PAYNE STEWART: THE GHOST FLIGHT OVER AMERICA

Controllers did what controllers do. They called again. They called on the previous frequency, in case the handoff had gone wrong. They asked other aircraft in the area to try raising N47BA on the emergency frequency. They watched the target continue to climb, straight and steady, past its assigned altitude and on up toward the mid-forties, and they understood very quickly that this was not a radio problem.

A radio failure means an aircraft that flies its cleared route and lands at its destination. What they were watching was an aircraft flying itself. The distinction is one of the oldest and worst in air traffic control, and it was made within minutes.

CNN carried it for the better part of four hours. Other networks broke in. There was no precedent for what was being broadcast: not the aftermath of a disaster, not a rescue with a chance of succeeding, but a disaster in progress, on a fixed schedule, with a known ending and nothing anyone could do about the interval.

Anchors filled the time by explaining hypoxia to a country that had never heard the word. Aviation experts were brought on to answer the only question viewers had — is there any chance? — and had to keep saying no. Somewhere in the middle of it, Payne Stewart's name was attached to the aircraft, and a technical emergency became a national one.

It is worth being honest about what that broadcast was. It held an audience for four hours on the strength of watching six people die slowly and invisibly, and a great many people who watched it have described feeling complicit afterwards. It also did something more useful than most disaster coverage ever manages: several million Americans learned that afternoon what happens to a human being at 40,000 feet without pressure, and why the mask drops before you help anyone else.

Different aircraft, different countries, different root causes — a failure nobody could identify in one case, a switch in the wrong position in the other. But the killing mechanism is identical, and so is the shape of the ending: an aeroplane in perfect trim, holding its heading beautifully, with nobody left aboard to fly it and fighter pilots alongside who can see everything and change nothing.

Hypoxia is the quietest thing that can go wrong on an aircraft. It removes the ability to recognise that anything is wrong before it removes anything else, which is why both of these crews — experienced, competent, doing nothing careless — never got the masks on. Every warning system, every checklist change and every training requirement that came out of these two accidents exists to buy crews back the seconds that the thin air takes away first.