«Jukka-Pekka Nikolajeff Trafin tutkimuksia Trafis undersökningsrapporter Trafi Research Reports 7/2014 Trafin tutkimuksia 7-2014 Analysis of the Bird ...»
To give one more example of bird strike definitions, Transport Canada has agreed with the Bird Strike Committee Canada (BSCC) to have their own criteria for ensuring harmonised reporting of bird strikes. According to that definition a bird strike has occurred, “if 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.” (Transport Canada, 2004).
2.2.2 Bat Strikes In some countries, strikes with bats are also categorised as bird strikes. Indeed, a strike with a bat can be very hazardous due to the bats’ habit of flying in large flocks and at night, when they are difficult to see. The bat’s anatomy brings some challenges as well, since bats do not have light and pneumatised bones like birds (Parsons et al., 2009). In Finland, however, bat strikes are not a flight safety issue.
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2.3 Which Part of the Aircraft the Birds Normally Hit?
According to earlier studies by Transport Canada (2004), 15% of birds hit the aircraft nose. The wings and the engines both sustain about 13% of bird hits. The aircraft fuselage gets 11% of the bird strikes and the landing gear about 9%.
The European Aviation Safety Agency (EASA) has made research about accidents caused by bird strikes between the years 1999 and 2008. The engines sustained damage in 44% of the accidents. The wings were second with 31% and the windshield third with 13%. The nose part of the aircraft was damaged in only 8% of the strikes leading to an accident (EASA, 2009).
The latest study, made by Mr. John Thorpe, was presented in last IBSC meeting in Stavanger, Norway in June 2012. Thorpe’s study included bird strike data from last 100 years. According to this data, the aircraft engine was damaged in 76% of the accidents. Windshield damage led to an accident in 7% of the cases. Thorpe’s study focused on airliners and executive jets.
2.3.1 Which Part of the Aircraft is the Most Sensitive?
EASA (2009) has performed a similar study where they compared bird strike accidents between the years 1999 – 2008. This study pointed out the parts of the aircraft where the bird strikes had caused damage. The result was that the engines sustained some damage in 44% of the accidents. When the different engine types were examined more carefully, it could be concluded that turbofan engines sustained damage in 53% of the cases. The corresponding figure for turbo propeller engines was 38%, whereas reciprocating engines were damaged in only 6% and turboshaft engines in only 3% of the accidents. The wings suffered some damage in 31%, the windshield in 13%, the nose in 8% and the fuselage in 4% of the bird strike accidents (EASA, 2009).
2.4 Why are Bird Strikes Dangerous?
It is good to remember that most of the bird strikes do not cause any hazard. The probability of dying in a bird strike is very small. It is estimated that fatal bird strikes only occur once in a billion flying hours (Miller et al., 2010). It can also be misleading to think that strikes with large birds would always be the most dangerous ones. Even a flock of small birds can easily break an engine, windshield or another aircraft structure and lead to a serious hazard to safety. The size of the bird does not directly correlate with the damage sustained either. In fact, mass density varies a lot according to bird species. To give an example, a Laughing Gull (Leucophaeus atricilla) is about one third of the size of a Herring Gull (Larus argentatus), but has significantly higher density. Another interesting example is the Starling (Sturnus vulgaris). They have a 27% higher mass density than gulls and can form flocks with up to 100 000 birds. This is why Starlings are sometimes called “feathered bullets” (Transport Canada, 2004 and EASA, 2009).
Simply by following the laws of physics, the mass of the bird and the aircraft velocity are the two values that affect the kinetic energy of the strike. Of these two values, the (squared) velocity actually has a stronger influence on the consequences than the
bird’s mass. Kinetic energy can be calculated using the formula:
Trafin tutkimuksia 7-2014 Kinetic Energy = (Mass / 2) x (Velocity) 2 This makes a significant difference between the various phases of flight. During take-off, engines are often set at maximum power, while during approach, they can be running at idle. High engine RPM makes take-offs more dangerous than approaches if a bird strikes the engine.
2.5 Aircraft Certification EASA has established detailed requirements for what an aircraft has to be able to endure when hitting a bird. Those requirements are shown in EASA certification CS-E800. According to the requirements, engines running at take-off thrust must
Take a single large bird (1.85 – 3.65 kg) without any hazardous effect on the engine.
Take flocking large birds (1.85 – 2.50 kg) without suffering no more than 50% loss of thrust and providing at least 20 minutes capability with some thrust variation.
Take flocking medium sized birds (the mass of the birds can vary) losing a maximum of 75% of the thrust.
Take small birds (mass 0.85 kg) losing a maximum of 25% of the thrust.
The airframe and, for example, the windshield have to be strong enough to sustain bird strikes as well. The certifications vary between different aircraft categories. To give an example, a normal passenger aircraft should be able to continue flying safely after hitting a bird of 1.80 kg at cruise speed (Vc). For some aircraft components, such as the empennage, which is an important part for aerodynamics and steering, this weight is doubled so that it must be able to sustain a bird strike of 3.60 kg.
There are no specific requirements for fuel tanks (EASA, 2009).
2.6 History of Bird Strikes Bird strikes became a safety problem as soon as people started to share the sky with birds. The first bird strike happened already in the year 1905, and the first fatality was caused seven years later in 1912. (LeMieux, 2009).
In 2009, EASA published estimations about the number of fatalities and hull losses caused by bird strikes. After the very first fatal bird strike in the year 1912, there have been 47 fatal bird strike accidents in commercial air transport operations, causing 242 fatalities and 90 hull losses. The actual figures are much higher, though, because military and general aviation are missing from EASA’s report. The cost of bird strikes is over one billion Euros every year, and the value of any human life lost is priceless (EASA, 2009).
This study was recently updated by Mr. John Thorpe (2012) in the IBSC meeting in Stavanger. Thorpe’s study shows that the number of fatal accidents caused by bird strikes has now risen to 55 and the number of fatalities to 276. Total hull losses have also increased to 108 (Thorpe, 2012).
A very recent example of a bird strike was the crash of a Sita Air Dornier Do 228 aircraft on 28th of September 2012 in Nepal. The aircraft was reported to have crashed shortly after taking off from Kathmandu airport. The pilot had told the air Trafin tutkimuksia 7-2014 traffic controller that they had hit a vulture. The forced landing was unsuccessful, and all 16 passengers and 3 crewmembers died (BBC, 2012).
2.7 Bird Strikes – Growing Problem in the Future Aviation is a rapidly growing business. During year 2011, a total of 2.8 billion passengers travelled by air using 38 million separate flights (IATA, 2012). The Airbus Company has recently published a Global Market Forecast (GMF) for the coming 20 years. The estimations in that forecast show a 4.7% annual increase in global passenger traffic. This means that 28,200 new transport or cargo category aeroplanes would be needed and 10,350 old aircraft would have to be replaced by new modern aircraft. The total number of transport category aircraft is estimated to be over 32,550 by the year 2031, which is 110 % more than today. At the same time, the number of cargo category aircraft is estimated to increase from 1,600 to 3,000, which is almost the double. All this is going to cost approximately USD 4.0 trillion (Airbus, 2012).
But do we know how many birds there are sharing the sky with us? Globally, the number of individual birds has been estimated at around 100 billion. To give some examples, it is estimated that there are about 20 billion birds living in North America, about 180 million in the British Isles and about 64 million in Finland (Transport Canada, 2004).
By looking at the numbers above, there is no doubt that bird strikes are a significant safety issue now and in the future.
2.8 Altitudes Where Bird Strikes Happen In commercial air transport, bird strikes usually take place during departures and approaches below 500 feet (Dolbeer, 2006). Many bird strikes also occur on the ground during take-off run or landing roll. According to Transport Canada research (2004), some 90% of all reported bird strikes where the phase of flight was given happened during take-off or landing. Normally the risk of bird strikes decreases when the altitude increases. However, there are always exceptions; some bird strikes have been reported even at flight level 370 (11,278 metres) (Layborne, 1974). During migration, birds have been reported to be seen even higher. To give an example, Bar-headed Geese (Anser indicus) have been seen above Mount Everest at 30,000 feet (9,144 metres) above sea level (ASL), and a flock of swans between Iceland and West Europe at 27,000 feet (8,229 metres) ASL. Mallards (Anas platyrhynchos) have been reported at 21,000 feet (6,400 metres) ASL and Snow Geese (Chen caerulescens) at 20,000 feet (6,096 metres) ASL. According to the observations, birds are normally flying between 5,000 – 7,000 feet (1,524 – 2,134 metres) above ground level (AGL) during migration, but the altitude can vary from 1,600 feet up to 11,500 feet (488 – 3,505 metres) (Transport Canada, 2004). In conclusion, a bird strike can actually happen at any altitude.
The National Wildlife Strike Database for Civil Aviation in the United States received 38 961 reports of bird strikes between the years 1990-2004. Of those bird strikes, 26% (n = 10,143) occurred above the altitude of 500 feet (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.
Trafin tutkimuksia 7-2014 Bird strikes above 500 feet were most often caused by waterfowl, gulls, terns, passerines and vultures. Bird strikes below 500 feet most frequently involved passerines, gulls, terns, pigeons, doves and raptors.
The strikes causing damage to aircraft most often happened at altitudes over 500 feet (66%). Dolbeer (2011) also realised that the relative number of damaging bird strikes had increased from 37% in the 1990’s up to 45% by the end of year 2009.
In the United States, bird strikes below 500 feet most often occur between July and October, when compared to the relative frequency of aircraft movements. Bird strikes are much more frequent during daytime than at night (Dolbeer, 2006). During autumn and spring migrations, however, some birds also fly actively at night time. It can be concluded that the risk of a bird strike is always present when you fly.
2.9 Climate Change and Bird Strikes The bird population is increasing rapidly. During the past decade, some species that used to live in the south are now moving northward and bringing new challenges to the agriculture and fisheries, as well as safety problems to aviation. In Finland, the new arrivals Barnacle Goose (Branta leucopsis) and Great Cormorant (Phalacrocorax carbo) are probably the best examples. The Great Cormorant (Phalacrocorax carbo) was nesting in Finland for the first time in 1996. Ten couples were counted at that time. By year 2009, the number of nesting couples had increased up to 16,000 couples. Another success story has been seen with the Barnacle Goose (Branta leucopsis). The first couple was nesting in Helsinki in 1989, and now the population is estimated to be between 3,000 – 5,000 couples only in the Helsinki area (Birdlife Finland, 2012).
In the United States alone, the goose population has grown from one million up to four million since 1990 (LeMieux, 2009). For example collisions with Canada Geese (Branta Canadensis), which are common in many places and are relatively large birds, have led to aircraft damage in about 67% of the strikes (Baxter and Robinson, 2007). A recent bird strike accident that attracted significant media attention in the USA was caused by a flock of Canada Geese. This accident happened on the 15th of January 2009 at 03:25 p.m. to US Airways flight 1549. The flight had just left LaGuardia Airport in New York when it hit the flock of birds. The accident caused the large transport category aircraft to ditch in the Hudson River in the centre of New York, but luckily everybody survived (LeMieux, 2009).
2.10 Understanding the Birds To understand why bird strikes happen, we should know what the birds are thinking, what they see, how they react in different situations and why they are acting as they are. This is not a simple task at all, but it is exactly what we should learn by using the valuable data obtained from bird strike reports. Only by knowing the bird species and when, where and why they hit the aircraft, we can initiate effective preventive actions. It is not only by chance that the birds are acting like they do; they always have a reason for their different behavioural patterns. That is exactly what we have to learn so that, in the future, we could be one step ahead of the birds and share the sky safely with them.
A recent study shows, for instance, that we could reduce the risk of bird strikes by changing aircraft fuselage colours to brighter ones (Fernandez-Juricic et al., 2011).