1. Introduction
Collisions between birds and aircraft are as old as aviation. The first recorded bird strike was experienced by the Wright Flyer III on 7 September 1905 [
1]. Bird strikes are regular events. Depending on the country, average bird strike rates between 2.83 and 8.19 per 10,000 aircraft movements were reported in civil aviation for the past years. Examples are provided in
Table 1.
Nevertheless, while collisions between birds and aircraft usually result in lethal consequences for the bird, aircraft damage is rare. Two to eight percent of all recorded bird strikes result in actual aircraft damage in civil aviation [
6,
7,
8,
9,
10]. Regarding operational impacts, between six and seven percent of all reports indicate a negative operational effect on the flight [
6,
7,
10]. It is estimated that bird strikes cause annual costs of at least one billion US
$ to the worldwide commercial aviation industry [
11]. Due to incomplete reporting, these figures have to be interpreted as conservative estimates [
12,
13,
14].
As accidents have demonstrated, collisions between birds and aircraft also bear the potential for catastrophic outcome for the involved aircraft. As of 11 November 2019, bird strikes were determined to have caused 618 hull losses and 534 fatalities since the beginning of aviation [
1].
To understand the factors contributing to the risk of bird strikes and find suitable measures for their prevention, broad data analysis is a prerequisite. This requires consequent reporting by the parties noticing bird strikes [
15]. Furthermore, international standards and common definitions are needed. Thereby, the focus lies on civil aviation in general and commercial aviation in particular. The first section of this paper deals with the current state of data availability and consistency. Subsequently, the factors determining the risk of bird strikes are introduced. Thereafter, measures taken on the ground, in the air and by regulatory means as well as their limitations are presented. Finally, current research and its potential to further reduce the risk of bird strikes is discussed.
2. Definitions and Data Availability
Bird strikes are defined as
a collision between a bird and an aircraft which is in flight or on a take off or landing roll [
16]. To include other animals colliding with aircraft, the term can be broadened to
wildlife strike. In general, statistics are provided for birds and terrestrial animals separately, for example by the aviation authorities of Canada, the United States of America (USA) and the United Kingdom (UK). One exception is Australia, where all flying animals, including flying foxes and bats, are included in the bird strike statistics [
2,
6,
7,
10].
This paper focuses on collisions involving birds and the term
bird strike is used. First, the vast majority of wildlife strikes occur with birds, for example: 98% in Australia, 95% in Canada and 95% in the USA [
2,
7,
10]. Second, terrestrial animals can be prevented from entering airport perimeters, for example by installing fences [
17]. In contrast, birds can enter airfields regardless. Furthermore, they do not only pose a risk on the airfield, but also in the approach and departure corridors. The related challenges are addressed in this paper.
International Civil Aviation Organization (ICAO) requests its contracting states to report bird strikes [
18]. Data are usually collected by the Civil Aviation Authorities (CAA). Its quality relies on consistent reporting by the parties involved in aircraft and airport operations: The pilots, maintenance crews, air traffic control and wildlife control. In recent years, the importance of complete bird strike reporting has been recognized and has since been encouraged or even enforced by many CAAs across the world. Within this context, the European Union (EU), which previously had no consistent reporting regulations among its member states, put into force mandatory bird strike reporting in 2015 [
19]. All parties involved in air traffic operations within the EU have been obliged to report observed bird and wildlife strikes [
20]. In Australia, mandatory reporting has already been in place for several years. Furthermore, in many countries, action has been taken to increase the motivation to report. This has resulted in increasing numbers of bird strike reports. For example, in the USA, where a mainly voluntary reporting system is in place, the ratio between all reported bird strikes and all bird strike occurrences increased from 41% to 91% for commercial aircraft in the period from 1990 to 2013 [
13]. When including airports, which handle general aviation and commercial traffic, the share amounts to 47%. In the UK, pilots have been required to report all bird strikes since 2004. Before, only damaging bird strikes had to be reported [
21]. The number of reports strongly increased since the implementation of this mandate [
22]. Both in the USA and UK studies, the reason for the rise is mainly attributed to better reporting, rather than increased bird strike risk. The authors of both studies (Refs. [
13,
22]) reason with the ratio between number of damaging strikes and all strikes. In case of an increased risk, the rise of reports would be expected to be similar for damaging and non-damaging strikes. However, in both countries, the proportions of damaging strikes fell. This is supported by the latest USA data for the period until 2015, as visualized in
Figure 1. In the subsequent years, a slight increase can be observed. Data from the years to follow will have to confirm if this represents the beginning of a trend in the opposite direction and bird strike risk is increasing.
Over the past few years, bird species hazardous to aviation have expanded and adapted to urban areas [
23,
24]. As air traffic is rising as well [
25], the likelihood of encounters increases due to a higher number of airspace users. However, due to better reporting, the increasing trend in the number of bird strikes does not necessarily—or at least not exclusively—imply a rising risk of bird strikes.
The bird strike data collected and the level of detail published vary among the different countries. For example, some countries provide the altitude distribution via flight phases, others in altitude bands of various intervals. Therefore, comparisons of bird strike rates in particular and statistics in general have to be performed carefully.
The subsequent chapters describe the factors contributing to the bird strike risk. The ICAO defines a safety risk as
the predicted probability and severity of the consequences or outcomes of a hazard [
26]. This definition is applied here.
4. The Severity of Bird Strikes
The consequences of a bird strike for the aircraft involved are depending on the circumstances of the individual collision. The major criterion is kinetic energy
where
refers to kinetic energy in
,
m to mass in
and
v to velocity in
.
With regard to mass, the number of birds involved, their biomass as well as parts of the aircraft hit, determine the consequences of a collision for the aircraft [
24]. Considering the velocity component, due to the high relative difference, mainly the aircraft’s speed is relevant.
Based on data from the Federal Aviation Administration (FAA)’s National Wildlife Strike Database for Civil aviation, Dolbeer performed a study to evaluate the consequences for damages resulting from bird strikes below and above 500 ft in 2011 [
32]. Even though the majority of strikes—approximately 75% in the period between 1990 and 2009—happen below 500 ft, only 55% to 65% of the damaging strikes took place in this altitude band. This indicates that a large proportion of strikes above 500 ft cause in damage, which is also reflected in
Figure 2. This observation is supported by a study performed for the European Aviation Safety Agency (EASA) in 2009 [
24] that takes into account data from civil aircraft from the UK and Canada for the period between 1990 and 2007. For these countries, 57% of all strikes happened during take-off and landing, 39% during climb and approach and approximately 1% during en-route flight for the observed period. The remaining 3% of all strikes happened during taxi and parking. The amount of damage per flight phase increases with increasing height: 3.7% of all strikes during take-off and landing, 7.9% of all strikes during approach and climb, and 34% of the en-route bird strikes caused damage. This can be explained by larger aircraft velocities at higher altitudes as well as by the fact that larger birds such as Canada Geese and Turkey Vultures fly at higher altitudes [
24,
52,
53]. The combination of these two factors lead to a significant increase in the kinetic energy of the impact and thus to a higher probability of damage with increasing height.
4.1. Parts Struck
The majority of bird strikes hit the large front-parts of the aircraft: the nose, the wings’ leading edges, and the engines. The shares of strikes to the various parts differ between different sources (e.g., [
2,
7,
31]). Exemplary,
Figure 4 presents the proportion of damaging and non-damaging strikes per aircraft component. The magnitude of damage resulting from a bird strike strongly depends on the part(s) struck. Small parts such as the pitot tube and lights are most vulnerable to damage due to their exposed positions and missing requirements on impact-resistance. The danger of hazardous consequences for the aircraft is especially high, when large or multiple birds are ingested into one or more engines because this can lead to partial or total loss of thrust. This is reflected by the accident statistics: Out of the 30 accidents involving hull losses and fatalities that happened since 1960, 23 were a result of one ore more engines struck [
1,
54,
55]. Currently, approximately 94% of the world’s aircraft fleet is equipped with two engines only [
49]. Due to the resulting smaller redundancy, the danger is larger when birds are ingested [
51]. Thereby, substantial engine damage is most likely during departure [
56]. Over the past years, two major crashes occurred due to the ingestion of birds in both engines of twin-engine aircraft. In January 2009, an Airbus A320 aircraft lost thrust in both engines during initial climb out of LaGuardia Airport after the ingestion of several Canadian Geese. The crew successfully performed an emergency landing on the Hudson river [
57]. In August 2019, a similar accident took place in Moscow when the crew of an Airbus A321 performed a successful emergency landing in a corn field after the engines failed due to ingestion of multiple gulls during departure [
58]. In both cases, all passengers and crew survived.
4.2. Risk of Accidents
The number of serious bird strike-related accidents are comparable to serious accidents due to other environmental causes, as
Figure 5 shows. This figure compares the share of fatal and hull loss accidents resulting from environmental hazards for the periods 1960–1999 and 2000–2015. To compensate for the different length of the compared periods, the shares and not absolute numbers of accidents are provided. Over the last few decades, technological improvements and additional safety equipment have been introduced to reduce the number of windshear and turbulence related accidents [
59]. The effect of these measures, especially on turbulence-related accidents, is visible in
Figure 5. On the other hand, the shares of serious accidents due to bird strike, lightning strike and thunderstorm increased.
4.3. Effect on Flight
Depending on the magnitude of the damage, there is a direct operational effect on the flight. In addition to the aircraft involved, airport operations and other airspace users may also be impaired.
Table 2 provides an overview of operational impacts for various countries and continents. In addition, the worldwide reports collected by ICAO [
31] are presented. While the share between
none and
unknown varies among the sources, the effect-categories have a similar influence.
Independent of their impact on a flight, an examination to ensure the airworthiness of the aircraft involved has to be performed before the next departure [
61]. Therefore, not only damaging but all recognized bird strikes affect operations and consequently result in costs. Furthermore, airport operations might be impaired—for example due to temporary runway closure to remove bird remains.
4.4. Costs of Bird Strikes
Little information is available about the costs resulting from bird strikes. This is related to the reluctance of airlines to report damage costs due to competitive reasons [
62]. Global estimates are from the early 2000s. For example, depending on the damage caused, Sodhi approximated in 2002 that the costs for engine repairs range from US
$ 250,000 to one million US
$ [
50]. Allan et al. approximated in 2003 the total annual costs for the world aviation fleet to be approximately one billion US
$ [
63]. Based on data obtained from United Airlines (UAL) for the years between 1999 and 2002, costs of a non-damaging strike sum up to approximately US
$22,417 per strike. This includes, for example, an aircraft check following a bird strike. They conclude that the average costs for a damaging strike amount to US
$225,329. More recent data are available from the FAA [
7], which is summarized in
Table 3. Some of the reported strikes between 1990 and 2015 include information about repair and indirect costs. Indirect costs result from lost revenues, passenger rebooking, aircraft rescheduling and flight cancellations. On average, the repair costs amounted to US
$ 164,595, the average indirect costs to US
$ 27,599 , resulting in total average costs of US
$ 192,194 per damaging strike. However, this information was included only in a small proportion of all reports, as
Table 3 indicates. Hence, these numbers might not be representative. Furthermore, due to incomplete reporting of strikes in general, the authors of the study presume a strong underestimate. Therefore, projected costs are based on the averages obtained from the reports that include cost information. These are also presented in
Table 3.
Another type of cost can be expressed in aircraft downtime. Based on the 5% of reports including information about aircraft downtime in [
7], an average of 101 hours per strike result. When including missing reports, the authors project an average of 4521 days of aircraft downtime per year due to wildlife strikes.
6. The Next Step
Over the past few years, the awareness has risen that increasingly includes the parties actually handling air traffic—ATC and the pilots—is vital to further reduce the risk of bird strikes in civil aviation [
28]. Currently, the controllers can provide general warnings on bird activity in the airport area based on visual observations or reports by pilots [
27,
97]. Pilots in commercial aviation can mainly enhance their situational awareness by studying current bird strike risk information in the form of BIRDTAMs (a special form of Notice to Airmen (NOTAM) which provides information on current bird strike risk [
98]) and bird migration reports, where available [
27,
98,
99]. Furthermore, they should stay alert throughout the flight and report observations on enhanced bird activity as well as experienced bird strikes [
27]. In general aviation, route planning should consider the avoidance of areas abundant of birds in addition. By flying at high altitudes, the probability, and by flying at low speeds, the impact of a potential bird strike can be reduced [
27,
99].
To introduce operational bird strike prevention further involving ATC and pilots, experiences from military aviation can serve as an example. As military operations are often performed at low altitude, military aircraft spend much more time in areas with high bird densities than civil aircraft. Hence, military operations are more vulnerable to bird strikes than their civil counterparts. For this reason, several air forces across the world have started to implement procedures to adjust flight planning based on current bird strike risk since the 1970s (e.g., [
100,
101,
102]). In the beginning, this mainly included flight restrictions during peaks of bird migration [
37]. With developments in technology and increasing data-sets to model and predict bird movement, a more dynamic and short-term planning to avoid high-risk air spaces at a given time has become possible [
28]. The military efforts have mainly focused on en-route intervention of flight operations for low-level training flights [
37,
102]. For civil aviation, an application of these procedural approaches at and around the airports would be useful and is seen as an important next step in bird strike prevention [
28]. In contrast to military aviation which has a certain flexibility in flight planning, civil aviation is bound to schedules [
103]. Therefore, regular flight restrictions in cases of high risk are unfeasible. On the other hand, dedicated real-time warnings of high-risk situations resulting in short-term delays could be applicable. Different levels of advice could be possible. First, the general situational awareness of the pilots could be raised. Second, aircraft taking off could be advised to adjust their rate of climb to quicker pass critical zones. In addition, third, in case of high collision risk, air traffic could temporarily be held back.
According to Annex 15 of ICAO, ATC shall provide current information on the
presence of birds constituting a potential hazard to aircraft operations [
104]. However, in order to be able to give precise warnings rather than general information on bird movement, additional surveillance technology is required.
An increasing number of airports have installed radars dedicated to tracking birds, so-called avian radars, over the past few years. They are designed to track individual birds as well as flocks of birds up to distances of 11 km and heights of 1.5 km [
105]. While initial installations covered two-dimensional positions only, systems providing three-dimensional positions are increasingly becoming available. Moreover, radar ranges are increasing and the data quality is improving. Thus far, these radars are mainly used by local wildlife control to detect hotspots of bird movements at the airfield. However, avian radars, possibly in combination with other surveillance technology such as thermal or video imaging, have the potential to serve as input for procedural, real-time bird strike prevention. A unique implementation of a radar-based bird strike advisory system for civil aviation is located at the Durban King Shaka International Airport, South Africa [
28,
106]. During summer, around three million swallows visit a roosting site which is located on the extended runway center line, 2.6 km from the airport. At dawn and dusk, the birds move in large flocks to and from this site. The radar is used to detect these movements. Based on the observed risk level, ATC is advised to temporarily hold back air traffic. Contributing factors to the successful implementation of the procedures are the detectability of huge swarms of birds by avian radars, the short and distinctive periods of threat and the relatively low number of aircraft movements at the airport [
28]. The general introduction of comparable procedures at other airports could be limited by the following factors. In contrast to King Shaka airport, bird strike risk is more random at other airports with respect to number of birds and time of day. The ability of avian radars to detect individual birds, even large ones, close to the ground as well as with increasing distance from the radar, is limited [
107,
108,
109]. Therefore, not all birds are observed by the radar and no warning for potentially critical strikes can be presented due to the missing information. Moreover, tracks of individual birds are more difficult to predict than those of swarms. This reduces the potential positive effect on safety and to superfluous warnings in case of falsely predicted bird movement. Furthermore, bird strike risk is distributed throughout the day. This could lead to increased workload for the controllers and to to unjustifiable reduction in runway capacity at high-density airports.
An ongoing FAA study has addressed the question of workload increase for controllers when involving them in the bird strike hazard reduction process [
110]. In human-in-the-loop simulations, controllers were presented with four test conditions in which they had to control air traffic at an airport. In the baseline scenario, bird activity information was provided as observations by pilots transmitted via radio, representing current procedures. In the three remaining conditions, information was provided in different ways via the controller’s Human–Machine Interface (HMI). Initial results indicate that the controllers appreciate the increased situational awareness. Moreover, the controllers reported a reduction in workload when receiving dedicated bird strike risk information via their HMI in contrast to information reported by pilots. A European study focuses on the potential effects on an airport’s safety and runway capacity when implementing procedural risk-reduction methods [
111,
112]. Fast-time simulations involving deterministic bird and aircraft movement revealed a potential for increasing safety and reducing cost with only a small impact on runway capacity [
112]. Ongoing research is evaluating the effect when the limited predictability of birds is taken into account.
Alternatively to ground-based warning systems, there are ideas to integrate radar-based alerting systems into the aircraft [
113]. Independent of the chosen approach, a close collaboration between research and operational personnel is crucial for a successful implementation of new measures [
114].
The presented initiatives to apply operational bird strike prevention based on the positive results from military aviation are preliminary. Nevertheless, they demonstrate the potential to further reduce the risk of bird strike by applying procedural measures.