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where the air is thin and oxygen levels are very low. The system uses air drawn from the compressors in the engines and regulated through valves in the fuselage to squeeze the rarified, high-altitude air back together, recreating the dense, oxygen-rich conditions at sea level. (Or close to it. Pressurizing all the way to sea level is unnecessary and would put undue stress on the airframe, so the atmosphere in a jet is actually kept at the equivalent of 5,000 to 8,000 feet, meaning that you’re breathing as you would in Denver or Mexico City—minus the pollution.)
    That’s all there is to it.
    Great, you’re thinking, but what about a loss of pressurization: the plastic masks dropping, people screaming…
    Yes, a cabin decompression is potentially dangerous. During cruise, depending on the altitude, there’s a differential of somewhere between 5 and 8 pounds per square inch between the pressure inside the plane (high) and the pressure outside (lower). You can think of the fuselage as a sort of balloon, with up to 8 pounds of force pushing against every inch of it. Introduce a hole or a leak into the picture, and you’ve got a problem. Loss of pressure means loss of oxygen, and if this happens explosively, such as from a bomb, the resultant forces can damage or outright destroy the plane.
    However, the overwhelming majority of decompressions are not the explosive kind, and they are easy for a crew to handle. Odd things have happened, such as the bizarre Helios Airways accident in 2005, but crashes or fatalities from pressure problems are extremely uncommon, even with a fairly rapid decompression brought on by a hole or puncture.
    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 ever be confronted by this spectacle, try to avoid shrieking or falling into cardiac arrest. Instead, 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.
    Up front, the pilots will don their own masks and commence a rapid descent to an altitude no higher than 10,000 feet. If the descent feels perilously fast, this isn’t because the plane is crashing: it’s because the crew is doing what it’s supposed to do. It might be jarring, but a high-speed emergency descent is not unsafe by itself.
    One afternoon I was working a flight from South America to the United States. 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 quickly losing pressurization. The captain and I put our masks on, took out the book, and began to troubleshoot. Part of that troubleshooting involved one of those steep descents. Commencing such a drop is a multistep process: set 10,000 in the altitude window; select “flight level change” from the autoflight panel; increase the speed command to a point slightly below maximum; deploy the speedbrakes; retard the thrust levers to idle… To the passengers, I’m sure it felt like a roller coaster, but everything was carefully coordinated. The autopilot was engaged the whole time, and no limits were exceeded.
    Should a pressure loss occur over mountains or other high terrain, pilots will follow predetermined depressurization routes, sometimes called “escape routes,” that allow for a more gradual descent, in stages. Even if crossing the Andes or the Himalayas, there is always the opportunity to reach a safe altitude before supplemental O 2 runs out.
I often travel from Louisville to New York, but the only flights serving this route are puddle jumper regional jets. I’m reluctant to fly on these planes because I feel they’re unsafe. Are they?
    The short answer is no. No commercial aircraft is