In-Flight Emergency: When the Masks Come Down

This past weekend's Southwest Airlines incident has everybody talking and wondering about inflight decompressions. What is a decompression, exactly? How deadly are they?

Let's start with a review of pressurization:

The cabin is pressurized so that passengers and crew are able to breathe normally, without the need for supplemental oxygen. Pressurizing the cabin effectively squeezes the air back together, re-creating the dense, oxygen-rich conditions on the ground. Or close to it, as during cruise the atmosphere in a jet is actually kept a bit higher than sea level, usually on the order of 5,000-8,000 feet. In other words you're breathing as you would in Denver or Mexico City—minus the pollution. (Pressurizing all the way to sea level is unnecessary and would put undue stress on the airframe.) Pressurization is maintained via air from the compressors in the engines and regulated through valves in the fuselage.

That's all there is to it. Something about the word "pressurization" makes people envision the upper altitudes as a kind of barometric hell. I've been asked, "If the plane wasn't pressurized, would my eyes pop out?" No. Cruising in an airplane is not the same as dropping to the Marianas Trench in a deep-sea diving bell.

As a plane climbs and descends, the level of pressurization will change, and is modulated by something called an outflow valve. I always find it funny when the pre-takeoff safety briefing includes the line, "If cabin pressure should change, an oxygen mask will drop...." They used to say, "In the event of a loss of cabin pressure..." but apparently this was deemed too scary. Scary, but a lot more accurate, because in fact cabin pressure is changing all the time.

Changing is one thing. Escaping uncontrollably is another. Introduce a sudden hole or leak into the picture, and you've got a problem; loss of pressure means loss of oxygen. If the leak is serious enough, the plane can depressurize completely. Essentially there are three types of depressurization: gradual, rapid, or explosive. The first two are fairly common and easy for a crew to handle. The Southwest mishap was a rapid decompression.

In-Flight Emergency: When the Masks Come Down

The first thing pilots do in such a scenario is pretty straightforward: they don their cockpit oxygen masks and commence a rapid descent to an altitude no higher than 10,000 feet, where everyone can breathe without a mask. Even with a total pressure loss there are several minutes of supplemental oxygen available for both passengers and crew, but the protocol is always to get down as quickly as possible. When this happens, passengers will later talk of "dropping" or "falling." In fact the crew was doing as it was trained to do. It might be startling, but a high-speed emergency descent is well within the capabilities of any aircraft and not, by itself, unsafe.

If, at some point cabin pressure reaches a certain threshold-–around 10,000 feet, give or take—the passenger masks will deploy from the ceiling, exposing everybody to the so-called "rubber jungle." This is typically the point where people begin shrieking and picturing their loved ones, but try to relax. The masks are there to assist you; the plane will be at a safe breathing altitude in just a few minutes.

A few months ago I was working a flight from South America to the US. All was quiet, high over the Caribbean, when suddenly there was a loud whooshing sound that seemed to come from nowhere and everywhere at once. I could feel my ears popping, and sure enough a glance at the instruments showed we were rapidly losing pressurization. The captain and I put our masks on, took out the book and began to troubleshoot.

Part of that troubleshooting involved a rapid descent to 10,000 feet. As we descended—or plummeted, as the passengers probably saw it—I kept an eye on the cabin pressure gauge. As the plane itself was falling, the equivalent cabin altitude was climbing. At a certain point, if we didn't get down fast enough, the rubber jungle was going to deploy automatically, at which point, I'm sure, at least a few people were going to panic.

But we did get down, and the masks remained tucked in their cubby holes.

Unfortunately, neither of the backup pressurization modes worked properly; fuel constraints made it impossible to finish the flight at 10,000 feet, and so we wound up diverting. We were replaced with a new crew and the passengers were swapped into a different plane.

We didn't make the papers, but almost anything is fair game these days, and I suppose we could have. We had a technical problem followed by a startlingly fast descent, followed by a diversion... Sounds like a good story, except there was never any emergency, and little to no danger.

Incidentally, should a pressure loss should occur over mountains or so-called "critical terrain," pilots will follow predetermined depressurization routes, (sometimes called "escape routes") that allow for a timely, if more gradual descent. Even if crossing the Andes or the Himalayas, there is always the opportunity to reach a safe breathing altitude before supplemental O2 runs out.

Now, as maybe you noticed, I haven't yet addressed that third kind of decompression: the explosive kind. Here the big thing isn't the loss of pressure per se, but the danger of resultant forces seriously damaging or destroying the plane. Bombs, for example, can tear apart an entire fuselage in fractions of a second. A bomb alone doesn't (necessarily) bring down a plane; the explosive decompressions caused by a bomb do. See Pan Am 103, et al. Large-scale structural failure, like the infamous fuselage burst of an Aloha Airlines 737 in 1988, in which a flight attendant was killed and the plane nearly lost, can be similarly disastrous.

But those are extremely rare occurrences, and that's not what happened to Southwest. Cabin pressure was equalized fairly quickly, at which point it could be reasonably assumed that the plane was going to stay in one solid piece and fly just fine. Which it did.

All of this, meanwhile, ties in to Jesus Diaz's recent piece about the FAA's ordered removal of oxygen masks from aircraft lavatories.

We can argue whether this removal was worth the trouble from a security standpoint—the TSA was afraid of terrorists somehow rigging up a deadly device from the oxygen plumbing—but while the likelihood of a rapid decompression is slight enough, even if one were to occur, the possibility of it killing or seriously hurting an un-masked passenger in a lavatory is slim.

As we've seen, should a decompression occur the crew is trained to descend very rapidly to an altitude where supplemental oxygen is no longer needed. Per regulation this cannot take more than a few minutes, which as a general rule is not enough time to kill anybody. A more likely danger is falling and hitting your head.

Also as we've seen, the drop-down passenger masks deploy automatically at relatively low cabin altitudes. They are not necessarily an indicator of a rapid or complete decompression, and in most cases you could ignore them at little or no risk — not that I would advise doing that, just in case.

Patrick Smith is an airline pilot, air travel columnist and the host of www.askthepilot.com. He lives near Boston.

Portions of the article above originally appeared on Salon.com.

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