By Tom Geoghegan
BBC News Magazine
When Tony Blair's Canadair Regional Jet was hit by lightning, he reportedly "didn't bat an eyelid", despite anxious faces on the media pack around him. Given the protection planes have against lightning, the prime minister had good reason not to worry.
Lightning is an electrical discharge in the atmosphere, very similar to a spark, and usually produced by a thunderstorm. It can occur within or between clouds or between cloud and ground or cloud and air.
The key to a plane's protection is its aluminium skin which conducts the electricity away, says John Sherlock, of lightning protection company Furse.
"If the lightning goes in the front of the plane, it then travels along the outer aluminium skin and exits the other end," he says. "Quite a few planes get hit, but because they're made of aluminium, it goes in one end and out the other.
"Although I've never been on a plane which has been hit, and don't want to be, passengers may just see a flash and before they know it, it's gone before they realise."
There will always be a bang, rarely a jolt and - depending on where you sit - maybe a flash, says Brian Burrows, an independent lightning consultant. Strikes usually hit on the extremities - the wing tips, the nose, fin or tailplane tips.
Many are actually triggered by the aircraft in heavily-charged clouds, in which case the lightning originates on the plane and flows away.
He says a commercial civil jet is usually struck about once a year, which amounts to about once every 3,000 flying hours.
To receive certification of being airworthy, aircraft have to undergo rigorous testing at Culham Electromagnetic and Lightning Group in Oxfordshire, where Mr Burrows worked. Planes must bear 200,000 amps of current - an average lightning strike is 30,000 amps.
The aluminium makes the structure of the plane inherently safe, he says, and it also protects the fuel tanks, which are usually in the wings. Although some planes such as the Eurofighter and Airbus use carbon fibre composites, it is safe if thick enough and used in conjunction with aluminium.
"It's not as good as aluminium but it's still good enough," he says.
But pilot avoidance of thunderstorms is also important, using a weather radar set. He or she should never fly straight through a storm because of the dangers of up and downdrafts, hailstones and lightning.
The electronics also have to be shielded from interference, which is done by the computers having built-in protection and by using back-up computers to do the same job if one should go down. Bundles of cables are kept in metal tubes or boxes.
Unable to recall a fatal incident involving British planes, Mr Burrows adds: "Protection is a complicated exercise but it works, as you can see by the fantastic safety record of aircraft."
But there have been lightning-related tragedies elsewhere. Fifty-one people were killed in China in 2000 when a Wuhan Airlines plane was struck by lightning on approach to Wuhan Airport.