Perspectives in ornithology: Competition in the air: Birds versus aircraft

Auk, The, Jul 2002 by Sodhi, Navjot S

THE FIRST KNOWN aircraft fatality that was directly attributable to a bird occurred in 1912, when a gull (Larus sp.) was caught in the control cables of an aircraft, causing it to crash. Since that time, aircraft have generally increased in size to carry more passengers. Birdaircraft conflicts are becoming more common recently, which is possibly due to increased numbers of both aircraft (e.g. an estimated 28 million jets now take off in the United States as compared to 18 million in 1980) and some kinds of bird species (e.g. Canada Geese [Branta canadensis], in the United States have quadrupled to 2 million since 1985). Between 1990 and 1998, there were an estimated 22,000 bird-aircraft collisions in the United States, which cost an annual $400 million in aircraft repairs. This bird-aircraft conflict takes place around the world, although the species, situations, and severity differ. It is estimated that at least 350 people have been killed in bird-aircraft collisions worldwide.

Understanding bird-aircraft conflict is critical due to monetary reasons and the potential threat to human life. Despite the severity of the situation, bird-aircraft conflict has largely remained on the fringes of rigorous ornithological investigations, and sound ornithological understanding is still required to find longterm management solutions for that conflict. I hope that this review will stimulate ornithologists to show more interest in this crucial issue.

HUMAN SAFETY AND ECONOMICS

Incidents.-On average, the aircraft of the U.S. Air Force incur 2,500 bird strikes annually (Lovell 1997). Out of those, one human death occurs per 2,000 strikes (Neubauer 1990). Most air crashes occur when a bird hits the windshield or is inducted into the engine. In terms of civilian aircraft, over 5,000 bird strikes were reported in the United States during 1999 alone. Between 1950 and 1999, 286 serious birdrelated accidents of military aircraft (in which the aircraft were destroyed or there were fatalities) occurred in 32 countries. Of those accidents, 63 were fatal, which resulted in 141 deaths (Richardson and West 2000). These birdstrike incidents, at least in some cases, are minimum estimates because pilots only report 20 to 30% of actual strikes (Burger 1985). Pilots are thought to underreport bird strikes either because they are unaware of the strikes or because of the inconvenience of filing reports (Solman 1978, Linnell et al. 1999, Brown and Hickling 2000). Sometimes, strikes by large bird species (>350 g) such as Brahminy Kites (Haliastur indus) and Cattle Egrets (Bubulcus ibis) go unreported by pilots (N. Sodhi pers. obs.). Therefore, runway carcass searches must supplement pilot reports to correctly evaluate the bird threat at airports.

Economic losses.-The cost of repairing an aircraft damaged by a bird strike can vary from very little to millions of dollars when an aircraft is lost. The aircraft component that is most frequently damaged by bird strikes is the engine. International Civil Aviation Organization's (ICAO) analysis shows that bird strikes damaged 200 engines on or near airports around the world in 1996. The cost of repair due to bird ingestion can range from $250,000 to $1 million, depending on the type of engine. However, there have been cases in which the cost of aircraft repair has been as high as $6 million, as was the case for an Air France Concord that was struck by a number of Canada Geese in 1995 on approach to the John E Kennedy International Airport in New York City. It is predicted that bird-aircraft conflict will become costlier due to the plans for increased numbers of wide-bodied jets in the air (Robinson 2000).

The cost of the bird management program at the Christchurch International Airport in New Zealand is about twice that of repairs to aircraft that are damaged by bird strikes. However, that does not include the costs of lost flight time, passenger disruption, and passenger safety (Chilvers et al. 1997). Annually, aircraft spend 461,000 h on ground in the United States due to bird strikes (Cleary et al. 1999). The cost of bird strikes in terms of human morbidity and mortality has not been rigorously investigated (Neubauer 1990). One human fatality can cost up to $2.5 million. Other studies show that bird management actions have halved the cost of repairs to aircraft that are damaged by birds (e.g. Solman 1973).

Military versus civil aircrafts.-Military aircraft are usually more vulnerable to bird strikes than civil aircraft because they typically travel at high speeds at low altitudes (30 to 300 m), where most birds fly. Approximately 54% of the bird strikes on military aircraft and 90% of those to civil aircraft around the world occur in or near to airfields (e.g. during take off) (Smith 1986, Neubauer 1990, Cleary et al. 1999). However, those figures should be viewed with caution because bird strikes en route can go unreported. Military aircraft are also vulnerable at bombing ranges where pilots do not always adequately detect approaching birds (Neubauer 1990). The number of reported bird strikes on military aircraft in the United States increased steadily between 1974 and 1987. However, that could have been due to heightened pilot awareness of the need to report collisions. Thus, bird-aircraft collisions are not uncommon and can result in loss of life and high costs.