A Schleicher ASK 21 glider is a craft of elegance and poise. Its slim wings, seductively curved cabin and tapering fuselage embody a balanced design that moulds modern materials into flowing aerodynamic lines. On the afternoon of 17 April 1999, one such beauty soared gracefully above countryside near Dunstable, England, with an instructor and a novice pilot on board. The student had been given the trial lesson as a 30th birthday present. Although large storm clouds loomed nearby, at 1608 hours conditions in the immediate vicinity were calm and the air was clear.

At 1609 hours a fearsome force suddenly and violently shredded large sections of the glider. The instructor later recalled a “very loud bang” and a distressingly “draughty” cockpit. Dazed and briefly unconscious, he realised that “something was seriously amiss… requiring unpleasant and decisive action.”

By the time he vacated the wreckage—noting on his way out that there was no need to eject the canopy, nor any canopy—his student had arrived at the same conclusion. Witnesses on the ground observed a bright flash and heard a loud crack, and craned their necks to see a ball of smoke and fine debris hanging in the space where the glider had been. Below this, the remnant of a fuselage plummeted earthwards at high speed, with larger sailplane fragments fluttering behind. Thankfully two open parachutes were among them, with deafened and soot-blackened aviators swinging underneath. They were the fortunate survivors of a curious and powerful phenomenon known as positive lightning.

Usually, lightning occurs inside towering cumulonimbus clouds, or between the bases of such clouds and the ground. The vast vertical energy transfers involved in storm cell formation cause a strong negative charge to develop at the bottom of the cloud, which in turn attracts a positive charge on the ground underneath. Ordinary cloud-to-ground lightning happens when this differential grows to a critical point, and negative charge flows abruptly to earth in an explosive flash of electricity.