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Sumesh Das said...

Brutal freak turbulence is the most plausible cause of the crash of Air France Flight 447. If lightning alone caused the crash questions would be asked about the design of the A330, a medium-sized long-range airliner that enjoys a high reputation with the world's airlines.

No major crash has been directly blamed on lightning for more than four decades. Large aircraft are regularly struck by lightning but very rarely suffer damage from the bolt, which passes along the exterior of the fuselage and is diffused into the open air, mainly from small tabs on the trailing edge of the wings. Recent US statistics showed that every commercial aircraft is struck by lightning at least once a year.

The violent turbulence in the heart of storms is a threat to even large aircraft, however. Airliners usually avoid them by flying over or around the biggest ones. Smaller aircraft are from time-to-time torn to pieces in storms or thrown on the ground by them when they are approaching to land or taking off.

The best-known case of turbulence causing a commercial airliner crash was when a BOAC flight from Tokyo to Hong Kong went down near Mount Fuji in 1966 after encountering a storm.

All 113 passengers and 11 crew on board were killed and the subsequent inquiry found the probable cause of the disaster was that “the aircraft suddenly encountered abnormally severe turbulence which imposed a gust load considerably in excess of the design limit.”

The early data from France and Brazil today does not make clear whether the Airbus power failed directly because of lightning or whether the circuits were cut by some catastrophic failure in the aircraft.

All that is known is that the aircraft was in a tropical storm system and that electrics failure was brutal and sudden. It killed all the multiple means by which modern airliners communicate with the ground and prevented the use of back-up power — otherwise the pilots or the automated system would have been able to send a distress message.

Lightning could, for example, cause a fuel tank to explode, as happened on at least one occasion in the 1960s. It is more likely that the extreme turbulence of a tropical storm could have upset the aircraft, causing an uncontrollable descent and an in-flight break-up that would have cut the power. That scenario has been followed by numerous smaller aircraft over the decades. Ronan Hubert, a French airline accident expert, said on French television that this was the likely sequence of events.

All that is known so far is that the automatic data link in the Air France aircraft reported an electrical power loss to the Paris central control, which monitors the company's flights. Over the ocean, the aircraft would not have been tracked by conventional radar. But the aircraft would be reporting its position to controllers at fixed intervals. The information is fed into a central system that provides radar-like tracking. A power failure would interrupt this data flow.

All modern airliners are designed with multiple systems to protect their sensitive electronic systems from lightning discharges that would otherwise cause power surges that would fry their software and blow circuit breakers. To be certified as airworthy, the plane-makers must show the authorities that the aircraft can withstand jolts of electricity equivalent to lightning.

June 2, 2009 1:04 AM  

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