The Air Up There
Aug. 16, 2005 — -- The puzzling loss of a Helios Boeing 737 near Athens on Sunday has prompted many passengers to start asking questions about in-flight oxygen systems, a subject most airlines do not like to discuss, frankly for fear of creating flyer anxiety. Yet, our systems and procedures are so good that the possibility of an extremely rare inflight depressurization causing an accident is all but unthinkable, and this reassuring success in aviation safety should be openly discussed.
First of all, the masks in the cabin drop when there has been a major loss of cabin pressure that's raised the so-called cabin altitude from around 7,000 feet (where we normally maintain it) to anything above 14,000 feet. The truth is, modern jets travel at high altitude, and high altitude is a hostile environment for human life. Period. But we've made it incredibly safe by applying more than a half-century of technology and procedural knowledge.
Let's start at the beginning, with a bit of a primer on pressurization and oxygen, and please bear with me, because this is the key to understanding what may have happened over Greece on Sunday.
First, we sustain life by oxygenating our blood, and we do that by taking in oxygen through our lungs. To do that, we not only have to breathe oxygen in with every breath, but that oxygen has to have at least a certain minimum atmospheric pressure behind it to be pushed through the blood-membrane tissues of the lungs and into our blood streams. That isn't easy at high altitude.
Oxygen makes up approximately 21 percent of the atmosphere's gases regardless of what altitude we're talking about, and that means that the pressure of just the oxygen in the air is that same 21 percent of the overall air pressure whether we're breathing at the beach or on the top of Mt. Everest. As you go up in altitude, the atmospheric pressure drops, and so does the already-lower pressure of just the oxygen in the air.
We keep the aircraft pressurized, by the way, by treating it like a metal balloon. We force air conditioned air under pressure into the cabin and meter the rate at which it escapes from the rear (through a butterfly-like outflow valve). If the air conditioners are turned off or all of them fail, the cabin depressurizes, but not explosively. If a cargo door were to blow open (an almost-never occurrence), the cabin pressure would suddenly go from the equivalent of 7,000 to the altitude your aircraft is cruising. That's an explosive depressurization.
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