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.

Now, from sea level to about 10,000 feet there is sufficient partial pressure of oxygen (still 21 percent of the atmospheric pressure at that altitude) to force oxygen into our blood and keep us alive and conscious. If you strap on an oxygen mask and breathe pure oxygen above 10,000 feet you'll also be fine and clear-headed until you find yourself standing at or above 30,000 feet. Somewhere between 30,000 and 40,000 feet the pressure around you becomes far too low to push those oxygen molecules across the membranes in your lungs, and you get hypoxic (altitude sickness). If you try to breathe 100 percent oxygen above 40,000 feet for very long without a special type of mask, you'll die.

Oxygen Masks Buy a Few Minutes to Act

But what about those yellow masks that are supposed to drop out of the ceiling of a jetliner if the cabin altitude rises above just 14,000 feet?

They are NOT there to let you fly continuously at depressurized cabin altitudes above 30,000 feet. Those emergency passenger supplemental oxygen masks are only there to help keep you oxygenated while your crew descends the airplane rapidly to more breathable altitudes. In fact, the more modern passenger oxygen systems only provide approximately 14 minutes of oxygen flow -- which is easy to appreciate when you realize your aircrew and aircraft are required to get you to or below 14,000 feet in four minutes from any cruise altitude you might have been using!

But, will those supplemental passenger oxygen masks keep you from passing out?

If you're above 35,000 feet, probably not. But they will help you wake up again safely as the plane descends into the teens going toward 10,000.

Then are the pilots going to pass out?

Not if they follow the rules, and this is the key: At the very first sign of a true cabin pressurization emergency the first step for pilots is to pull out the quick-don oxygen mask which sits by each pilot position, sweep it over their head and into place, assure it's providing 100 percent oxygen, and check to make sure the internal microphone works to communicate with air traffic control and your fellow crewmembers. The pilots also have a different type of oxygen system, one which uses a secure mask to push positive oxygen pressure into their lungs above approximately 29,000 feet. That's bedrock basic procedure in all high-altitude airplanes. The second step for the pilots is to decide whether to make an emergency descent, and if the cabin altitude is above 14,000 feet, the answer is almost always "Yes," and "Now!"

Pilot Action in Greece Crash Still a Mystery

An emergency descent, however, did not happen with the Helios jet, and we don't know why.

So what did happened to the Helios 737 and its crew?

Like all air accidents, there is never just one cause, but it is highly likely that a major contributing factor in Sunday's crash was depressurization and an inability of the pilots to breathe pure oxygen under pressure in time. Perhaps the pilots delayed reaching for their masks because they were troubleshooting a pressurization and air conditioning problem and waited too long. Maybe they put on the masks in time only to find they weren't getting any oxygen because, for instance, the crew oxygen bottle was empty, or the valve was turned off. If so, most captains would have left their left seat to try to turn on the valve (which is located behind the right backside of the copilot's seat). That entire sequence, by the way, would have consumed 20 or 30 seconds, which is just about the maximum amount of time a pilot would have before drifting into a mental fog leading to unconsciousness (time of useful consciousness). Of course, it's entirely possible that a completely different combination of events led to the unconsciousness of the first officer and (presumably) the captain, as well as the passengers. An added potential tragedy is the fact that properly done preflight checks of the oxygen system will always turn up a closed oxygen valve or empty bottle before the plane takes to the air.

One thing is certain in this tragedy that took 121 lives: Human physiology is well understood, and without additional oxygen, under pressure, an aircraft flying more than an hour above 30,000 feet will have no conscious humans left aboard.

Aviation has all the procedures and equipment in place to safely traverse the hostile environment of high altitude 100 percent of the time. What likely failed in this tragedy was strict compliance with various aspects of that system.