Moderate Bird Activity Analysis of the “Strike-Chain” from a Bird’s Perspective By LT Justin “Toto” Davis, USN
A Bird Strike, or from a Bird’s Perspective, a Helicopter Strike We share the skies with an ancient and learned adversary. When it comes to air superiority, humans are incredibly late to the game. Insects were the first animals to evolve flight, probably as early as 400 million years ago (160 million years before the first dinosaurs). The Pterosaurs, the group including Pterodactyls, followed suit at around 220 million years ago. Next up, the ancestors of the modern birds we know and love and hit today were well-adapted to a flying lifestyle by 130 million years ago. Then, finally, bats established themselves as fliers by about 55 million years ago (Alexander 76).
flight maneuverability--was the best predictor for how often a species is struck. Species with lower wing loading (smaller birds with greater maneuverability) were found to collide with aircraft at a much higher rate compared to species with higher wing loading (larger birds with reduced maneuverability) (Fernández-Juricic et al. 7). This may seem counter-intuitive, but many small-bodied birds have higher energy requirements and end up spending a good portion of their lives actively foraging for food at or below 500 feet AGL (Fernández-Juricic et al. 9). Does that altitude sound familiar? All that time in the air leads to a much higher likelihood of a strike overall. In addition, these birds usually gather in large flocks, (up to 100,000 individuals for European starlings) so while a single strike would likely not cause damage, flying through an entire flock would be ruinous. Such was the case in the single most fatal bird strike ever recorded, when a Lockheed L-188 Electra passed through a flock of these same 3-oz. European starlings on takeoff from Boston in 1960, killing 62 passengers on board.
Humans, though? We finally parted with solid ground a little over 100 years ago when Orville and Wilbur Wright made magic happen over Kitty Hawk, North Carolina in 1903. It should be no surprise that shortly after the advent of powered flight, our aircraft rapidly surpassed the ability of birds to detect and react to our presence in the air. Indeed, only a few years later in 1914, we experienced our first fatality as the result of a bird strike. Incidentally this was also the same year the U.S. Navy began flying airplanes. After all, modern military aircraft (especially helicopters) flying through the bird-rich lower altitudes, are a completely novel threat to birds from an evolutionary standpoint. With this in mind, would we be able to dissect what goes wrong in a bird strike from the bird’s perspective and use that information to possibly avoid a last-minute strike altogether?
Now, which birds inflict the greatest amount of damage? This is a simple matter of Newtonian Force = Mass x Acceleration. In this case, the larger-bodied birds pose the highest risk-the geese, ducks, vultures, and birds of prey, to name a few groups. Despite these birds ranking low in overall strike frequency, their sheer body mass and their abilty to achieve higher airspeeds make them a very serious threat to helicopters (Pfeiffer et al. 5). Additionally, when you have a large species that also tends to fly in flocks, like Canada geese, the chances of having a catastrophic incident increases dramatically. This was the case when a U.S. Air Force Boeing E-3 Sentry aircraft ingested several geese and crashed two miles after takeoff from Elmendorf AFB in 1995, killing all 24 crew members on board.
Probability versus Severity Before diving into this question, some context is needed. As with most situations in military aviation, bird strikes are a matter of probability versus severity. So, what groups of birds are struck most often? It turns out that for rotorcraft the groups that score the highest in strike frequency are the small-bodied songbirds (larks, starlings, blackbirds, sparrows, etc.) and the shorebirds (gull, killdeer, etc.) (Pfeiffer et al. 13). Naturally, there are many variables that play into this but perhaps the most enlightening reason comes out of one recent study that found that a bird’s wing loading--a measure of the bird’s in59
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