Last week, flying home from Halifax, I read an excellent piece of journalism in Vanity Fair (which, truth be told, I picked up mainly to ogle Tina Fey). I must admit a certain lack of awareness of my surroundings when reading it, though: a jet in mid-flight is not the place to read a story about a catastrophic plane crash. Not because I’m superstitious, but because each page had the words ‘Air Crash’ in bold letters at the top, and that’s the kind of shit that makes other passengers jumpy. Anyway, the article is called “The Devil At 37,000 Feet” and I highly recommend you visit Vanity Fair to read the entire thing.

The core of the article was the time line of minor mistakes and small missteps that compounded until eventually a private jet collided with a passenger 737 over Brazil; the former managed to land safely, but the latter plummeted to the ground, killing all aboard. The story was tragic and frustrating, but it was also incredibly informative because of how author William Langewiesche told it, and revealed a key secondary theme: the accuracy of two arrows.

As it happened, these two flights were aimed directly at each other. The private jet should have been a thousand feet below the 737, but for a multitude of reasons it was not, and so they were on the same path. The problem was that they were on precisely the same path. At that altitude jets fly themselves, and with jets as new as these they flew at exactly 37,000 feet, exactly in the middle of flight corridors, with precision only an autopilot (working with high-tech GPS and altimeters) could achieve. As Langewiesche described it:

“Until recently, head-on airplanes mistakenly assigned the same altitude and route by Air Traffic Control would almost certainly have passed some distance apart, due to the navigation slop inherent in their systems. But this is no longer true. The problem for the [private jet] was that the [737] coming at them on the same assigned flight path had equipment that was every bit as precise.”

In the past, even if two pilots tried to hit each other it would be almost impossible, due to the hugeness of the sky. Langewiesche relates this here:

“In the United States a controller doing simulation research once mentioned to me the difficulty of directing two airplanes into each other even if you try. I answered that I was not surprised. Even the largest airplanes are small, and the starting point of collision avoidance has traditionally been a reality known as the theory of ‘the big sky.'”

And so, the systems designed to make flights safer (as they almost invariably do) in this case remove the possibility for luck to play a part in avoiding a collision.

At the end of the article Langewiesche explains this to some Caiapo Indians — on whose Amazonian land the plane crashed — using arrows as the analogy:

I asked the Caiapós to consider that in all the sky above the forest only these two airplanes had been in flight. It was as if in a space the size of the Caiapó village—no, all the way out to the road—you had shot two arrows in opposing directions, and they had collided. What were the odds? In the past it never would have happened. Even if you had assigned them identical flight paths, the arrows would have passed some distance apart because of the inherent inaccuracies of flight. But now better feathers have been invented, and have become required equipment for the high-speed designs. As a result, the new arrows are extraordinarily accurate, which allows more of them to be shot around, but with increasing reliance on tightly coupled systems of control. The sky is just as big as it ever was, but the margin for error has shrunk. And when the systems fail? That is what happened over the Caiapós’ land. The paradox was precision. Mistakes were made, the Devil played, and two arrows touched nose to nose.

Again, I strongly encourage you to read the full article. Don’t worry, it won’t make you scared to fly…though it may give you pause before booking any flight that crosses Brazilian airspace. The website also contains an interview with Langewiesche, and downloadable audio files of both planes’ cockpit recordings.