How birds can bring down a plane

Dozens of passengers have had an amazing escape after their plane came down in New York. One theory is that the jetliner hit a flock of birds. It may sound like a freakish event, but "bird strikes" are an age-old problem for the aviation industry.

The plane ditched in the Hudson River shortly after take-off

Even the earliest pioneers of flying machines, the Wright Brothers, had trouble with birds.

In 1905 they wrote in their diary: "Chased flock of birds for two rounds and killed one which fell on top of the upper surface and after a time fell off when swinging a sharp curve."

No-one was hurt in that incident, but seven years later another aviation trailblazer was not so lucky.

Nicknamed "The Birdman", Calbraith Rogers was the first person to fly across the US. Months after he completed his historic journey, he flew his biplane into a flock of birds and crash landed, dying of a broken neck.

According to reports from the time, a seagull jammed the controls in Rogers' plane.

Danger of flocks

With modern passenger planes, the technology is clearly more advanced, but the dangers posed by birds have not gone away.

Airport authorities around the world try to scare birds away - usually by driving around the airport in a truck with speakers, blasting out calls from birds of prey.

Kevin Poormon, a senior research engineer at the University of Dayton in the US state of Ohio, says the Federal Aviation Administration (FAA) stipulates planes must be able to withstand a strike from an 8lb (3.6kg) bird.

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"Aircraft are being struck every day by birds - the reason you don't hear about them so much is they are designed to take these impacts," he told the Associated Press.

"But once you get to large flocks or large birds striking at a critical moment, that's where these events hit the news."

It is thought that the plane involved in Thursday's incident had both of its engines taken out after hitting a flock of Canada geese - which can weigh from about 3lb to 12lb.

According to the Bird Strike Committee, a US-based organisation, a 12lb Canada goose struck by an aircraft at lift-off would generate a force equivalent to a 1,000lb object being dropped from a height of 10ft (3m).

Rarity of 'double strike'

Chris Yates, an aviation expert for Janes Information Group, says plane engines are very delicate and a bird such as a Canada goose being sucked into the engine would prove catastrophic if it smashed the rotor blades.

"The debris will be spread around the rest of the engine," he told the BBC.

"We're not talking about bird bones here, we're talking about large chunks of metal crashing around inside an engine - it can impact virtually any part of the engine."

But he says instances where both engines are hit by birds are extremely rare - and bird strikes are a problem for large passenger jets only on take-off and landing, because jets generally fly at a higher altitude than birds.

Overall, statistics from the US authorities compiled by the FAA suggest some 219 people have died since 1988 in incidents involving animals colliding with planes.

The FAA received almost 76,000 reports of bird strikes between 1990 and 2007.

For the 18-year period, reports were received of 11 deaths in eight separate incidents involving bird strikes in the US.

sources above found at http://news.bbc.co.uk/2/hi/americas/7832539.stm