«Jukka-Pekka Nikolajeff Trafin tutkimuksia Trafis undersökningsrapporter Trafi Research Reports 7/2014 Trafin tutkimuksia 7-2014 Analysis of the Bird ...»
4.4 Time of Day When the Bird Strikes Occurred Bird strikes occur at different times during a day. This study focused on bird strikes in Finland. Because Finland is located far up in the north, the day is very long in the summer and very short in the winter. Therefore, the times given in the reports analysed do not provide exact information on the visual conditions (day vs. night). In fact, during summer time (from middle of May to middle of August), the nights are Trafin tutkimuksia 7-2014 so bright that the term “night time” is misleading. On the other hand, during winter months (from middle of November to middle of February), the night time is so long and dark that the term “day time” may be misleading.
The proportion of reported bird strikes between 18:00 – 06:00 increased from year 2000 (20%) to 2011 (32%). At the same time, the number of reports in which the time was unknown remained almost the same. The reason for this is not clear, but it could be related to weather. Weather has a significant role in bird flying activity. For example, arctic migrations are heavily dependent on the weather.
4.5 Arctic Migrations This study shows that the migrations have an influence on the number of bird strikes. In all studied years, 2000, 2006 and 2011, there is a clear peak in bird strikes in April. In the year 2006, the first reported bird strikes happened in April. In autumn, the bird strike numbers for September are almost the same as for April. Figure 11 shows the number of bird strikes by month in the years 2000, 2006 and 2011.
However, corresponding figures published about the number of bird strikes in the UK between years 2005 and 2009 are different. In both countries, the high season for bird strikes seems to be in June, July and August, but in the UK, the migration months do not show up as clearly as in Finland (EASA, 2009).
The arctic migrations are a massive spectacle of nature, which gathers together even several hundred thousands of birds. The autumn migration is actually even more interesting than the spring migration. This is because birds do not have to hurry in moving south as long as the weather is good enough in the north. Birds do not start their migration as long as the temperature is suitable for them and some food is available. In Finland, the autumn migration may last from the beginning of May until the end of February, which actually covers almost the whole year. September is the most common month for the most species to move south.
The spring migration has been better studied than the autumn migration. Among the first signs of spring are the flocks of cranes, swans, geese and waterfowl flying from south to north. The time frame of spring migration is shorter than that of autumn migration, because in spring, the birds seem to be in a hurry to get to their breeding areas quickly, to find a partner, build a nest, mate and lay eggs. The northern summer is short and the birds have no time to waste. The last arrivals of spring migration are hardly settled down when the first birds are already starting their autumn migration (Päärni, 2010).
Especially arctic spring migrations may vary significantly from one year to another.
If the weather is bad during the heavy migration period and there are only few days with favourable weather, this may increase the risk for bird strikes.
The migration of arctic waders is a good example of this. The waders are flying very high if there is a good tailwind and the weather is good. The migration routes are then largely unknown, because the birds cannot be monitored from the ground. On the other hand, if the weather turns bad and the wind starts blowing from the north, this forces the birds to fly lower and they can be well observed from the ground.
Even if the weather is good, the wind direction below the altitude of 1 km may cause the birds to divert in a sideward direction. Wind from south–south-east to south-west pushes the birds towards the coastal areas of the Gulf of Finland and inlands to Trafin tutkimuksia 7-2014 south-eastern Finland. On the contrary, wind from south-east to west pushes the birds to the open sea areas of the Gulf of Finland and towards Estonia, which means that the birds do not even enter the Finnish airspace. This has an influence especially on arctic waders, but also affects other birds.
Arctic migrations in the autumn are again different. First, the whole migration period is longer and it takes place from July to October, whereas the spring migration only lasts one month. Various species of geese are easiest to follow. If the wind blows from east to north-east, the goose migration activity is heavy in Finland. If the wind turns to blow from north to north-west, the number of birds observed is significantly lower. The main flocks pass Finland from the Russian side, and only some birds fly in the Finnish airspace.
Birds nesting in Finland always have to fly through Finland, and the migration occurs when the weather is good. Various small birds are best observed when they have to fly into a headwind. This causes them to fly low and they often form massive flocks in the south-western peninsulas, such as Porkkala and Hanko. In a good tailwind, the birds are flying higher and the number of observations is lower than in a moderate headwind. (Koistinen, 2013).
4.6 Weather As mentioned in the previous section for arctic migrations, the weather has a significant impact on bird activity. It is unfortunate that weather details were so poorly reported. Local weather details provided by pilots, air traffic controllers or airport maintenance staff are highly valuable when analysing and studying bird strikes.
Weather details were reported a lot better in the year 2000 than ten years later in 2011.
The weather influences the birds’ flight strategy, foraging behaviour and most of all, their flight altitudes. Especially those species using thermal convections fly at totally different altitudes in good weather. In contrast, the birds that fly more powered flight are not that much influenced by the weather (Shamoun-Baranes et al., 2006). The impact of the weather can be more significant than the time of day. This is a significant fact especially during migrations. The birds can be waiting for an optimal wind for days and build up massive flocks. During low pressure, birds are waiting on fields or just a few individuals will migrate in a fairly narrow front. But as soon as the high pressure comes, the birds will immediately start to move (Päärni, 2010). In Finland, the wind direction has to be from north-east for autumn migration and from west for spring migration.
4.7 Phase of Flight When the Bird Strikes Occurred Approach is the most sensitive phase of flight as concerns bird strikes. In the Finnish bird strike reports analysed during years, 2000, 2006 and 2011, more than one third of the strikes were reported to have happened during approach. The altitude can explain this, as during approach, the aircraft is flying low for a fairly long time, whereas in take-off, it climbs to a safe altitude at a relatively steep angle. Some bird strikes also happen during initial climb, which means the period from rotation to the moment when climb is established. It has been noted that birds very often prefer to stay at the end of a runway, which leads to a higher risk for large aircraft which need a longer take-off distance (Chilvers, 1997).
Trafin tutkimuksia 7-2014
In this study, there were no reported bird strikes during initial climb or take-off run for the year 2000. Ten years later in 2011, however, 42 (19%) of the reported bird strikes took place during initial climb. One reason for this could be that the pilots reported the flight phase more accurately in the year 2011 than in 2000. Nevertheless, it could also be due to the fact that many of the aircraft operating in 2011 were larger than those operating ten years ago. For example, according to the data collected on aircraft movements at Finnish airports, large wide-bodied Airbus A330 and A340 aircraft made a total of 17 take-offs and landings in year 2000, while ten years later in 2011, those aircraft types had 4,210 take-offs and landings. Aircraft movements, also reviewed by aircraft and engine category, are shown in Tables A1.7, A1.8 and A1.9.
4.8 About Heights and Altitudes Bird strikes normally occur below the altitude of 500 feet during take-offs and approaches. In the year 2000, 101 (59%) of the strikes happened at or below 500 feet altitude. In the year 2011, 93 (42%) of the strikes were reported to have taken place at or below 500 feet altitude.
The National Wildlife Strike Database for Civil Aviation in the Unites States received 38,961 reports of bird strikes between the years 1990–2004. Of those bird strikes, 10,143 (26%) occurred above 500 feet altitude (Dolbeer, 2006). Later a trend was found suggesting that the number of bird strikes above 500 feet was increasing.
In Dolbeer’s (2011) later study, it was revealed that the number of bird strikes occurring above 500 feet had actually increased from the year 1990 up to 30% between the years 2005 to 2009. This study shows that exactly the same trend could also be seen in Finland based on the bird strike reports received by the Finnish Transport Safety Agency. The share of bird strikes above 500 feet increased from 71 (41%) in the year 2000 up to 129 (58%) in the year 2011. This means a total increase of 29% from 2000 to 2011.
Bird strikes above 500 feet were most often caused by waterfowl, gulls, terns, passerines and vultures. As for the bird strikes below 500 feet, they happened most frequently to passerines, gulls, terns, pigeons, doves and raptors (Dolbeer, 2011). According to the bird strike reports received by the Finnish Transport Safety Agency in the years 2000, 2006 and 2011, bird strikes above 500 feet were most often caused by swallows, swifts and Sand Martins (Riparia riparia).
The strikes causing damage to aircraft most often occurred above 500 feet altitude (66%). Dolbeer (2011) also discovered that the relative number of aircraft-damaging bird strikes had increased from 37% in 1990 up to 45% by the end of year 2009.
According to this study, 12 (7%) of the bird strikes caused some damage to the aircraft in the year 2000. Of the aircraft-damaging bird strikes, 7 (58%) took place at or below 500 feet, 4 (33%) occurred above 500 feet and in one case (8%), the altitude was unknown or not reported.
During the year 2006, 8 (6%) bird strikes were reported to have caused some damage to the aircraft. Two (25%) of the aircraft-damaging bird strikes happened below 500 feet and four (50%) above 500 feet. In two cases (25%), the altitude was not reported or not known.
Trafin tutkimuksia 7-2014
In the year 2011, 11 (5%) of the bird strikes caused some damage to the aircraft.
Three (27%) of the aircraft-damaging bird strikes happened at or below 500 feet altitude. The same number, three (27%), took place above 500 feet. In five (45%) of the reports the altitude was not unknown or not reported.
Contrary to Dolbeer’s (2011) study, the number of bird strikes causing aircraft damage decreased in Finland by 29% during the years studied, from year 2000 to 2011.
4.9 Trends When analysing the results for the years studied, 2000, 2006 and 2011, some trends
and similarities can be observed:
Time of year when the bird strikes occurred. Summer is clearly the worst season especially for bird strikes that happen at airports or near vicinity. This can be explained by the fact that airports provide an ideal environment for birds for nesting and finding food. Large grass areas attract various insects and small rodents, which in turn bring the birds looking for food. Later, when young birds are leaving their nests, they are simply not careful enough and can easily be hit by aircraft. The number of aircraft operations is not higher in summer.
Altitude where the bird strikes happened. Most of the reported bird strikes occurred below 2000 feet. This fact also correlates strongly with the phase of flight. If the aircraft starts its approach to an airport from 2000 feet by using a three-degree glide, it will be flying at the altitude where bird strikes most frequently occurred for about 9 km. Departing aircraft normally climb in a 10degree “nose up” angle, which means that the aircraft reaches the altitude of 2000 feet after flying a distance of about three kilometres.
Size of bird that hit the aircraft. Small birds are causing the major part of all bird strikes reported in Finland. This could be explained by the fact that small birds often eat insects, which they can easily find at airports in the summer. Airports seem to be an ideal environment for small birds. One reason could also be that small birds are not perceived as significant a flight safety issue as they could be.
In addition, individual small birds or minor flocks of them are not as easily seen by pilots or air traffic controllers as larger birds, and preventive actions may be taken too late or not at all. Most preventive actions are directed towards medium and large birds. For example at Helsinki-Vantaa International Airport, there is a very strict zero tolerance against any geese. They will be immediately eliminated.
Identification of bird species. Different bird species were poorly identified. The number of identified birds remained low during the years studied, and it even dropped from 30% in the year 2000 down to 10% in the year 2011. The reason for this is simple: pilots, air traffic controllers and airport maintenance staff just do not know the species of birds, and for example DNA testing is not used.
Types of aircraft that were hit by birds. Turbofan powered aircraft are most sensitive to bird strikes. This is explained by the fact that most flight operations are carried out by turbofan aircraft. Turbofan aircraft are fast moving and relatively quiet as well. The power plant is often located under the wing, fairly low above the runway and taxiway surface. Many birds can be easily sucked into the engine due to this.