Storm threat to modern aircraft

It is possible that the fury of an equatorial storm brought down Air France flight 447.

The plane's flight path seems to have taken it through what meteorologists call the inter-tropical convergence zone.

This is where two air masses meet, sending huge storm clouds more than 40,000ft (12,000m) into the sky.

Eight years ago, former British Airways captain Roger Guiver was confronted with an enormous storm during a flight from Cape Town to London Heathrow.

"You take weather like that extremely seriously," he says. "You don't go anywhere near it."

There are two potential dangers - lightning and severe turbulence.

Lightning

Lightning can strike anywhere - the charge flows around the plane's skin and can damage electrical systems.

Wing tip
"Static wicks" on aircraft wings are meant to dissipate electricity

But aircraft wings have what are called "static wicks" which dissipate the electricity safely.

Bored, long-haul passengers looking out of the window at the wings will spot them - thin, aerial-like structures, trailing in the slipstream.

Roger Guiver says one dramatic warning of a possible lightning strike is St Elmo's Fire - static that flickers over the windscreen as the plane flies through a storm.

But lightning almost never causes air crashes, at least directly.

The respected Aviation Safety Network database lists just 15 incidents in more than 50 years of aviation history.

The worst was the loss of an Iranian Air Force Boeing 747 in 1976 near Madrid. Lightning ignited vapour in a fuel tank, causing an explosion.

Electrical faults

If lightning did strike AF 447, it is more likely to have caused the electrical faults mentioned in automated maintenance messages which were sent out over a satellite network shortly before the plane disappeared.


[Turbulence] is not frightening for us, but it's awful for the passengers at the back
Roger Guiver
Former British Airways captain

But electrical problems may in turn have resulted in the crew losing vital control systems, or a fire.

The pilot's main weapon against turbulence is weather radar - the receiver mounted in the nose of the aircraft can pick up signs of storm clouds ahead, which are displayed in the cockpit.

Crews aim to fly at least 10 miles (16km) around the worst storms, for reasons of safety and passenger comfort.

But it is not an exact science. Weather radars detect moisture primarily, and sometimes struggle to identify ice crystals, which can be present in the worst storm clouds.

At night, storms cannot easily be seen by eye.

Turbulence

Pilots try to avoid ending up in big storm clouds because of the forces they can impose on a plane.

At cruising height, the plane must be kept to precise speeds - the altitude means changes of speed can cause stalls.

Air France Airbus A330-200 believed to be the missing plane - archive image from AirTeam Images

But in a storm, the plane can be lifted up or thrown down in the turbulence, making it difficult for the autopilot to fly within the limits.

"It's not frightening for us, but it's awful for the passengers at the back," says Roger Guiver.

Investigators will want to discover if the Air France Airbus suffered such severe turbulence that it caused catastrophic structural damage - the loss of a rudder, engine mounting, or even a broken wing.

This would be extremely rare.

The wings on newly designed aircraft are literally tested to destruction by bending them at least 50% beyond the kinds of forces produced in a storm.

As ever with air accident investigations, finding the cockpit voice recorder and flight data recorder is key.

They may be up thousands of metres below the surface.

But the US National Transportation Safety Board says the homing beacons they carry should be detectable down to 14,000ft (4,300m).

Retrieving them is tricky but, as military salvage experts point out, these days there is no place on the ocean floor where remotely operated vehicles cannot go.

The boxes and key sections of wreckage may be winched up, so that investigators can begin the task of explaining the fate of flight AF 447.