Chapter 7 — Bird- and Mammal-strike Statistics

Introduction

Aviation-industry decisions delicately balance safety and budgetary concerns while attempting to assess exposure to, probability of and severity of wildlife strikes. Developing effective risk-management strategies therefore relies heavily on the collection and analysis of data derived from bird- and mammal-strike statistics.

This chapter evaluates available data, and examines important trends that may help stakeholders reduce the risk of wildlife strikes.

Getting the definitions down

To ensure consistent statistics, it’s important that all parties reporting wildlife strikes adhere to the same criteria. According to the Bird Strike Committee Canada, a bird strike is deemed to have occurred whenever:

  • a pilot reports a bird strike;
  • aircraft maintenance personnel identify damage to an aircraft as having been caused by a bird strike;
  • personnel on the ground report seeing an aircraft strike one or more birds;
  • bird remains—whether in whole or in part—are found on an airside pavement area or within 200 feet of a runway, unless another reason for the bird’s death is identified.

Strikes against other classes of wildlife—primarily mammals—are interpreted with less formality, but embrace the spirit of definitions established for bird strikes.

The case for mandatory reporting

To ensure the highest quality of wildlife-strike statistics, it is crucial that agencies responsible for maintaining databases receive as much information as possible about every strike—even non-damaging strikes and near misses. While damage information is useful in quantifying costs to the aviation industry, non-damaging strikes and near misses are of equal statistical significance when developing a complete picture of the risk at any particular location.

Despite progress made by North America’s aviation industry in reporting wildlife strikes, many continue to go incompletely reported or unreported altogether. Wildlife-management experts believe only 20 percent of all strikes are reported; reporting rates are likely lower in many developing countries where strike reporting is inconsistent or non-existent.

Strike reporting is not mandatory in most jurisdictions. Transport Canada and the FAA actively encourage reporting by aviation industry stakeholders, but currently have no regulatory authority to compel them to do so. Three additional factors contribute to the non-reporting of wildlife strikes:

  • Some industry stakeholders believe strike reporting creates information liabilities, raising public fears about the potential for strike-related accidents.
  • Stakeholders assume incorrectly that others have reported a strike.
  • Pressured to meet tight on-time performance, industry personnel do not complete strike reports because of misplaced beliefs that wildlife strikes are not an important safety issue and do not have a significant economic impact on the industry.

In 1999, the NTSB recommended to the FAA (in Safety Recommendation A-99-91) that there be a requirement for “all airplane operators to report bird strikes to the Federal Aviation Administration.” The FAA rejected the recommendation on the grounds that:

  • a regulation would be difficult to enforce;
  • existing reporting procedures are sufficient to monitor trends; and that
  • the problem should be addressed by bird-management programs and airportplanning initiatives.

Regardless of the FAA’s stance, there is ample evidence to indicate that safety would be greatly enhanced through a regulatory requirement to report all wildlife strikes.

Reporting wildlife strikes

Accurate wildlife-strike reporting requires that many industry stakeholders provide input to the data gathering process. The following sections present a brief overview of the strike-reporting process and the impact it may have on wildlife-strike statistics. A full description of the reporting process (including examples of strike-reporting forms) is contained in Appendix C—Bird- and Mammal-strike Reporting Procedures.

Schematic Illustrating Bird/Wildlife Reporting Functions in Canada

Figure 7.1 Schematic Illustrating Bird/Wildlife Reporting Functions in Canada

Who should report wildlife strikes?

Any number of stakeholders may provide either some or all of the information necessary to complete a wildlife-strike report; in fact the truth of an individual wildlife strike may only become clear once the contributions—no matter how small—of various witnesses have been gathered. The greater the amount of information gathered, the more precise the data analysis will be, enabling airport wildlife-management personnel to optimise strike-reduction strategies. The functions of and interactions between various strike-reporting stakeholders are depicted in Figure 7.1 and discussed in the following paragraphs.

Pilots report many strikes to ATS providers and may then complete strike reports for submission to Transport Canada. Commercial pilots may also report to their airlines. Pilots are often unaware of or unable to determine all the circumstances of a strike; they may be unsure of the species of bird involved, extent of the damage to the aircraft or resulting repair costs.
 

Air-traffic service providers may learn of a strike by radio reports from pilots or airport wildlife-management personnel. In the event of any operational impact, ATS providers must report a strike through Transport Canada’s Civil Aviation Daily Occurrence Reporting System (CADORS).
 

Aircraft maintenance personnel occasionally discover wildlife-strike damage that may not have been previously detected during aircraft inspections.
 

Airlines often submit strike-report summaries directly to Transport Canada. These reports are derived from information submitted by pilots and aircraft maintenance personnel, and also include information on operational effects, aircraft damage, repair and other associated costs.
 

Airport maintenance and safety personnel may discover dead birds or mammals during regular FOD inspections of runways and taxiways. Unless another cause of death is evident, it is assumed that aircraft struck the animals. This strike information should be reported to an airport operator or directly to Transport Canada.
 

Wildlife-management personnel may find dead birds on or near runways while conducting day-to-day operations. These experts also identify struck wildlife species to supplement reports from other sources. This strike information should be reported to ATS personnel, the airport operator or directly to Transport Canada.

Airport operators should collate all airport strike data for submission to Transport Canada.

What information should be reported?

The ideal method of reporting a wildlife strike is to use the Transport Canada Bird/Mammal Strike Report (see Appendix C). In practice, reporters often don’t have all the information to complete every part of the form, and yet it cannot be stressed enough that each form should be filled out to the fullest extent possible.

Damage inflicted to a general aviation aircraft by a single hawk.
Damage inflicted to a general aviation aircraft by a single hawk.

In reviewing U.S. and Canadian forms, its interesting to note Transport corresponding FAA form does not make this provision.

The Transport Canada Bird/Mammal Strike Report requests the following information:

  • type and time of incident,
  • type of aircraft and engines involved,
  • phase of flight operation,
  • parts struck,
  • effects on the flight,
  • weather conditions,
  • types and numbers of birds/mammals involved,
  • specific engine damage,
  • costs of the incident, and
  • additional comments and remarks.
Bird identification

If bird-hazard reduction measures are to be undertaken it is essential to know:

  • what species of birds are present at airports,
  • what species are being struck by aircraft, and
  • what species are causing damage.

Accurate identification of struck species is also becoming more important in response to liability and due-diligence issues, and in development of tools and techniques to manage species involved in strikes.

Identification of living birds

The identification of living birds is relatively straightforward but requires skill and practice. Airport and ATS personnel should be familiar with large and flocking species that frequent airfields and pose potential threats. Binoculars and modern bird guides are required; Transport Canada also distributes posters that illustrate key species found at Canadian airports. However, detailed biological studies necessary in development of effective airport wildlife-management programs require specialized and professional ornithological knowledge.

Dr. Henri Ouellet in the Canadian Museum of Nature laboratory.
Dr. Henri Ouellet in the Canadian Museum of Nature laboratory. Dr. Ouellet developed the Keratin Electrophoresis feather identification process for Transport Canada.

Identification of bird remains

Following a bird strike, there is often little to identify a bird; remains may include a relatively intact carcass or be limited to blood smears in an engine. Investigators call on the range of identification techniques described below to determine whether a bird strike occurred and, if so, precisely what species was struck.

Comparison with museum specimens

Experienced ornithologists examine feathers by eye to determine the species or group involved; findings can be verified through comparison with specimens in a museum collection. It’s estimated that 75 percent of struck birds can be identified using this technique.

Microscopic examination of feathers

Feather samples that cannot be identified by eye are examined under a microscope, where a feather's fine structure—its barbs and barbules—is revealed. Pioneered by Drs. R. C. Laybourne and C. J. Dove at the Department of Vertebrate Zoology at the Smithsonian Institution in Washington, D.C., this technique can be used to identify the family or genus of bird involved, but usually does not provide species identification.

Keratin electrophoresis

Electrophoresis is a technique whereby the biochemical structure of feathers is analyzed to identify a bird species. Feathers are made of keratin, a substance similar to human hair and fingernails; keratin proteins provide a fingerprint which is consistent within a particular species. In keratin electrophoresis, feather proteins from an unknown sample are compared with samples from known specimens—a technique developed by Dr. Henri Ouellet of the Canadian Museum of Nature with funding from Transport Canada. The Museum database contains 3,500 profiles from over 800 species of birds. Unfortunately, the service once provided by Dr. Ouellet is not available at this time.

DNA analysis

Following serious engine ingestions, only small amounts of blood or tissue may remain—just enough for DNA analysis. Using modern genetic techniques, the DNA can be amplified through polymerase chain reaction (PCR) to obtain samples large enough for analysis. The mitochondrial cytochrome “b” gene is commonly used to identify organisms based on their genes’ nucleotide-coding sequence.

The Birdstrike Avoidance Team at the Central Science Laboratory (CSL) in the U.K. is developing this DNA technique for use with bird-strike samples. Comparing birdstrike material with genetic-library sequences shows that a 97- to 99-percent match efficiency is possible if the sequences are from the same or congeneric species. Birds from the same family give matches 87 to 95 percent of the time, but more distantly related species cannot be matched reliably. Dr. J. R. Allan and co-workers at the CSL estimate that this technique could become operational in the U.K. for a relatively small amount of money; a reference library of the most commonly struck families in Europe could be developed for as little as USD$15,000; each sample would cost about USD$150 to process.

Bird- and mammal-strike databases

Bird-strike statistics are maintained by civil-aviation regulatory agencies in many countries; some maintain separate military and civil-strike databases, while others maintain combined databases; however, there is no standard practice whereby these databases are combined or shared.

It falls to a database manager to ensure that multiple reports of the same strike— submitted by different sources at different times—do not skew the data. Where duplicate reports occur, each bears close scrutiny, as together they are likely to provide a better understanding of a specific incident. Careful data collation and verification is essential in maintaining accuracy of a strike database and any trend information derived from it. Once again, it’s extremely important to recognize that bird strikes be recorded to the fullest extent possible. In countering the wildlife-strike problem, priorities can be defined and solutions implemented only following effective data submission, compilation and analysis.

Strike-database information is analyzed to determine a number of trends including:

  • wildlife species that create problems overall and at particular locations,
  • problematic times of the day and year,
  • yearly strike trends by location,
  • phase of flight when strikes are most likely to occur,
  • types of aircraft most likely to be struck,
  • parts of the aircraft most likely to be struck,
  • effects of strikes on aircraft,
  • percentage of strikes that are damaging and affect flight,
  • costs associated with strikes, and
  • altitude at which strikes occur.

Three major wildlife-strike databases are:

  • the Transport Canada bird/mammal strike database,
  • the United States FAA database, and
  • the International Civil Aviation Organization (ICAO) database.

Many European countries also have sophisticated reporting systems and databases; however, as this book focuses on North America, discussion will be limited to the databases noted above. One further point: neither reporting parameters nor software are standardized among current databases, making exchange of data an extremely difficult and time-consuming task.

Transport Canada

The Aerodrome Safety Branch of Transport Canada maintains this country’s bird/mammal-strike database. Annual summary reports of bird strikes have been published and distributed to stakeholders in essentially the same form since the early 1980s—the longest continuous series of comparable bird-strike data in existence. These reports include information on:

  • strikes that occurred at Canadian sites,
  • strikes to Canadian aircraft at foreign locations, and
  • strikes to aircraft operated by the Department of National Defence in Canada and abroad.

It wasn’t until 1997 that these reports included information on near misses and mammal strikes. Analysis of the most recent nine-year period (1991-99) indicates that there were 6,848 bird strikes in the Transport Canada database. Of those, 5,891 involved civil aircraft and 957 involved military aircraft.

U.S. Federal Aviation Administration (FAA)

In the United States, wildlife strikes are voluntarily reported to the FAA on a standard form (FAA Form 5200-7; see Appendix C). Although FAA personnel have monitored these reports since 1965 to determine general patterns of wildlife strikes, no quantitative analyses of these data were conducted until 1995. Through an interagency agreement, the U.S. Department of Agriculture’s National Wildlife Research Center is now responsible for maintaining the FAA strike database and analyzing its data. Detailed annual reports are now published, providing a wealth of information. The reports are cumulative and contain data for the ten-year period from 1990 to 1999 covering 28,150 wildlife strikes—27,433 bird, 681 mammal and 36 reptile strikes.

International Civil Aviation Organization (ICAO)

As the world civil-aviation body, ICAO has maintained an international bird-strike database—the ICAO Bird Strike Information System (IBIS)—since 1980. Each member country is responsible for submitting yearly bird-strike data; ICAO analyzes the data and produces an annual report. Because reports are received from dozens of countries in as many as five languages, the production of the annual statistics usually lags by two years.

The IBIS database contains information on 89,251 bird strikes from around the world for the period 1980 to 1999 inclusive.

Major bird- and mammal-strike accident database

Apart from the compilation of bird-strike statistics is the collection of bird-strike accident figures. Two well-known researchers in the field have independently developed separate wildlife-strike accident databases.

John Thorpe, retired from the Civil Aviation Authority in the UK—former Chairman of the Bird Strike Committee Europe and honourary Chairman of IBSC—has compiled a worldwide database of all known serious civilian aircraft accidents involving birds. Dr. W. John Richardson of LGL Limited, Canada, has created a database of military- and civil-aircraft incidents involving birds.

Serious incidents are defined in these databases to include:

  • loss of life,
  • injury to occupants,
  • destruction of aircraft,
  • loss of or damage to more than one engine,
  • damage to one engine together with ingestion in another,
  • uncontained engine failure,
  • fire,
  • significant-sized holes (e.g., windshield, nose, radome),
  • major structural damage, and
  • particularly unusual features, such as complete obscuring of vision, multiple- or significant-system loss and propeller, helicopter rotor or transmission damage.

Highlights

Combining all relevant data from 1912 to 2003, birds are known to have caused 42 fatal accidents, 231 deaths, and the destruction of 80 civil aircraft. This data is undoubtedly underreported, since records from earlier years are incomplete or nonexistent. In all likelihood there have been many unreported accidents caused by birds. This is particularly possible in accidents involving small general-aviation aircraft, since investigations of these incidents are generally not as intensive as those involving commercial aircraft.

Major aircraft accidents

There are several major hull-loss accidents that are worthy of mention:

  • On October 4, 1960, the worst bird-strike accident to date occurred when a Lockheed Electra encountered a flock of European Starlings just after becoming airborne from Boston’s Logan International Airport. Starlings fly in dense flocks of individual birds weighing about 80 grams each. Numerous birds were ingested into three of the four turboprop engines. The number-one engine had to be shut down; numbers two and four lost power. The aircraft lost speed, stalled and crashed into Boston Harbor. Of the 72 persons on board, 62 died and 9 were injured.
     
  • On November 23, 1962, a United Airlines Vickers Viscount struck a flock of Whistling (Tundra) Swans migrating at 6,000 ft above Maryland. The leading edge of the tailplane was dislodged and the aircraft became uncontrollable, crashing and killing all 17 people on board. Major movements of large flocking birds such as ducks, geese, swans and cormorants can present significant hazards to aircraft, day and night.
     
  • On November 12, 1975, at J.F.K. International Airport in New York, a remarkable accident occurred. An Overseas National Airlines DC-10-30 with 139 persons on board struck gulls at V1 speed. The number-three engine exploded causing a severe wing fire. Takeoff was rejected and the aircraft quickly burned out. Remarkably, there were no fatalities and only 11 minor injuries, most likely due to the passengers’ familiarity with emergency evacuation procedures— they were all airline employees.
     
  • On September 15, 1988, a Boeing 737-200 ingested Speckled Pigeons into both engines at liftoff from Bahar Dar—an airport 5,800 ft above sea level in Ethiopia. Both engines failed and the aircraft attempted an emergency landing in open country 10 km from the airport. Unfortunately, the aircraft struck a riverbank and burned. There were 35 fatalities and 21 injuries among the 104 passengers on board.

Major aircraft incidents

While actual hull-loss accidents are dramatic in their scope, they are far outnumbered by serious incidents in which hull losses were barely avoided—incidents that are just as important when developing risk-management strategies. Modern safety management experts recognize that risk-mitigation strategies cannot be developed through aircraft-accident statistics alone; statistically, serious accidents comprise 10 percent or less of meaningful safety data. Increasingly, the aviation industry is embracing other techniques to evaluate hazards:

  • gathering safety data from other sources such as non-punitive reporting systems and incident-evaluation reports, and
  • risk-analysis tools that evaluate the potential severity and potential for reoccurrence.

All serious wildlife-strike incidents need to be carefully reviewed and analyzed using an established risk-management protocol. Unfortunately, a separate database of these serious incidents does not exist at this time, nor does a risk analysis of their potential severity and reoccurrence—potential highlighted in the following examples:

  • On the night of January 9, 1998, a Delta Airlines B727 departed Houston, Texas. At about 6,000 feet the aircraft struck a flock of migrating Snow Geese and suffered extensive damage to all three engines, the leading-edge slats, radome and airspeed pitot tube—damage due in part to the aircraft’s involvement in a trial to assess the efficiency gains of high-speed departures. The crew successfully returned to the airport and there were no injuries. However, the potential for catastrophe is clear. 
  • On October 26, 1992, a KLM B747 landing at Calgary International Airport struck a flock of Canada Geese just before touchdown. The aircraft landed successfully and there were no injuries. The aircraft suffered major uncontained damage to the number-one engine and damage to the leading-edge slats. Multiple-bird strikes during this critical phase of flight—in close proximity to the ground—have the potential to lead to disaster. Later sections of this chapter document the frequency of engine ingestions, rejected takeoffs, and precautionary/emergency landings that indicate just how common these close calls really are.
Analysis of bird-strike statistics in civil aviation

The following sections present analysis of bird-strike statistics collected in Canada and the U.S. between 1991 and 1999. Given that the population of the U.S. is about ten-times greater than Canada and that the U.S. has the highest per capita use of aircraft in the world, it’s reasonable to assume there would be ten times more bird strikes per year in the U.S. In fact, annual reported bird strikes averaged 2,857 in the U.S. from 1991 to 1999 compared with 761 in Canada—a ratio of 3.75:1. This higher ratio may be due to the substantially higher Canadian reporting rate, the result of a longer history of concern for the problem, a more aggressive public relations program and a more formal regulatory and policy structure that—until recently—included government ownership of most airports.

When comparing strike statistics it’s important to remember that data are not always representative of actual strike statistics, since many strikes go unreported. For instance, an airport with more reported strikes than another may actually have a better wildlife-management program—and therefore fewer actual strikes—than the airport with fewer reported strikes. The former airport might simply be more thorough at reporting. Numbers of strikes are also a function of the number of aircraft movements. To standardize strike statistics and enable an accurate method of annual data comparison at airports, strike rate—expressed as the number of strikes per 10,000 movements—is the measurement that’s been adopted by the wildlifestrike community.

In the following analyses, the summary data—presented in tables and figures—are based on information reported in the Transport Canada and FAA summary publications. Because each strike report does not contain information on every birdstrike parameter, totals for various categories vary. For example, not all strike reports identify the type of bird struck or the part damaged, so the totals presented are only from reports that did include these data.

Phase of flight

Most bird-strike databases contain statistics noting the phase of flight during which strikes occurred. These statistics are important because each flight phase has a different level of risk. The two most critical are takeoff and landing; overall accident statistics show that most accidents occur during these two phases of flight. From a wildlife-strike perspective, an aircraft is much more vulnerable during takeoff than when landing.

At takeoff, an aircraft’s engines are operating at high power settings, and the aircraft is heavier due to a full fuel load. During takeoff there is very little time—perhaps two

Phase of Flight at Time of Bird Strikes
Figure 7.2 Phase of Flight at Time of Bird Strikes. Canada and U.S. (1991-1999) (Canadian data include military strikes)

to three seconds—to react to a wildlife strike, evaluate aircraft or engine damage and decide to reject takeoff or continue to fly. Successful rejected-takeoff and engine-out takeoff maneuvers require precise flying skills and good crew co-ordination, since aircraft performance under these circumstances is limited. Any multiple-system failures caused by a wildlife strike—such as loss of lift-enhancing devices or more than one engine—can render an aircraft unflyable.

There is significantly less risk involved during landing. Impact force and potential for damage are reduced because an aircraft is approaching at lower speeds, under reduced power and carrying a diminished fuel load.

The statistics on bird strikes by phase of flight for Canada and the U.S. between 1991 and 1999 are summarized in Figure 7.2. In comparing the overall statistics, the two nations are similar—37 percent of strikes in Canada occur during takeoff versus 39 percent in the U.S. However, a breakdown of the statistics reveals a different story— in Canada, 31 percent of strikes occur during the takeoff run, 6.5 percent during the climb-out. In the U.S., 20 percent of strikes occur during the takeoff run and 19 percent during the climb-out phase. The difference suggests the two databases may be using slightly different definitions of the takeoff run and the climb-out phases.

Relatively few strikes occur when aircraft are en route at higher altitudes—3.8 percent in Canada and 3.6 percent in the U.S. There are again substantial differences between Canadian and U.S. data for strikes during descent and approach—19 percent vs. 41 percent respectively—and during landing roll—22 percent vs. 16 percent. The overall figures for the landing phase are closer—41 percent in Canada and 57 percent in the U.S. Once more, definitions may vary between databases.

Altitude

Aircraft are most likely to encounter birds during takeoff and landing phases, as the majority of bird flights occur within a few hundred feet of the ground. The highest recorded strike in the FAA database involved an unidentified species of bird reportedly struck by a DC-8-62 at 39,000 ft on October 23, 1991.

Altitude (AGL) Percent of
Known Total
0 40
1-99 15
100-299 11
300-499 5
500-999 7
1000-1499 5
1500-3999 10
>4000 6

Table 7.1 Altitude of Bird Strikes in the U.S. (1991-1999)

U.S. data on bird strikes at altitudes above ground level (AGL) are summarized in Table 7.1. The figure is based on 20,893 reported strikes with known altitudes during the period 1990-1999:

  • 40 percent occur while the aircraft is still on the ground—primarily during takeoff and landing roll,
  • 15 percent of strikes occur between one and 99 ft above ground, and
  • 16 percent occur between 100 and 499 ft AGL.

In total, 71 percent of these strikes occur on, or immediately adjacent to, airport properties. Above 500 ft, the number of bird strikes decreases proportionally as altitude increases.

Bird strikes that do occur above 500 ft AGL generally involve flocking birds, particularly migratory waterfowl that can exceed 5 kg. Multiple strikes to several parts of an aircraft are not uncommon in these incidents, creating potential for loss of more than one engine and damage to other major aircraft systems. While chances of a bird strike at altitudes above 500 ft AGL are statistically low, the potential consequences of a high-altitude bird strike may be more significant.

As this data indicates, it is imperative to reduce the numbers of birds at and around airports. This strengthens the case for both effective airport wildlife-management programs and control of sites such as bird-attracting landfills near airports (see Chapter 8).

Monthly Bird Strike Distribution in Canada and the United States
Figure 7.3 Monthly Bird Strike Distribution in Canada and the United States (1991-1999)
(includes Canadian aircraft at foreign locations and Canadian military aircraft)

Time of year

The frequency of bird strikes varies with the time of year. The percentages of Canadian and U.S. strikes that occur each month are plotted in Figure 7.3 for the years 1991 to 1999. In Canada, relatively few strikes occur during winter months—two to three percent per month from December to March. The number increases in spring when migrating birds return from the south—five to nine percent per month from April to June. Peak numbers occur in summer—14 to 17 percent per month from July to September. These rates are thought to be high for two reasons: large numbers of birds are present after the nesting season—particularly naïve young birds that have no experience with aircraft—and birds begin to migrate from the far north in late summer. Fall strikes—12 percent in October and seven percent in November—mark the period when substantial numbers of birds are still present, but many migrating birds have left Canada. The significance of migratory bird strikes is important. Given the weight and numbers of birds in a flock, knowledge of migratory paths and times is critical to reduce the probability and severity of bird strikes.

The annual pattern of bird strikes in the United States is similar to that in Canada, with some exceptions. Peak strike numbers also occur from July through September— 10 to 14 percent per month—but the number of winter strikes is higher at four to six percent per month from December to March. The higher winter rates reflect the large number of southern airports in the U.S. where migrant birds spend winters.

Time of day

Bird strikes occur at all hours of the day—the vast majority of Canadian strikes during daylight hours. This is not surprising, since fewer birds fly at night when fewer aircraft are flying as well. The 1999 hourly distribution of bird strikes in Canada is presented in Figure 7.4, demonstrating the substantial numbers of bird strikes occurring at all hours of the day. Small increases are evident in the morning—between 08:00 and 10:00—and early evening—15:00 through 17:00—when the numbers of scheduled flights peak.

 Daily Bird Strike Distribution in Canada in 1999
Figure 7.4 Daily Bird Strike Distribution in Canada in 1999 (includes Canadian aircraft overseas and Canadian military aircraft)

  CANADA UNITED STATES
Aircraft
Part
Number
Struck
Number
Damaged
Percent
Damaged
Number
Struck
Number
Damaged
Percent
Damaged
Windshield 514 33 6.4 4,195 321 7.7
Wing/Rotor 855 113 13.2 3,030 941 31.1
Fuselage 682 31 4.5 2,665 146 5.5
Nose 750 40 5.3 3,061 235 7.7
Engine 608 96 15.8 3,887 1542 39.7
Propeller 266 12 4.5 819 92 11.2
Radome 251 32 12.7 2,645 405 15.3
Landing Gear 303 8 2.6 1,180 153 13.0
Pitot 43 29 67.4 0 0 0.0
Other 977 168 17.2 1,174 626 53.3
Total 5,249 562 10.7 22,656 4,461 19.7

Table 7.2 Aircraft Parts Most Commonly Struck and Damaged by Birds Canada and U.S. (1991-1999)

Birds tend to be most active at dawn and dusk, but as sunrise and sunset times vary throughout the year these strike patterns are obscured. Consequently, daily strike-rate patterns revealed in the data are strongly influenced by peak aircraft-activity times. There is also variation in the temporal distribution of strikes among airports. Recent analysis also suggests that North American strike rates may in fact be higher at night.

The temporal patterns of mammal strikes are quite different than those of birds. The FAA database reported 681 mammal strikes during the 1991 to 1997 period; of the 522 mammal strikes in which time was known, 63 percent occurred at night—13 percent occurred at dawn and dusk, and only 24 percent during the day. These patterns reflect the nocturnal and crepuscular behaviour of most mammals that frequent airports in the U.S. and Canada.

Part of aircraft struck

The data presenting parts of aircraft struck by birds is partially related to type of aircraft involved and phase of flight. Data from 1991 to 1999 for Canada and the United States are summarized in Table 7.2. Overall, the fuselage, nose, radome, windshield, wing, rotor and engine are the parts most frequently struck. The numbers of strikes to windshields and engines is proportionally higher in the U.S. than Canada, although the reason is not apparent.

There is marked variation in the likelihood of a strike causing damage. The overall percentage of reported strikes causing damage is 10.7 percent in Canada and 19.7 percent in the U.S. It is not clear whether this difference is real or merely a statistical anomaly; each country uses similar aircraft and the species of hazardous birds are generally the same. It is possible that damaging strikes in the U.S. are more likely to be reported than non-damaging strikes; this would account for the apparent discrepancy between Canadian and U.S. figures.

Effect on Flight Number of Incidents % Total Incidents
No Effect/Continued Flight 4224 61.6
Precautionary/Forced Landing 608 8.9
Aborted Takeoff 173 2.5
Engine Ingestion 137 2.0
Engine Shutdown/Failure/Fire 30 0.4
Vision Obscured 61 0.9
Rupture Skin/Airframe 73 1.1
Other Effect 114 1.7
Unreported 1442 21.0
Totals 7002 100.0

Table 7.3 Effects of Bird Strikes on Aircraft in Canada (1991-1999)

Strikes most likely to cause damage are those involving:

  • engines: 16 percent and 40 percent in Canada and the U.S. respectively,
  • wings and rotors: 13 percent and 31 percent, and
  • radome: 14 percent and 15 percent.

Multiple engine strikes are the most dangerous to aircraft safety; they’re also the most expensive to repair.

As one might guess, mammal strikes involve different parts of aircraft than bird strikes. Overall, 607—85 percent—of the reported mammal strikes in the FAA database caused damage to various aircraft parts:

  • landing gear were damaged in 63 percent of incidents: 251 occurrences in which 158 involved damage;
  • propellers were damaged in 91 percent of incidents: struck 109 times and damaged 99;
  • wings and rotors were struck 83 times, each resulting in damage; and
  • engines were damaged 98 percent of the time: 59 cases of damage in 60 strikes.

Effect on flight

Bird strikes are of greatest concern when they cause damage and affect the flight of an aircraft. The Canadian experience from 1991 to 1999 is summarized in Table 7.3.

Aircraft Type 1991 1992 1993 1994 1995 1996 1997 1998 1999 Total
DeHavilland Dash-8 79 51 71 77 120 69 96 70 97 730
Boeing 737 115 32 62 53 54 36 61 45 38 495
DC-9/MD-80 59 30 38 46 60 47 35 15 27 357
Airbus A320 13 36 49 47 34 36 49 61 30 355
Boeing 767 27 16 22 17 31 25 8 19 11 176
Boeing 727 28 24 8 14 24 14 22 11 6 151
British Aerospace BA146 18 16 14 23 20 18 9 10 10 138
ATR 42 26 7 18 13 10 18 10 11 19 132
Fokker F28 9 0 6 6 18 17 15 16 16 103
Regional Jet CL65 0 0 0 0 11 28 27 26 1 93
Beech King Air 2 3 12 12 7 2 20 20 31 109
Canadair Challenger 11 8 9 2 6 10 0 11 8 65
Boeing 757 3 1 5 0 12 9 7 10 0 47
Boeing 747 3 5 1 10 3 8 5 11 10 56
BA Jetstream 31/41 5 3 5 7 8 0 7 12 5 52
McDonnell-Douglas DC-10 10 0 3 4 8 2 5 2 1 35

Table 7.4 Civil Aircraft Most Commonly Struck by Birds in Canada (1991-1999)

Readers should be aware that there might be more than one effect on any particular flight. In 83 percent of cases, strikes had no effect and flights continued. Precautionary landings were necessary in nine percent of reported bird strikes— many involving emergency procedures on the ground. Aborted takeoffs occurred 173 times—2.5 percent of cases.

Ingestions by engines occurred in two percent of cases, resulting in 30 engine failures, fires and precautionary engine shutdowns. Altogether, one percent of total reported strikes resulted in potentially serious engine problems.

Both Canadian and U.S. data regarding the effects on flight caused by mammal strikes differ from those involving birds. Only 36 percent of the 414 flights with full reported data proceeded without effects. Of these flights, 19 percent—79—involved rejected takeoffs, while 12 percent—49—resulted in precautionary landings.

Types of aircraft struck

All types of aircraft are susceptible to wildlife strikes, although vulnerability may differ. The types of aircraft most frequently struck in Canada are summarized in Table 7.4. The number of strikes per aircraft model is related to:

  • the number of aircraft in service,
  • the number of takeoffs and landings, and
  • the airports used by that specific type of aircraft.

For example, the most frequently struck aircraft in Canada is the Dash-8—a short-haul aircraft that makes repeated daily takeoffs and landings at many smaller airports lacking effective wildlife-management programs.

Engine ingestions

The greatest concern regarding bird strikes to jet passenger aircraft is extensive damage and loss of power that can result when birds are ingested into engines. Unfortunately, engine manufacturers do not have access to all data on damaging events—a fact that hinders their ability to build more resilient engines. Following an examination of approximately 6,000 bird-ingestion events involving CF6 and CFM high-bypass turbofan jet engines, Tom Alge of GE Aircraft engines recommended that all bird ingestions resulting in engine damage be reported to manufacturers. Non-damaging ingestions—revealed during routine maintenance—are also not reported consistently. Alge found that of the 6,000 ingestions:

  • 40 percent took place on takeoff,
  • 10 percent on initial climb,
  • 13 percent on final approach, and
  • 35 percent on landing roll.

Although the frequency of ingestions was similar during departures and arrivals, departure ingestions resulted in damage at twice the rate as that incurred during arrivals.

A 1995 FAA study by Banilower and Goodall examined bird ingestions involving modern high-bypass turbofan engines used on A300, A310, A320, B747, B757, B767, DC-10 and MD-11 aircraft. Between 1989 and 1991 there were 644 ingestion events during 3,163,020 operations by 1,556 aircraft—a worldwide ingestion rate of 2.04 events per 10,000 aircraft operations. The ingestion rate in the U.S. was 0.70 per 10,000 operations compared to 2.52 ingestions per 10,000 operations in the rest of the world. During this three-year period there were 31 multiple-engine ingestion events—a rate of 9.8 per million operations. The FAA study reported that 47 percent of engines that ingested birds suffered some damage; about half of these cases involved significant damage.

The data also showed that ingestion risk fluctuates by location. Canada, the U.S. and some European and Pacific Rim countries enjoyed the lowest risks. The highest occurred at airports in Africa and some South American, Asian and European countries—locations that would gain immediate and significant benefit from effective wildlife-management programs.

Wildlife species involved in strikes

Determining which species of birds and mammals are struck adds value to the design of all aircraft components, as well as airport wildlife-management programs.

  CANADA UNITED STATES
Bird Group Total #
of Strikes
% of
Identified
Strikes
Total #
of Strikes
% of
Identified
Strikes
Non-Passerines        
Waterfowl (i.e. ducks,geese, swans) 273 6.5 1366 11.7
Waterbirds (i.e.heron, crane, loon, coot) 37 0.9 51 0.4
Raptors 341 8.1 1320 11.4
Owls 102 2.4 250 2.1
Shorebirds 307 7.3 834 7.2
Gulls and Terns 1614 38.5 3266 28.1
Pigeons and Doves 125 3.0 1373 11.8
Gallinaceous Birds (i.e. grouse/pheasants) 27 0.6 62 0.5
Other Non-Passerines     54 0.5
Passerines (perching birds)        
Crows 65 1.6 208 1.8
Swallows 291 6.9 297 2.6
Blackbirds 20 0.5 671 5.8
Starlings 160 3.8 591 5.1
Snow Bunting 300 7.2 33 0.3
Other Passerines 528 12.6 1253 10.8
Identified Bird Totals 4190 100 11,629 100.0
Unidentified Birds Struck 2658   14,084  
Total Birds Struck 6848   25,713  

Table 7.5 Identified Bird Groups Commonly Struck in Canada and U.S. (1991-1999)

The species and numbers of strikes in Canada and the United States between 1991 and 1999 are summarized in Table 7.5. The table contains information on a total of 15,819 reported strikes. The species or group involved is identified in 61 percent of reported strikes in Canada and 45 percent in the U.S. By far, the most frequently identified group involved in strikes are gulls and terns—38.5 percent of reported strikes in Canada and 28 percent in the U.S. The overwhelming majority of these strikes involve gulls; less than one percent involve terns. Waterfowl are reported in 12 percent of U.S. strikes, but only 6.5 percent in Canada. Diurnal raptors—such as hawks, eagles and vultures—are involved in 11.4 percent of strikes in the U.S. and 8.1 percent in Canada. Pigeons and doves figure prominently in U.S. strike data— 12 percent compared to only three in Canada. The U.S. has much larger dove populations, and these numbers swell in winter when Canadian doves migrate south. Overall, the passerines—perching birds—constitute 33 percent of reported Canadian strikes and 26 percent in the U.S., although figures vary among species; Blackbirds and starlings are more frequently identified in the U.S., whereas swallows and Snow Buntings are commonly struck in Canada (Table 7.5).

During the 1991 to 99 period, 152 mammal strikes were reported in Canada and 681 in the U.S. The most commonly reported species struck in Canada are:

  • Rabbits: 24 percent,
  • Striped Skunk: 13 percent,
  • Coyote: 12 percent,
  • Fox: 11 percent, and
  • White-tailed Deer: 7 percent.

In the U.S., 65 percent of reports refer to deer—11 percent to Coyote.

Species/Groups Cause Damage Affect Flight Aircraft Downtime Monetary Loss
Number Per Cent Number Per Cent # Hours Per Cent Cost* Per Cent
Gulls/Terns 581 29.8 456 32.9 19,326 20.9 11.4 19.1
Waterfowl 640 32.9 305 22.0 38,268 41.3 33.5 56.1
Raptors (incl. Owls) 334 17.1 208 15.0 24,276 26.2 8.6 14.5
Pigeons/Doves 135 6.9 141 10.2 5,578 6.0 3.8 6.4
Blackbirds/Starlings 73 3.7 91 6.6 1,240 1.3 0.7 1.1
Other Waterbirds 24 1.2 13 0.9 699 0.8 0.2 0.3
Shorebirds 85 4.4 77 5.5 2,994 3.2 1.2 2.1
Corvids (Crows, etc.) 20 1.0 18 1.3 77 0.1 0.0 0.1
Sparrows 19 1.0 36 2.6 20 0.0 0.0 0.0
Grouse/Pheasants 16 0.8 12 0.9 93 0.1 0.0 0.0
Miscellaneous 21 1.1 31 2.2 86 0.1 0.2 0.3
Total Known 1,948 100 1,388 100 92,657 100 59.6 100
Unknown Species 1,889   1,110   21,437   17.8  
Total Birds 3,837   2,498   114,094   77.4  
  * in millions of U.S. dollars

Table 7.6 Identified Bird Groups Commonly Struck. Canada and U.S. (1991-1999)

Damaging strikes

The likelihood that a particular bird strike will cause aircraft damage is related to the size of bird—its weight—and its flocking behaviour, which determines how many individuals are likely to be struck. In both Canada and the U.S., gulls are the most frequently struck bird group—28 to 39 percent (Table 7.5). Gulls are involved in 30 percent of damaging bird strikes (Table 7.6). Waterfowl—primarily ducks and geese—are involved in 33 percent of damaging strikes, but only 12 percent of the overall strikes in the U.S. Raptors, including owls, are also involved in a higher percentage of damaging strikes—17 percent as opposed to 11 percent of the total number of strikes. Pigeons and doves account for 11 percent of U.S. strikes—only 6.4 percent of these cause damage. Other passerines account for 11 percent of overall strikes but less than one percent of those resulting in damage (Tables 7.5 and 7.6).

Relative costs by species

The FAA database presents information on the numbers of hours of aircraft downtime and reported costs of strike incidents. Table 7.6 illustrates how the otherwise strong influence of gull strikes appears to drop:

  • waterfowl are involved in 41.3 percent of total downtime and 56 percent of damage costs,
  • raptors: 26.2 percent of downtime, 14.5 percent of costs,
  • gulls: 21 and 19 percent respectively, and
  • pigeons and doves: six and 6.4 percent.

Hazardous species

When determining bird species that pose the greatest hazard to aircraft safety, a number of factors must be considered including:

  • numbers of birds present,
  • weight and density of the birds,
  • flocking behaviour of the birds,
  • behaviour of birds at and near an airfield, and
  • their responses to aircraft.

There has been little study of the behaviour of birds in response to approaching aircraft. Based on the Canadian and American experience, it’s clear that waterfowl, gulls, raptors, pigeons and doves are the most hazardous species on a continent-wide basis. This is not to say that other species or groups are not more important at specific airports.

A review of the ICAO worldwide database shows similar trends. For example, during the three-year period between 1994 and 1996 there were 743 bird-strike incidents that caused serious damage. The bird species or group was known in 419 of these cases—gulls were the leading cause of serious damage in 32 percent of incidents, followed by raptors—including owls—at 21 percent and waterfowl at 20 percent.

Conclusions

To summarize, it’s important to emphasize the value of bird-strike statistics and the importance of collecting strike data—data that provides:

  • a fundamental risk-analysis tool for developing strike-reduction strategies;
  • a means to evaluate performance of wildlife-management strategies;
  • cost information for documenting the importance of the bird-strike problem;
  • justification for expenditures necessary to address the wildlife-strike problem;
  • a key planning tool needed to form the basis of airport wildlife-management programs;
  • data required by airframe and engine manufacturers to assist in design of safer and more bird-proof engines and airframes;
  • information needed by insurance companies; and
  • information needed by airport operators to demonstrate they have shown due diligence in addressing bird and mammal strike problems at their facilities.

The collection and evaluation of wildlife-strike data is a cornerstone of a safer aviation environment.

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