A birdstrike risk assessment model and its application at Ordos Airport, China

Study area and data survey method

Ordos Civil Aviation Airport, our study site, is located in the southwestern part of the Inner Mongolia Autonomous Region, China. It is characterized by mid-temperate continental climate and is located on the northeastern edge of the Mu Us Desert. Its main characteristics are a long winter and short summer, but has four distinct seasons. The mean annual temperature is 6.2 ℃, and the mean annual precipitation is 358 mm, mostly concentrated between June and August. The mean annual wind speed is 3.6 m/s¹⁷. There are five main land cover types in Ordos civil aviation airport and its surrounding areas: farmland, residential areas, woodland, shrub grassland and wetland.

We used the line transects method and point counts method to investigate the environment and birds within our study area. It is a commonly used bird survey method, and is also widely used in the survey of birds in and around airports. The line transects were 1000 m × 100 m and the walking speed was 1.5–2.0 km/h. We observed and recorded birds with 10 × 50BA and 30 × 77BA Leica telescopes, SLR digital cameras (Canon 5D Mark III) with telephoto lenses (Canon 100–400 mm). The point count method we used had an observation radius of 200 m. We observed birds with 10 × 50BA Leica binoculars and a 30 × 77BA Leica fixed-mount spotting scope^{18,19,20,21,22}. The flight altitude was estimated using a visual comparison method: an altimeter was used to measure the height of trees and buildings in the observation area and these heights were used to estimate the flight altitude of observed birds. Bird identifications were based on A Field Guide to the Birds of China²³.

The survey areas were divided into three areas: A, B and C. Section A was located within the boundary of the airport. Section A surveys consisted of five shrub grassland transects, with one transect on the runway and one on the apron. Section B was the area within 4 km of the center of the airport (but excluding Section A). There were five woodlands, nine shrub grassland, two farmland and four residential areas. Section C consisted of all areas located with 8 km of the center of the airport, excluding Sections A and B. In Section C we established three woodlands transects, three shrub-grassland transects, two farmland transects and four wetland transects. The species, quantity, distribution, cluster and flight altitude of birds in 39 transects or point counts set up within 8 km of the airport and its surrounding areas were investigated monthly by the method of sample strip or sample point (wetland using sample point method). A total of 468 individual point count or transect surveys were conducted over the study year.

Birdstrike risk assessment model

Investigating the bird situation in the airport and surrounding areas is a prerequisite for birdstrike prevention. The establishment of a scientific and standardized risk assessment process for birdstrike prevention (Fig. 2) is helpful for the systematic evaluation of birdstrike risk. This model is based on the ISO 31000 risk management process²⁴—risk identification, risk analysis, risk assessment, risk response, risk recording and reporting, communication and consultation, monitoring and review. A flow chart for bird strike risk assessment was constructed.

The occurrence of a birdstrike is a matter of probability. The consequences of a birdstrike are a matter of severity, with loss of aircraft or life occurring in extreme cases. Together they combine to determine birdstrike risk, and thus our five risk factors are meant to capture severity and likelihood. The first risk factor is the comparative number, which is important for the simple reason that if a bird species collides with an airplane, a greater number of birds have more serious consequences for an airplane. Among the bird strike events between 2007 and 2014 with the largest record impact energy, half of them involved species in the family Anatidae, and they were all birds with a relatively large comparative number^25,26. The second risk factor is bird weight. The greater the weight of a bird, the greater the force generated by an aircraft impact, and the severity of birdstrikes will also increase. Flight altitude is an important factor in the analysis of birdstrike risk¹². According to ICAO data, we use 40 m as the critical value of the risk zone. If the average flight height of a bird species is closer to the critical value, the risk of birdstrike will be higher¹². Our fourth risk factor is a clustering coefficient, which relates to the living habits of a bird species to move in large groups. If a bird species often gather in large numbers, then the possibility of encountering an aircraft and causing a birdstrike event is greater. This is due to the nature of the collective behavior of birds while flying in flocks of murmurations. Following large, tight formations, birds make fewer independent moving decisions, being forced to constantly react to the movements of their neighbors and having their view partially obscured. They may not have space to avoid oncoming aircraft, or may lack the freedom and alert to choose a successful escape path leading to a higher probability of collision with the aircraft^27,28. About 80% of birdstrikes occur during the take-off, climb, approach, and landing phases of flights^12,13,29,30, so the distance between bird activity from the flight zone is also an important factor in assessing the probability of birdstrikes. Combining with the above analyses, a risk assessment matrix based on the five factors of bird number, weight, flight altitude, cluster coefficient and range of activity was proposed to assess the risk level of bird species in the airport and its surrounding area within 8 km.

Risk factor assignment

1. 1.

   Comparative number = (the number of individual birds/the number of individuals with the most number of birds) × 100.

2. 2.

   Comparative weight = (estimated weight of all birds of a single species/the largest weight of all birds of any species) × 100.

3. 3.

   Risk coefficient of flight height:

   Flight height H (m)	Risk coefficient of flight height
   H > 100	0.1
   100 ≥ H > 50	0.5
   50 ≥ H > 30	1
   30 ≥ H > 5	0.5
   5 ≥ H	0.1

4. 4.

   Clustering coefficient assignment:

   Number of individuals of a cluster	Cluster coefficient
   N > 100	1
   100 > N ≥ 20	0.5
   20 > N ≥ 3	0.2
   3 > N ≥ 1	0

5. 5.

   Activity range risk coefficient assignment: according to the bird species observed area, it could be divided into three levels: activities in flight area, activities within 4 km from flight area, activities within 8 km from flight area but not within 4 km. If a bird species has activity in each area, the nearest one to the flight zone will be used as the input for the risk assessment model. The birds distributed in these three regions were assigned 0.9, 0.6 and 0.3 respectively.

Risk assessment matrix

The expert evaluation method is used to determine the numerical range³¹ (Table 1).

According to the very low, low, moderate, high and very high levels of possibility and severity (Table 1), the level of potential threatening birds are divided into three risk levels: high danger (level 3), moderate danger (level 2), and low danger (level 1). (Table 2).

Adjust the risk level of individual bird species according to the actual situation of the airport:

1. 1.

   If the bird is a raptor, increase the risk level by one.

2. 2.

   The risk level for bird species that are seen crossing a runway or passing through the sky above the runway more than three times should be increased by one.

Raptors fly fast, and collisions with airplanes can have very serious consequences. Among the birdstrike events with the largest record of birdstrike impact energy from 2007 to 2014, half of them were raptors. However, because their weight is actually low compared to birds like ducks, and their solitary habits, the risk level calculated by this method is often lower than the actual risk, so the risk level of the raptor is increased by one level. Most birdstrikes occur when the aircraft takes off and lands. If the bird’s movement often crosses the runway or the nearby sky, it is more likely to cross an aircraft’s flight trajectory, and therefore is very dangerous for the aircraft. For this reason, when a bird species is seen crossing the runway and flying over the top of the runway three times, the risk level of that species should be increased by one.

Each airport should adjust their assessments based on locally collected empirical data on strike likelihood and severity as well as ongoing bird monitoring at the airport and its surrounding environment.

Source: Ecology - nature.com