February 25, 2009

With two aircraft brought down in two months and an ever present danger around the world, what is being done to minimise the risks of bird strikes? Ben Hargreaves investigates

There is an almost unbearable pathos to transcripts of final communications between a doomed airliner and the control tower. In the case of US Airways flight 1549, which successfully ditched in the Hudson River in New York last month, the narrative is one of remarkably bad luck – followed by cool thinking and decisive actions that saved the lives of all 155 people on board.

The flight, which ran into trouble just three minutes after its take-off from LaGuardia airport, made a national hero of former airforce pilot Chesley B Sullenberger III. Sullenberger, who guided the Airbus A320 to rest in the Hudson, avoiding bridges and gliding into the river at the correct angle so the plane would not break up, knew straight away what the problem was with the aircraft. At 8.27pm, the transcript records the captain saying: “Hit birds. We lost thrust in both engines.” Having lost all power, and unable to reach runways at either LaGuardia or nearby Teterboro airports, Sullenberger informs the control tower calmly: “We’re going to be in the Hudson.”

It was by no means unheard of for a flock of large birds, in this case Canada geese, to get sucked into the engines. Last November, a Ryanair flight for Rome from Frankfurt suffered a similar serious bird strike, when it was hit by a flock of thousands of starlings, and was forced to make an emergency landing at Rome-Ciampino airport, which led to the collapse of the plane’s left landing gear. The pilots of the Ryanair craft, like Sullenberger, won plaudits for bringing the aircraft down safely.

Bird strikes have been thrust into the public consciousness since these two incidents, and it’s possible, if you fly regularly, that you have experienced one. Most never result in catastrophe, or are even reported. “Most bird strikes do very little damage,” says John Ling, the engineering programme manager for transport at the IMechE. He says that since a plane can still fly on one engine to suffer severe bird strikes in both engines simultaneously had to make Sullenberger and his crew “desperately unlucky”.

Dr Stuart Bounds, head of dynamic modelling at Atkins Aerospace in Bristol, makes the analogy that bird strikes are similar to cycling along a country road in the summer. “Every now and again an insect hits you, and it’s fine, or at worst you swallow one and you keep going. But very rarely a bee hits you – and that can actually hurt at speed.”

Engineers at aero engine manufacturers work hard to ensure that engines can withstand a bird strike, using guns to fire physical models, or even “euthanised” birds, into gas turbines to assess their ability to withstand damage. Aerospace engineers also test the plane’s wings and fuselage to determine resistance to bird strikes. This is an important part of the work that Atkins Aerospace carries out for its clients, which include Airbus.

“We do a lot of work on the airframe in terms of bird strikes,” says Bounds. In these days of high computing power, the modelling of a strike is carried out through simulation, with physical testing being used to validate the computer model. Bounds and his team use linear finite element analysis packages to model components and test their resistance to a strike. At BAE Systems, engineers also use computer packages to model bird strikes on military aircraft, although there is no substitute for final physical testing. BAE has even developed synthetic birds manufactured from gelatine to be fired from gas guns into aircraft components such as windshields, windows and wings. This helps to ensure the repeatability of physical tests, the company says.

Of particular concern to aerospace engineers are leading edge parts of the wing, which are particularly vulnerable to bird strikes during take-off and landing. Birds hitting the aircraft are not just a headache in terms of safety — strikes hard enough to damage the airframe mean the plane has to be grounded and repaired.

Local media in New Zealand reported earlier this month that a bird strike punched a fist-sized hole in the wing of an Air New Zealand A320 during a flight from Melbourne to Christchurch. The accident was apparently not discovered during the flight but only on later inspection of the aircraft. Technical staff repaired the damage overnight and the aircraft returned to service.

As Bounds and his team model bird strikes, they look to ensure that components will not be compromised by the impact of 4lb birds hitting them at cruising speed.

The “bird” used in simulation is typically cylindrical with hemispherical ends. At speeds of 350 knots, such as when a plane is taking off or landing, the bird essentially behaves like a fluid on hitting the aircraft. The key is to ensure that the strike does not compromise vulnerable parts of the aircraft on the leading edge. “The energy of a bird hitting the leading edge at 300 knots is roughly equivalent to dropping a car from a metre,” Bounds says. A bird strike on the wing could potentially knock out aircraft systems, destroy control surfaces such as slats or ailerons, or even damage the fuel tanks. A hit on the fuselage could result in depressurisation of the cabin. Windscreens also need to be bird-proof as the cockpit is vulnerable.

Engineers at Atkins also simulate other types of dangerous event such as as impact from runway debris and tyre bursts. A bursting tyre caused the Concorde crash in 2000 that killed more than 100 people.

The engineers might focus on the vulnerability of the underside of the aircraft to such accidents. In the case of Concorde, debris from the tyre punctured the lower wing cover on the bottom of the airliner, resulting in a massive fuel leak and explosion. The plane was only allowed to fly again once a perforated Kevlar lining was incorporated into the base of the fuel tanks to protect them. The lining had to be perforated to allow the fuel to circulate around the wing. On Concorde, the fuel was used to help cool the wings’ skin during supersonic flight.

Advances in computer power are likely to improve the quality of bird strike simulation and allow for a greater understanding of how newer materials such as carbon or glass fibre composites behave during a strike. “We have a good understanding of metals but with composites there is more to be learnt,” says Bounds. “There is a continued effort to build computer models of composites and look at the impact of bird strikes.”