Newton, MA—The National Transportation Safety Board is investigating whether one or both engines of American Airlines Flight 587 failed after ingesting a flock of birds, according to AP news reports on Tuesday.
Such a bird strike could have triggered a massive chain reaction, ultimately downing the Airbus A300, says Joe Metrisin, a senior structural engineer at Florida Turbine Technologies, a maker of gas turbine engines for commercial and military aircraft.
"If it occurred at altitude, a bird ingestion would be consistent with the situation, unless something came loose from the airframe," says Metrisin.
"The bird would probably strike a fan blade, breaking it off," explains Metrisin, who uses simulation software to predict the structural integrity of fan blades, including their performance in bird-plane collisions. "Then either the blade might penetrate the casing of the engine, hitting either the engine mount or part of the wing and causing that to fail. In another scenario, the imbalance caused by the loss of the blade could inflict further damage on the engine, such as a disk rupture, which would be catastrophic."
That might explain why witnesses reported seeing a flaming engine break away from the plane in mid-air.
Bird strikes are not uncommon. According to the Bird Strike Committee USA, a group formed in 1991 to promote the development of new technologies to reduce wildlife hazards, 5,900 bird-plane collisions were reported in 2000. Damage to aircraft due to bird strikes and other wildlife encounters is estimated to be in the hundreds of million dollars annually.
The deadliest recorded bird strike to date occurred in 1960, when a jet was struck by a flock of starlings and plunged into Boston Harbor. Sixty-two people lost their lives.
Although the FAA requires that jet engines undergo rigorous tests to ensure that they can withstand bird strikes of varying degrees, design trade-offs abound.
"In order to protect the aircraft, the engine case is designed to contain a released blade. But containment of the blade can actually cause additional blades to be liberated, which may lead to catastrophic engine failure," says Metrisin.
He also stresses that it is impossible for design engineers to prevent every worst case scenario. "Take a Canadian goose, for example," says Metrisin. "Those things weigh about 20 lbs, and there’s almost no way you can design engines to withstand that type of impact. They would be so heavy that the plane would never get off the ground."