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A Schleicher ASK 21

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.

This commonplace ‘negative’ lightning has a high voltage but a relatively low current. While it can certainly be dangerous, there are numerous reports of people being struck by lightning and surviving⁠⁠— sometimes more than once. Similarly, lightning hits aircraft on a surprisingly routine basis, with several documented incidents occurring every year. Modern metal-skinned aircraft are designed to deflect the charge harmlessly through their outer conducting surfaces.

A cumulonimbus anvil cloud.

The extent of the devastation wreaked upon the 1999 Dunstable glider was unusual. Investigators partly attributed this to the sailplane’s layered composite glass fibre construction. The lightning bolt passed through adhesive-bonded layers of glass fibre in the wings, stripping them apart in a spectacular process known as explosive delamination. Rapid heating of gases in the voids of the structure generated shock waves that flung apart layers of glider like a vigorously and instantaneously peeled airborne onion.

Yet this didn’t explain the full magnitude of the damage. The conducting metal linkages of the flight controls should have provided a relatively easy route for the electrical discharge to pass through the glider, but metallurgical examination of the debris revealed some strange anomalies. Although one connecting bolt had experienced extreme temperatures of 1000 degrees Celsius, other components had been bizarrely deformed despite receiving much less heat. One hollow control rod was crushed into a solid bar by an intense magnetic field, something that could only have been generated by energies far exceeding those of ‘normal’ negative lightning. It was clear that some higher power had been at work.

Suspicion rapidly settled on the phenomenon known as ‘positive lightning’. Awareness of positive lightning’s significance has gradually increased in recent decades, and it is now believed to comprise up to 5% of all lightning strikes. The negative charge at a storm cell’s base is balanced by a strong positive charge at the cloud’s anvil-shaped top, up to 60,000 feet above the ground. While there is also a positive charge on the ground immediately underneath the storm cell, significant charge differentials can develop between cloud tops and negatively-charged land surfaces much further away. Occasionally these differentials are sufficient to spark a positively charged lightning strike⁠⁠— a huge high-energy arc capable of hitting the ground more than ten miles from the storm itself, often under clear skies and bright sunshine.

Vast energies are required to deliver these bolts from the blue. Research suggests that positive lightning can generate currents and potentials ten times greater than negative strikes: up to 300,000 amps and 1 billion volts, or approximately 300,000.21 gigawatts of power in a single discharge. Following the 1999 Dunstable incident, researchers in lightning test establishments in the US, UK, and Germany tried to replicate some of the glider’s extreme damage by unleashing increasingly huge electrical discharges onto unsuspecting test articles. Despite their best efforts, the hollow metal rods remained resolutely uncrushed. Maniacal cackling professors and hunchbacked assistants were not available for comment.

A formerly straight control rod from a positively stricken aircraft.

Many of the more serious lightning strikes known to have adversely affected human interests⁠⁠— whether flights, forests, power grids, or the Hill Valley clock tower⁠⁠— are now believed to have been positive. Reassuringly, all modern passenger aircraft incorporate conducting strips and other lightning mitigation measures, designed to protect vulnerable electrical and fuel systems. It should be noted that the Dunstable glider had no such protection, and that a lightning induced wing-shredding event on a commercial flight is considered exceedingly unlikely. Nonetheless, the fact that many aircraft safety standards are based on assumptions derived from puny negative lightning, rather than high power positive strikes, leaves some cause for concern.

Not everything about positive lightning is negative. The phenomenon may represent the key to unlocking some important mysteries of meteorology, and is associated with intriguing scientific curiosities such as sprites, jets and ELVES⁠—bizarre forms of high altitude lightning first imaged by observant space shuttle astronauts. And there are some highly speculative theories that may allow humans to eventually source useful energy from the sparkier parts of thunderstorms. Science, as ever, is charged with discovering both the helpful and the harmful aspects of this up-and-coming phenomenon.