Debris is seen at the crash site of Air Algerie flight AH5017 near the northern Mali town of Gossi, July 24, 2014. Poor weather was the most likely cause of the crash of an Air Algerie flight in the West African state of Mali that killed all 118 people on board, French officials said on Friday.STRINGER/Reuters
Bad weather doesn't cause plane crashes, at least not for modern, high-flying jetliners.
While very powerful storms can still tear the wings off a small plane and icing, especially for turboprops, remains a dire risk to the avoided, the gravest danger posed by weather comes when pilots fail to avoid it or press on despite the additional dangers posed by extreme winds, hail, rain and lightning.
Weather is being touted by some French officials as the "probable cause" of the crash of a Swiftair MD-83, which was leased to Air Algérie and was flying an overnight route from Burkina Faso to Algiers when it crashed in Mali soon after its pilots told air traffic control they wanted a diversion. Huge thunderstorms lay across the aircraft's route – the roiling tops of some soaring to more than 16,000 metres – well above the maximum altitude an MD-83 can reach.
Pilots routinely divert – or are diverted by air traffic control – to go around thunderstorms or thread through the gaps in a line of storms. The advent of nose-mounted weather radar that paints a picture of the path ahead to pilots has made storm avoidance far easier.
Nevertheless, bad weather can catch unwary or unlucky pilots by surprise, forcing them to cope with sudden or unusual flying demands.
The compact crash site and massive destruction of the Swiftair MD-83 are indicative of a high-speed impact but until details emerge from the flight data and cockpit voice recorder it remains unknown how and why the pilots lost control of the twin-engined, medium-range, 18-year-old airliner high above the southern Sahara early Thursday morning.
Most aviation accidents, including those where bad weather is a factor, happen on landing or take-off. Only very rarely do pilots fail to cope with bad weather at high cruising altitudes.
The most recent, now notorious, case involved three Air France pilots who completely lost control of a perfectly flyable and undamaged Airbus A-330 on an overnight flight from Rio de Janeiro to Paris five years ago. Most other aircraft crossing the line of huge equatorial thunderstorms that night diverted around the most intense weather. On the Air France A-330, the pitot tubes – small detectors that jut out from the fuselage to determine airspeed – iced up, resulting in the autopilot disconnecting and requiring the pilots to fly manually.
Confused and disoriented, the two junior pilots couldn't agree whether to try to climb or descend and seemed unaware that the aircraft had slowed, stalled, and was now falling towards the sea 10 kilometres below. The senior pilot, who had been taking a mandatory sleep break, had returned to the cockpit but he too failed to comprehend that the aircraft was in an aerodynamic stall – in effect had ceased flying.
Shortly before the aircraft slammed into the sea, killing all 228 on board, one junior pilot said: "You're going up. Go down, go down, go down." The other junior pilot replied: "Am I going down now?" and the captain, standing behind the other two said: "No, you're going up."
After spending more than two years and more than $50-million to recover the flight recorders, details of cockpit confusion emerged. "The pilots showed a "total incomprehension of the situation," said Alain Bouillard, the lead investigator with France's accident investigation agency, the Bureau d'Enquêtes et d'Analyses.
While AF447 stands as an extreme example of how a weather-related initial problem could sometimes spiral into a major disaster, it isn't the only one.
There have, for instance, been occasions when large hailstones in severe thunderstorms destroyed jet engines.
In 1977, a Southern Airways DC-9, the predecessor of the MD-83 flown by Swiftair, attempted an emergency landing on highway in Georgia after losing both engines. The pilots apparently misread their own weather radar mistakingly believing they had diverted to an area of moderate rain as they attempt to weave through a line of thunderstorms.
Instead the aircraft encountered massive hail – big enough to shatter the cockpit windows – and torrential rain. Lighting struck one wingtip. Both engines flamed out after sucking in massive amounts of rain and hail.
The pilots desperately asked air traffic control for any airport within gliding distance. "We've lost both engines–how about giving us a vector to the nearest place. We're at seven thousand feet," they radioed. Minutes later, they managed to put the stricken DC-9 down on a straight stretch of rural highway, but it hit a gas station. The pilots, 60 passengers and nine people on the ground were killed. Twenty two people survived the crash.