What happens if an airliner suddenly loses cabin pressure?

Let’s just say it’s not like it is in the movies

Airliners carry their own portable atmosphere. How much can they afford to lose?

It may have been speculation about the cause of the Air France Flight 447 crash this summer that led Faithe Silver, of Tampa, Florida, to ask: “How often do commercial airliners experience cabin depressurization in flight?”

We assume Ms. Silver doesn’t mean the ordinary pressurization and depressurization that happens on every flight after takeoff and before landing. She wants to know what happens if the cabin rapidly depressurizes. Is it as dramatic as in the movies?

Pilot and author David Lombardo writes in his book Advanced Aircraft Systems: “Hollywood has fostered an image of rapid cabin depressurization that has come to be known as explosive decompression. Someone on the ground shoots a hole into the side of an aircraft and it results in total loss of cabin pressure with paper, food trays, and baggage flying everywhere.”

Right, that’s what we’re talking about.

“Reality simply does not work that way,” explains Lombardo. “A bullet hole in a cabin wall would have no perceived effect on cabin pressure…. A bullet hole is far smaller than the opening of the outflow valve [through which cabin air escapes during routine depressurization]. In fact, such a hole would account for less air leakage than what is normally lost around door and window seals.”

That’s not to say sudden decompression isn’t a danger. The FAA (in its Advisory Circular 61-107A) provides a helpful chart showing just how long crewmembers are able to perform flight duties with an insufficient supply of oxygen. In an aircraft at 22,000 feet, passengers and crew would have 5 minutes of “useful consciousness” after rapid decompression. But at 43,000 feet, the time drops to a mere 5 seconds, hardly long enough to don an oxygen mask. (The same circular notes “One pilot does not need to wear and use an oxygen mask if both pilots are at the controls and each pilot has a quick donning type of oxygen mask that can be placed on the face with one hand from the ready position and be properly secured, sealed, and operational within 5 seconds. If one pilot of a two-pilot crew is away from the controls, then the pilot that is at the controls must wear and use an oxygen mask that is secured and sealed.”)

To see how oxygen deprivation affects the brain, take a look at this PBS feature about climbers scaling Mount Everest. Subjects were tested at various stops along the summit. (As a control, they were tested at base camp as well.) Tests included simple verbal puzzles (“If I say that Jack stole Ann’s ball, who is the thief?”), and sentence repetition. Climbers did fine at sea level, but as they reached higher altitudes, they found it difficult to answer even simple questions.

NASA Technical Report CR-1223, which gives the lowdown on “Rapid (Explosive) Decompression Emergencies in Pressure-Suited Subjects,” includes a review of “human accidents involving explosive decompression.” The effects are unpleasant, to say the least. After regaining consciousness, subjects reported a feeling of unsteadiness “like being drunk,” lingering pains in the chest “like after a K.O.,” and intense throat pain.

On July 13, 2009, a Southwest Airlines Boeing 737-300 (Flight 2294) depressurized after a one-foot hole appeared in its upper fuselage (an accident investigation is under way). The aircraft, en route to Baltimore from Nashville, was diverted to Charleston, West Virginia, where it landed safely. The cabin depressurized about 30 minutes into the flight at 34,000 feet, and no injuries were reported, although the NTSB noted “The damage left a hole measuring approximately 17 inches by 8 inches.”

In the event of a larger hole in the fuselage, there’s more risk of hypothermia than decompression. “[T]he cabin temperature would drop from 70˚ degrees F to as low as -60˚F,” writes Lombardo. “With temperatures that low, it is only a matter of seconds before hypothermia sets in and everyone begins to freeze to death.”

So it isn't the pressure drop that gets you after all. It's the cold.

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