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What Happens When A Plane Loses Cabin Pressure?

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Place your own masks before helping others with theirs Credit: Jeff Greenberg

Frequent fliers have heard it dozens of times over: “Should the cabin experience sudden pressure loss, oxygen masks will drop down from above your seat. Place the mask over your mouth and nose.”

The pre-flight safety announcement may give nervous fliers the jitters – that at any point in a flight we might need help breathing – but the importance of such a briefing was highlighted this week after pilots of a domestic flight in India “forgot” to pressurise the cabin, causing passengers to suffer nose bleeds and ear damage.

The suspected error prompted the deployment of oxygen masks in the cabin.

So in what circumstances – and how often – are passengers likely to have to heed the advice to “pull the strap to tighten and continue to breathe normally”? We investigate.




Why do planes pressurise their cabins?

Aircraft cabins are pressurised using cooled and filtered air bled from the engines, keeping the air pressure inside the cabin at the equivalent of an altitude of 8,000 feet – Boeing’s Dreamliner technology has lowered that to 6,000 feet, making the cabin atmosphere more pleasant – even though commercial aircraft often fly at 40,000 feet.

The dry cabin air might cause passengers to become a little dehydrated, but happily they are able to breathe unassisted, and continue watching the in-flight film, quaffing a tomato juice, or browsing the duty-free catalogue.

But this changes when there is a loss of cabin pressure – either slow or sudden. This can happen for a number of reasons.

Technical problems with the pressurisation system are one cause, but cracks in windows or the fuselage, incorrectly sealed doors, and breaches in the aircraft due to an explosion are also all potential triggers, allowing cabin air to escape.

Small air leaks would likely cause a slow loss of pressurisation, in which case the pilot would have time to make an emergency descent to a safe altitude, of between 8-10,000 ft. Bumpy, yes, but hopefully not fatal.

The controlled descent taken by the Southwest plane after its window smashed Credit: Flightradar24.com

How do oxygen masks work?

Is it about safety, or is it just an elaborate way of intoxicating passengers to keep them pliant while their stricken plane plummets to Earth?

That, in case you’re scratching your head, is the verdict of Brad Pitt’s character Tyler Durden in the film Fight Club:

“You know why they put oxygen masks on planes? Oxygen gets you high. In a catastrophic emergency, you’re taking giant panicked breaths.

“Suddenly you become euphoric, docile. You accept your fate. It’s all right here [points at an emergency instruction manual on a plane]. Emergency water landing – 600 miles an hour. Blank faces, calm as Hindu cows.”

Crashes or fatalities from pressure problems are extremely uncommon, even with a fairly rapid decompression brought on by a hole or puncture,” explains Patrick Smith, a pilot and author of Cockpit Confidential.

If cabin pressure falls below a certain threshold, the masks will deploy from the ceiling, exposing everybody to the so-called ‘rubber jungle’. Should you be confronted by this spectacle, strap your mask on and try to relax.

“The plane will be at a safe altitude shortly, and there are several minutes of backup oxygen for everybody.

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Pass it on: Popular Science

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