Steve Connor: Bad weather should not bring down a modern jet
Tuesday 02 June 2009
The best guesses as to what went wrong with Flight AF 447 from Rio de Janeiro to Paris are that it either experienced severe turbulence that led to catastrophic structural damage or that it was struck by lightning which caused the total failure of its electronic controls and communications.
The Met Office in Britain said that the aircraft was flying through the intertropical convergence zone near the Equator where there are a series of high-pressure cells that generate the sort of weather conditions leading to tropical thunderstorms.
The cumulonimbus clouds of such thunderstorms – or rather the rainfall associated with them – can be picked up by the aircraft's weather radar and pilots are trained to fly around them. The Met Office said that the top of the clouds in the intertropical zone in that area reached about 45,000 feet at the time of the accident but that two earlier flights in the same area had not reported any electrical activity.
However, a spokesman for the Airbus company said the aircraft might have been struck by lightning as it entered the storm region. It is well known that lightning can strike some distance away from the centre of such storms.
Experiments show there are two types of lightning strike. One is caused by a build-up of static electricity on board the aircraft as it passes through storm clouds and the other is a direct contact with lightning. Although lightning can be terrifying for passengers, it rarely causes problems. Sometimes it can result in burn marks on the fuselage or it can interfere with the aircraft's electrical systems.
A particular problem with modern aircraft is that they use composite materials that do not conduct lightning as well as aluminium. This can cause these parts of the aircraft to crack when struck, but again this rarely causes problems. Modern aircraft are also designed to be protected against the transient fluctuations in voltage caused by the indirect effects of lightning, including the magnetic fields and potential differences that occur between different parts of the aircraft's airframe as it passes through a storm.
The Airbus A330 is a "fly-by-wire" aircraft meaning that much of its flight is controlled by computer but this is designed with double or even triple backups to prevent any catastrophic failure during a thunderstorm.
Its fuel system is also designed to minimise the risk of fuel vapour ignition during a lightning strike, which is known to have caused about a dozen fatal accidents over the past 40 years, such as the loss of an Iranian Air Force Boeing 747 in 1976 near Madrid, which killed 17 people.
Another possibility is that the aircraft experienced such heavy turbulence that it suffered a catastrophic mechanical failure and broke apart. Maximum storm turbulence usually occurs between 12,000 and 20,000 feet, well below the aircraft's cruising altitude of 35,000 feet.
But again, modern aircraft are designed to withstand the sort of turbulence they are expected to experience, which means the sudden disappearance of AF 447 cannot be easily explained by a catastrophic mechanical break-up in mid-air.
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