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Non Avian Wildlife Hazards to Aircraft

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Article Information
Category: Wildlife Strike Wildlife Strike
Content source: SKYbrary About SKYbrary
Content control: SKYbrary About SKYbrary

Summary

This article describes the hazard to the safe flight of aircraft from wildlife other than birds. Airport Risk Assessment and Control of bird hazards is usually carried out within the wider context of wildlife hazards. As with birds, most animals quickly adapt to the normal noise and activity conditions of a chosen habitat and have no innate fear of aircraft or vehicles on airports.

Non-Avian wildlife hazards to aircraft usually involve ground dwelling mammals, which limits the potential consequences of impact, but bats - the only flying mammals - are a notable and occasionally significant exception.

Relative Size and the Degree of Hazard

Non-avian wildlife hazards are almost exclusively mammals. The relative sizes of the objects in any aircraft/mammal conflict is understandably significant. An encounter between a particular animal and a large transport aircraft might produce relatively insignificant damage whereas if that aircraft is much smaller, the consequences of an impact may even lead directly to an accident. A few animals such as bears and large deer species are so large that the safety of an aircraft of any size would be at risk if one was hit. Fortunately, many larger animal species rarely feature in aircraft impact events although some species like deer, elk and moose, which see safety in staying close to other animals, can raise the likelihood of an aircraft encounter having an impact outcome by just this tendency.

It has been suggested that an appropriate general guideline for a minimum size for reporting of non-flying animals actually or potentially hazarding aircraft taking off or landing should be 1 kg. See "Animal Ambush at the Airport: The Need to Broaden ICAO Standards for Bird Strikes to Include Terrestrial Wildlife".

Geographic variation in hazard

Much more so than with birds, the likelihood of an aircraft / mammal impact or near miss is extremely location-specific. Mammals have habitat and foraging requirements which are specific to species, and the runway areas of airports are generally open and treeless habitats which are unattractive to many larger animals because of their resultant vulnerability to visual detection by predators and humans. Although it is also the case that many mammal species are mainly active during the hours of darkness, aircraft manoeuvring areas are well lit by the lights ahead of moving aircraft and vehicles. Of course the natural habits of all animals can be, and are, modified by the perceived attractions of closer-than-normal association with human activity, especially the inevitable availability of edible waste in the vicinity of airport buildings. Such food supply may directly draw in animals which can constitute a direct hazard to aircraft or may indirectly attract larger predators which are themselves hazardous. In some parts of the world, some animal species undertake communal seasonal migrations similar to those made by birds and local awareness will be the only guide to any resultant increased hazard.

Hazard Control

The only option for large animals at airports used for scheduled service public transport flights is exclusion. Fortunately this has been aided by the trend towards securing airport perimeters with fencing of a sufficient scale and robustness to deny unauthorised human access. However, such perimeter fencing is not yet universal and is certainly not present at many smaller airports used by business and general aviation and for aerial work and leisure flying.

Mammals Which Regularly Feature in Aircraft Incidents

The chances of recorded incidents involving particular species, obviously depend upon not only propensity to record but on both the extent to which a species is present and the density of aircraft movements. For these reasons, animal species recorded regularly in such incidents are biased towards those found in North America and Europe. Deer species are top of the list overall with over 40 deer strikes per year regularly occurring in North America, many to light aircraft on private flights at airstrips where the operator does not attempt to exclude animals and is not required to. White-tailed and mule deer are most commonly involved. Coyotes are the next animal most often encountered in North America whereas in Europe, red foxes are often the most common sighting at airports of all sizes although they are only quite infrequently hit by aircraft.

Aircraft Certification

Unlike the case of birds and aircraft jet engines or windscreens, there are neither requirements nor manufacturers minimum standards for the parts of aircraft which may impact animals. Landing gear is a particular case. Nose gear impact with, say a fox, at speed may cause significant damage to a smaller aircraft, even up to the size of a 50 seat regional jet. No direct consideration of the effects of impacts of this sort on landing gear assemblies is given by their designers and this includes their important secondary functions as the platform for lower speed directional control (nose gear) and for braking (main gear).

Bats - a Special Case

As the only flying mammals, the hazard to aircraft from bats is similar in nature to that from birds in respect of the potential for airborne impact. However, bats generally fly only between dusk and dawn - although not necessarily only when it is fully dark. Bats can be conveniently divided into two classes, ‘small’ and ‘large’, for hazard assessment, subject to the caveat applied to birds about the hazards of flocking. Generally smaller ‘insectivorous’ bats (so called because they feed exclusively on insects) are found throughout the world whereas generally larger ‘fruit bats’ or ‘flying foxes’ (which are vegetarians) are found everywhere except the Americas. As with birds, bats’ flight habits do depend upon prevailing weather conditions but the insectivorous bats in particular are highly focussed on feeding opportunities and will readily fly even in light rain if insects are on the wing in numbers. Conditions like this often occur during thunderstorms over land in warm weather when a bird hazard would be unlikely. Insectivorous bat activity usually peaks in the two hours from near darkness and again around dawn. Dawn flight activity is more likely to extend into full daylight. Fruit bats are more likely to begin flight whilst it is only beginning to get dark and continue doing so during the night.

Bats as an Aircraft Hazard?

Given all the above information, especially the almost complete absence of daytime flight, it will be evident that bat strikes by aircraft are a relatively rare occurrence and unsurprising that strikes sufficient to directly cause an aircraft accident are so far unknown. Not only do bats fly mainly at night but the ones most likely to be encountered, the insectivorous bats, navigate by means of an ‘on-board radar system’ entirely analogous to primary radar and hibernate in winter when insect food supply diminishes. Fruit bats, whilst characterised by large communal roosts, use exclusively trees and caves rather than buildings for this purpose and transit at low level to and from their feeding sites of tree-borne fruit. This behaviour places most of them at an even lower risk of an aircraft impact than insectivorous bats.

Airports prone to Wildlife strikes

The following map shows the aerodromes listed on SKYbrary where non-avian wildlife hazards have been noted :

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