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

Auk, The, Jul 2002 by Sodhi, Navjot S

All things being equal, a solitary individual will cause less damage to an aircraft than will a flock. The number of birds that strike aircraft varies with species. Usually, ducks, geese, herons, owls, and doves collide with aircraft as individuals. However, shorebirds and starlings usually hit aircraft in flocks.

Is THERE A DANGEROus TIME?

Numerous factors can affect bird strikes on aircraft. Below, I discuss some of the more important factors.

Timing of bird strikes.-At the Christchurch International Airport in New Zealand, bird strikes peak at midmorning (0900), and there is another, smaller peak at night (2000) (Chilvers et al. 1997). However, strikes by sparrows peak at about 0800, whereas those by gulls peak at midday. At the John E Kennedy International Airport, most gull strikes occur between 0500 and 0900 (Burger 1985), but non-gull strikes do not show any diurnal peak time. Although approximately 10 to 17% of bird strikes can occur during night (Neubauer 1990, Satheesan and Grubh 1992), nocturnal birds are generally ignored in bird strike monitoring and control. That may be partly due to difficulty in sampling nocturnal birds and, in some cases, difficulty in accurately assigning the timing of strikes to bird carcasses that are found at airports.

Effects of weather.-For the U.S. Air Force aircraft, 61% of bird strikes occur during clear weather, when both birds and aircraft are more active (Neubauer 1990). To save energy, migratory birds usually use tail wind to fly. However, wind speed does not significantly affect bird strikes (Manktelow 2000). There is a positive correlation between bird strikes and mean monthly rainfall at Lihue Airport (Linnell et al. 1996). That correlation is probably because of increased seed production along the runways during the rainy months, which attracts granivorous birds. Similar results have been found in the United Kingdom (Manktelow 2000).

Seasonal variation.-The chance of a bird strike is 5 X higher during the migratory season than at other times (Jerome 1976). Bird strikes with the U.S. Air Force aircraft usually peak coinciding with the spring and fall migration (Neubauer 1990). Other authors have reported similar results (e.g. Blokpoel 1976). A large number of fatigued birds probably results in more bird strikes during migration. Heightened pilot awareness during the migratory season may also be at least partly responsible for more reporting.

More bird strikes occur in April than at any other time of year at the Christchurch International Airport. That is the time when fledglings are abundant, and they are possibly less successful at evading aircraft (Chilvers et al. 1997).

CAN PILOTS Do ANYTHING?

Approximately 90% of bird strikes occur 2,000 m (Satheesan 1990). For military aircraft, 56% of bird strikes occur at

IMPROVEMENTS To AIRCRAFT DESIGN

Aircraft speed is a major factor in crashes due to bird strikes (Niering 1990). That is because the kinetic energy that is dissipated during a bird strike increases with the aircraft speed. There is probably no jet engine in the world that can ingest as large a bird as a Canada Goose and still fly (Eschenfelder 1990). Based on bird-strike data, efforts are underway to improve aircraft so that they can withstand a greater impact (Niering 1990). Those efforts include new material designs for aircraft engine compressor blades, stronger windshield design, and more damage-resistant wings. For military aircraft, windshields need further strengthening modifications, and some of the older aircraft are probably still vulnerable during bird strikes (Neubauer 1990). Previous lessons are sometimes taken into account when making recommendations to improve aircraft design. When a DC10 remained in the air for 10 min after two of its three engines were hit by birds in 1973, the Bird Strike Committee of Europe recommended that European airbuses should have three engines instead of two (Solman 1978).