This article is based on EASA Safety Report - Bird population trends and their impact on Aviation safety 1999-2008 kindly provided to SKYbrary by EASA.
The report is not an exhaustive review of the subject of bird strikes in aviation; however, it aims to provide an overview of the problem, some estimates on the factors affecting future trends and also to highlight particular issues related to accidents in aviation attributed to bird strikes. The full Report is accessible from Further Reading.
Although bird strikes are documented to be an issue as old as aviation, its significance as a hazard has not been mitigated. The first documented bird strike that resulted in a fatality was in 1912 when the Wright Flyer encountered a flock of gulls whilst conducting a demonstration flight. Since that flight it is estimated that 47 fatal accidents have occurred due to a bird strike involving commercial air transport. The total number of fatalities is 242 people and 90 hull loses. The total number of fatal accidents in military aviation is believed to be much higher.
During the decade of 1999-2008 in total 71 accidents occurred due to a bird strike. Of these only 6 led to fatal injuries (See Figure 1).The highest number of accidents occurred during the take-off phase (48%), followed by the approach (30%) and the en-route phase (15%). In total 84% of bird strike accidents occurred during the take-off, approach and landing phases.
Aircraft Bird Strike Certification Requirements - Fields for Improvement
From an aviation regulatory perspective, birds are divided into three categories; classified as large, medium and small birds. These bird categories are used to describe the various certification criteria for airframe and engines. Recently many researchers have raised their concern that past airworthiness standards have been outpaced by changes in bird population and species (avifauna).
Climatological and other environmental changes affect bird populations and their biological behaviour. This change is not reflected throughout aircraft certification requirements. To this end, large bird certification requirements have recently been extended to include provision for large flocking bird tests, in order to take into account recent concerns about changes in the European avifauna.
Bird strike certification requirements appear to have been reactive to past occurrences, however new occurrences have shown that there are particular areas of concern, such as:
In several occurrences aircraft fuel tanks have been penetrated resulting in fuel leakage. Apart from the general 1.8 kg bird weight requirement, there are no particular certification requirements for fuel tanks and this issue needs to be assessed.
Light Non-Commuter Aircraft
There are no bird strike related certification requirements for light non-commuter aircraft and light helicopters although this category of aircraft is most likely to operate continuously under 8,000ft amsl where almost all bird strikes occur. The high proportion of accidents involving slow moving aircraft (turboprop aircraft and helicopters) resulting in damage to the windshield may also justify a review of the bird strike requirements for light aircraft. The EASA Final Report on 'Bird Strike Damage and Windshield Bird Strike (2009)', accessible under Further Reading, highlights that the "lack of any bird strike requirements for small rotorcraft has been identified from research and analysis of accidents as a safety issue. In particular, a lack of windshield bird strike protection capability was seen as the focal area for regulatory enhancement".
Very Light Jets
Due to their high speed, certification requirements for Very Light Jets follow those of commuter light aircraft requiring the windshield to be able to withstand a strike with a bird of a mass at least 0.9 kg (2lb) at maximum approach flap speed. This requirement was insisted upon by EASA and the related Certification Review Item.
Bird Population Trends and Patterns
Although in recent years the overall bird population has declined in Europe by over 10% the bird strike hazard for aviation has not reduced proportionally. The reason is that not all birds pose the same hazard to aviation safety, as this depends on the size of the birds and their foraging or migratory patterns. Birds may pose a threat to aviation due to their individual size or due to their tendency to fly in large flocks. It is likely that the smaller the birds are, the greater their need to travel in flocks in order to avoid predators.
In the past decades there has been a change in the number and the composition of the bird population as well as in the habitat of some of the species. Some bird species have adjusted to the urban environment while others have experienced a significant increase in their population. Furthermore, climatological changes have allowed new species to forage and breed in geographic areas which were not particularly suitable to them several decades ago. The ban of organochloride pesticides has also enabled some bird species population to increase from their low levels in the 1970’s. Finally it is also interesting to note that some of the wildlife protection programmes have introduced a population increase of some large bird species which were almost extinct a few decades ago. For example, 24 of the 36 largest bird species (weight greater than 2 kg) in North America have shown significant population increases in the past 30 years and only 3 species have shown declines. Using various other sources of raw and derived data it can be concluded that most of the occurrences (95%) occur below 2500ft amsl and around 70% occur below 200ft. Various sources quote different percentages for each altitude threshold, but they all concur that most occurrences take place very close to the ground (See Figure 2).This highlights the fact that the risk of bird strikes can be mitigated by measures taken primarily at an aerodrome level, such as avifauna assessment and management.
Reporting of Bird Strikes
If any improvement is to be realised in better assessing, mitigating or controlling bird strike effects on aviation safety, then it is of outmost importance that reporting of bird strike occurrences improves significantly. Some EASA Member States, most notably the UK CAA, have undertaken several steps to this direction. However there appears to be no combined effort in Europe to collect all relevant occurrences, especially those that did not lead to a bird strike.
Training for Bird Strike Avoidance
In addition there are no prescribed training requirements for flight crews in regard to bird strikes and bird strike avoidance. It is possible that not all flight crews are aware of the significance or some of the aspects of the problem.