There remains some confusion as to what was responsible for the crash-landing of an Aeroflot aircraft in Moscow on Sunday, but it is understood the Sukhoi Superjet100 was struck by lightning.
The Russian-made aircraft returned to the airport shortly after take-off, bounced hard on landing and burst into flames, killing 41 of the 78 people on-board.
The investigative committee is said to be considering three possible causes - poor training of the flight crew and ground personnel, a technical malfunction and bad weather. The plane's pilot Denis Yevdokimov told media that lightning had knocked out radio and electronic guidance systems, forcing pilots to fly the aircraft by hand.
But in itself a lightning strike is not usually sufficient to cause a plane crash. Indeed, it is believed aircraft are struck from above on a regular basis without incident. One study puts the rate at once per aircraft for every 1,000 flying hours - around once a year.
Typically, a bolt will hit an extremity, such as a wing tip, or the nose, and the current will travel through the aeroplane’s metal shell before leaving from another point – the tail, for example.
Patrick Smith, pilot and author of Cockpit Confidential, a book on the ins and out of air travel, agrees that planes are hit by lightning far more frequently than one might think.
“An individual jetliner is struck about once every two years, on average”, and aeroplanes are designed accordingly. “Once in a while there’s exterior damage – a superficial entry or exit wound – or minor injury to the plane’s electrical systems, but a strike typically leaves little or no evidence.” You might not even notice it, he says.
Professor Mamu Haddad, professor and director at Cardiff University's Morgan-Botti Lightning Laboratory, which works on understanding lightning strikes on aeroplane construction materials, explains further.
Modern aircraft, he says, are made from lightweight carbon composite covered with a thin layer of copper – Boeing Dreamliners and Airbus A350s have this construction – and act as effective Faraday Cages, meaning that the space inside the metal (i.e. the aircraft cabin) – is protected from electric currents.
Most important, he adds, is that the fuel tanks in the wings are not exposed to any lightning sparks – hence the surrounding metal, structural joints, access doors, vents and fuel filler caps must be able to withstand any burning from a bolt of lightning, which can have temperatures of up to 30,000C.
Strikes are most likely to happen when a jet is passing through cumulonimbus (storm) clouds, between two and five kilometres (6,500-16,500 feet) from the ground. And, like Patrick Smith, Prof Haddad says that fliers need not be concerned.
“Lightning can be up to 200,000 amps – at a low current people might hear noise, or see a flash of light through the window, but they won’t feel anything," he said. "One effect on the aircraft body might be some local melting, where the lightning struck, but the aerospace industry is highly conservative, and testing so rigorous, that passengers aren’t at risk.”
Rare though they may be, there have been several fatal incidents involving lightning strikes. In January 2014, four charred bodies were reportedly pulled from plane wreckage in Indonesia after a light aircraft owned by Intan Angkasa Air was hit by lightning and crashed. Bambang Ervan, an Indonesian transport ministry spokesman, confirmed to an Australian news site that all four people on board the aircraft were killed instantly.
In 2010, two people were killed when a Boeing 737-700 from Bogota was struck by lightning and split into three pieces as it landed at San Andres island in the Caribbean. At the time, aeronautical specialists explained that the lightning alone was unlikely to be the cause of the accident, but combined with a sharp change in wind direction, or an air pocket linked to lightning when a plane is near the ground, it could cause a crash.
Another serious case, resulting in 81 deaths, happened in 1963, when a lightning strike over Maryland caused a wing to explode on a Boeing 707 flown by Pan Am. The Federal Aviation Administration, the US equivalent of the Civil Aviation Authority, subsequently introduced changes to fuel tanks and discharge wicks aboard all aircraft. The crash is often cited as the last caused by a lightning strike
Non-fatal incidents are far more common. Famous cases include the flight taken by François Hollande, the French President, to crucial talks with Angela Merkel in Germany in 2012. The presidential Falcon 7X was struck by lightning just four minutes into the flight; Mr Hollande eventually arrived in Berlin 90 minutes late, on a different plane.
Patrick Smith remembers having a close encounter with lighting when he was at the helm of a 37-seat aeroplane.
“Lightning from a tiny embedded cumulonimbus cell got us on the nose," he said. “What we felt and heard was little more than a dull flash and a thud. No warning lights flashed, no generators tripped off line. Our conversation went:
‘What was that?’
‘I don’t know.’
‘Might have been.’
Mechanics would later find a black smudge on the forward fuselage.”
In other words, an incident is likely to be over in a flash, literally, leaving passengers on board unaware. It’s often those on the next flight who could be delayed, as the plane undergoes post-lightning safety checks.