Safety : Feathers Strike Fear

A tiny spot appears On the horizon, closes in with startling rapidity and WHAM! Depending on the speed and stage of flight, there may be little or no time to take pre-emptive action against getting hit by a winged menace. At times, the bird emerges undetected from the dust or smoke filled air. Often, the damage is negligible, perhaps a bloodstain or a minor dent. On the other hand, the impact could trigger a catastrophe—major damage to the airframe, injury to the crew, even engine failure and a crash.

Day in and day out, at airports around the globe, birds are a prime source of worry, especially for pilots. A bird strike (also called a bird hit, or BASH—Bird Aircraft Strike Hazard) is a collision between a bird (or bat) and an aircraft. Over 60 per cent of all bird hits occur below 30 m, less than 8 per cent above 1,000 m. Aircraft are regularly struck by birds at their most vulnerable periods—during takeoff and landing—when close to the stall speed, the safety margin is minimal, and the already preoccupied pilot must make a hurried decision. Flocks of birds are especially dangerous and can cause multiple strikes. Ask Chesley B. Sullenberger, captain of the US Airways Airbus A320 who, on January 15, flew through a flock of Canada geese a minute after takeoff and lost both engines. Exhibiting great skill and presence of mind, he managed to ditch in the Hudson River and save the lives of all 155 passengers and crew.

Though bird strikes are a growing hazard, fatalities for civil aircraft are remarkably low. It has been estimated that there is just one fatal accident involving a jetliner in one billion flying hours. The multi-engine configuration is obviously crucial, since an affected engine can be shut down and the aircraft recovered safely. A single engine military jet pilot may not be so lucky—often ejection is the only course of action, if there is sufficient time for the parachute to open. Military jets frequently operate at very low level, putting them squarely in the maximum danger zone. No wonder fighter flying abounds with accidents attributable to birds.

The first recorded bird strike fatality involved Calbraith Rodgers who, in 1911, achieved fame by flying across America from coast-to-coast in 49 days. On April 3, 1912, while flying an exhibition routine near Long Beach, California he apparently struck a seagull, which fouled his controls and caused him to crash into the breakers and drown. Since then, fatal accidents have occurred with depressing regularity. The greatest loss of life directly linked to a bird strike was on October 4, 1960, when a Lockheed L-188 Electra taking off from Boston, flew through a flock of starlings, damaging all four engines. The plane crashed into the harbour, with 62 fatalities out of 72 passengers and crew. Worldwide, since 1960, more than 25 large aircraft have been lost due to bird strikes. Since 1988, more than 219 people have been killed and more than 200 civilian aircraft destroyed in such accidents. Military statistics are harder to come by, but the toll of fighter aircraft (and lives) is undoubtedly several times higher.

Minor Damage, Major Damage
In the majority of bird strikes, 85 per cent or more, nothing happens—the bird bounces off the plane or a small bird is swallowed by the engine without discernible effect. Just 12 to 15 per cent of bird strikes result in significant damage. It mainly boils down to the equation for kinetic energy: energy is proportional to mass times velocity squared. Greater the difference in the velocity of the plane and the bird, greater the impact on the aircraft. A 5.5 kg bird hitting an aircraft travelling at 240 km/h at lift off generates the same force as a 450 kg weight dropped from a height of 3 m. Design requirements for modern airliners include the ability to withstand collision with one four pound (1.8 kg) bird; the empennage (tail) must withstand twice this.

Far worse than the airframe taking a hit is when a bird strikes an engine fan blade. Jet engines are like gigantic vacuum hoses sucking in any object in their path. The initial stages of the engine consist of many compressor blades, not very thick, and hence fairly vulnerable to damage. A bird impact could displace a blade such that it strikes another blade and a cascade could occur, resulting in practically instantaneous engine flameout. By specification, an engine does not need to continue running after ingesting one 1.8 kg bird; it must merely stop or be shut down without exploding or catching fire. Jet engines are particularly vulnerable during takeoff when the engine is rotating at very high speed.

Apart from the damage directly caused by bird strikes, accidents could occur as a consequence of bird avoidance action. Around the world, cases of emergency landings, aborted takeoffs and other hair-raising incidents due to collisions with birds are mounting. In May 2008, a Boeing 747 cargo plane departing Brussels ingested a bird into one engine. The captain made a splitsecond decision to abort takeoff but was unable to stop before the end of the runway. Although there were no fatalities, the plane broke in two and was destroyed.

Bird strikes are five times more likely to occur on planes with engines mounted under the wing, such as the Boeing 737 or the Airbus A320, than on planes with engines mounted on the fuselage, like the Boeing MD-80, probably because the airflow over the MD-80 causes birds to get blown away from the engines.

More Bash From More Birds
Is the number of bird strikes rising? It is. There were 4,300 bird strike cases involving US civilian aircraft in 2003; these jumped to around 7,600 in 2007. In India, 251 cases of bird hits were reported in 2008, against 217 in 2007. In Delhi alone, 39 cases were reported in 2008, against 30 in 2007. Since reporting of civilian bird strikes is voluntary in many countries, most strikes probably go unreported, especially when there is little or no damage. Some experts estimate that an astounding 80 per cent of bird strikes are not reported.

The reasons for mounting bird strikes are not far to seek—in-creasing flying intensity, growing populations of some large species of birds and increasing familiarity of the birds with human activity. Besides, modern turbofan engines are rather quiet. Whatever little noise they produce is to the rear; there is hardly any noise in front of the aircraft from where the birds emerge. So birds have less ability to detect and avoid aircraft. The situation is likely to get worse as airlines replace their older three- or four-engine fleets with more efficient and quieter two-engine aircraft, increasing the chance of a major accident because the plane has less backup power.

In North America, there has been a remarkable growth in populations of many large birds—such as the great blue heron, osprey, bald eagle, snow goose and Canada goose—because of wildlife conservation measures. Cleaning up the environment, banning DDT and cleansing water bodies are all fine things, but they have resulted in incredible surges of many species hazardous to aviation. In 1890, about 60 European starlings were released in Central Park, New York City. Today, starlings are the second most abundant bird species in North America with a latesummer population of over 150 million. Starlings are regarded as feathered bullets because they have a body density 27 per cent higher than most birds. The non-migratory Canada goose population increased from one million birds in 1990 to over 3.5 million in 2007.

In India, though no such dramatic increase in bird numbers has been noticed, human habitations have inched closer and closer to airports, with poor garbage disposal practices attracting huge flocks of scavenging birds, often in the direct path of aircraft.