seatbelt, even if it’s really loose. It’s good for turbulence protection, which happens all the time, and it’s very good if you have a deep depressurization: you still might bounce around, but you won’t get sucked out,” he says. “The other thing is, as soon as that mask comes down, put it over your face and pull on the cord to make sure that the oxygen is flowing. Put yours on before your infants, because if you’re gonna pass out, then you won’t be able to help anyone.”

The Alaska incident has renewed the discussion around “lap babies,” or infants under two that parents keep on their lap rather than in a separate seat. Flight attendant unions have for years called for the practice to be banned, saying that babies are at high risk during turbulence and decompression events.

“When an explosive decompression happens, it creates a violent and immediate suction that can eject people and items out of the plane or throw them around the aircraft cabin,” says Sara Nelson, president of the Association of Flight Attendants-CWA, representing 50,000 flight attendants at 19 airlines. “In an event like this, it is incredibly important to have your seatbelt on when seated at all times and that every passenger including children under the age of two have their own seats and are properly secured in them.”

The other important thing during any kind of emergency is to carefully follow the instructions of the crew, even if there is no announcement about the situation from the pilot, as was the case with the Alaska Airlines flight, according to passenger reports.

“I imagine the cockpit crew was quite busy,” say Patrick Smith, an airline pilot flying Boeing 767s and the author of the book and blog “Ask the Pilot.”

“There was an emergency descent to perform, as well as the necessary checklists and coordination with air traffic control. Also the pilots would’ve been wearing oxygen masks, which makes communications more difficult. It’s possible one of the pilots did make a PA, but it simply wasn’t heard in the noise. Or, being as busy as I imagine they were, they may have relayed information to the flight attendants, and left it up to them in the interest of time.”

Smith adds that depressurization incidents are usually easily handled and rarely dangerous: “Unless the plane decompresses explosively causing serious structural damage,” he says. “The biggest danger in the Alaska Airlines incident would’ve been the door plug colliding with the tail structure. But this didn’t happen, leaving the rest of it pretty routine.”

But even though those on board escaped with little or no physical injuries, there could be some psychological trauma, according to Graham Braithwaite.

“I think the National Transportation Safety Board Chair has pointed out that in this case, it’s probably the psychological injury that will be more significant,” he says. “It’s a pretty frightening experience for the people on the aircraft and they will no doubt be getting professional counseling.”

One thing aviation does extremely well, he adds, is learn from incidents like this one: “It’s one of the reasons we’ve achieved the level of safety that we have today. Every airline will look for where the learning is — there were some things that were done very well here in terms of how the crew responded to the event, as well as what might have gone wrong.

“The cabin crew in particular have reminded us of just what an incredible job they do when managing safety on board airplanes,” says Braithwaite. “So if it means that more people will listen to the safety briefing, or pay attention to the suggestion to keep their seatbelts fastened, that will have been a positive.”

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