THE pilot is counting down over the intercom: "One minute...30 seconds... 20..." I am 6 kilometres above the Atlantic Ocean and about to lose a lot of weight.

They call this the vomit comet - and with good reason. When the pilot reaches the end of his countdown, he will pull back on the joystick, sending the aircraft into a 45-degree climb. A kilometre or so higher into the blue, he will cut back the engines. The plane's momentum will take it soaring upwards until our ascent slows and stops and we begin to drop out of the sky. After a short while, the pilot will bring back the power and pull out of the dive. And then he'll do it all again and again and again. In the next 3 hours, we are going to experience 31 of these gut-wrenching parabolas.

These manoeuvres are not just for fun: flights ...