Moderate Bird Activity - Analysis of the “Strike-Chain” from a Bird’s Perspective - LT Justin “Toto” Davis, USN

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).

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?

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 inflight 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.

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.

A Breakdown of the “Strike-Chain”

So, how is it that in such a vast, three-dimensional space, birds and aircraft continue to smack into each other? After all, you never see birds, bats, and insects taking each other out unintentionally in flight. While there are many variables at play, the root of the problem goes back to the biological limitations by which birds are constrained when trying to share the skies with our machines. From our perspective it seems simple. We see the bird (hopefully from a distance) and then maneuver away. However, what does the bird see? How does it perceive an aircraft? How does it try to maneuver away from an approaching helicopter (if it sees us at all)? By studying the mechanics of a bird strike from the bird’s perspective we might be better equipped to reduce the frequency and severity of strikes by adjusting our in-flight behavior to minimize some of these biological limitations that constrain these birds. In other words, by gaining an understanding of how birds detect, perceive, and react to our presence, we may be better equipped to interrupt the steps which can lead to a bird strike. To help the tacticians among us, these steps can be called the “StrikeChain” and are summarized by the acronym DAM: Detect, Assess, Maneuver.

1. Detect. In the first step, the bird must detect the aircraft by either sight or sound. In general, the earlier the detection occurs, the better. Visual detection will vary based on the bird’s visual acuity (eye size), aircraft speed, time of day, etc. Sound detection will vary based on both wind and aircraft flight parameters (velocity, altitude, etc.). As previously stated, birds’ visual and auditory systems are not adapted to detect the speeds and sounds by which modern aircraft operate (Lima et al. 62). With that in mind, it’s likely that, in many cases, the unfortunate birds were removed from the gene pool long before they even knew an aircraft was there. Fortunately, in terms of breaking the “Strike-Chain,” our lights, engines, and rotor blades do most of the early warning work for us passively. Unfortunately for us, however, this seemingly positive effect also carries with it some grave side-effects as we’ll see below.

2. Assess. The bird must assess the aircraft as a threat once it has been detected. This threat assessment is usually predicated on whether or not a collision is imminent. However, a few problems exist here for the bird. The chief issue is that helicopters, with their size and shape, do not resemble any existing natural predators a bird might have. Hence, they’re likely not able to draw an immediate relationship between the unrecognized threat of, say, an approaching Knighthawk helicopter versus the natural threat of an approaching Redtailed hawk, at least until a collision is unavoidable. What is even more troubling however, is that given the sheer volume of aircraft and birds sharing the skies together, birds have simply become habituated to the presence of aircraft. This means that birds likely perceive aircraft to be benign obstacles rather than recognizing them to be the certifiable threats to their existence that they are. This desensitization to aircraft ultimately causes birds to allow for dangerously close approaches with aircraft, so that by the time an imminent collision is recognized, evasive maneuvering is no longer effective (Lima et al. 68). In terms of breaking the “Strike-Chain” for this step, there is little we can do. Unless we wage active war on birds with helicopters, they will continue to perceive us as a non-threat, and will thus continue to pose a BASH risk. That said, it should be noted that there have been reports of eagles and ospreys aggressively approaching small planes and helicopters (Lima et al. 69). While this is a rare behavior and likely prompted by defense of a nearby nest it is still an aggressive act and should not be taken lightly.

3. Maneuver. If indeed the bird assesses an aircraft as a threat early enough, it may initiate the aforementioned evasive maneuvers. As the third step in the “Strike-Chain,” this is also where we as pilots and aircrewmen have the most control. While little research has been conducted for this part of the “Strike-Chain,” there have been some preliminary studies and anecdotal evidence from pilots from which we can glean some insights. These can be categorized into two general modes of evasion: ground and airborne.

Ground evasion carries a high degree of risk, particularly for us low-flying helicopters. In this situation, after groundbased birds detect an aircraft they then take off in an attempt to evade the unknown threat. This may cause them to actually fly directly into the path of the approaching aircraft they were trying to avoid in the first place. Highly maneuverable birds with short takeoff distance requirements, are more likely to get airborne quickly and thus create a significant risk to helicopters if the flock is large enough. Large-bodied birds, on the other hand, may be more likely to stay put due to the longer takeoff distance requirements needed to get airborne (Fernández-Juricic et al. 9). However, should a group of large-bodied birds take off and intercept the flight path of a low-flying helicopter, the results could be, and have been, incredibly traumatic. Little can be done to prevent a scenario such as this developing, outside of avoiding wildlife areas and areas of land close to bodies of water. A nigh impossible task for us Navy/Marine Corps rotorcraft.

Airborne evasion maneuvers are even more unpredictable due to variations in species, altitude, relative airspeed, and distance, to name just a few factors. But generally speaking, relative distance and altitude are perhaps the best variables to focus on when determining how a bird might react to your presence. At greater distances, a common tactic of some less maneuverable but faster flying species (geese, ducks, shorebirds, etc.) is to simply accelerate away from the aircraft with no change in flight direction, obviously a bit of a lost cause. Other long-range tactics involve changing the direction of flight by flying an S-type maneuver (the most common) or by conducting a 360-degree loop with the birds returning to the original heading after the aircraft has passed (Lima et al. 69). These long-range evasive maneuvers may even take place before the pilots and aircrewmen have noticed the birds were there.

As the proximity to the aircraft decreases to within 100 to 200 meters most birds will be forced to rely upon ingrained anti-predatory behaviors to avoid a collision (Lima et al. 69). These anti-predatory responses are normally used to evade attacks from more natural predators (hawks, eagles, etc.), but some birds will employ them as a last-ditch effort to avoid a fatal collision with an advancing helicopter. Hence, having a general understanding of them may help in our prediction of how a bird might react when in close quarters with your aircraft. In general, the altitude of you and bird from the ground will provide the biggest indicator of how a bird might maneuver in relation to you. These altitudes will naturally vary based on any number of variables but generally, at higher altitudes (not in the terrain flight environment) diving is likely the preferred method of avoidance for co-altitude birds. According to one study, at altitudes above 150’ AGL, birds were found to usually attempt to dive/descend to avoid a collision (Lima et al. 69). With this in mind, at higher altitudes a climbing turn away from the bird is probably the best course of action to avoid a strike, with an emphasis on the climb.

Lower altitudes (below 150’ AGL) are even more pernicious, however. One study observed a variety of responses, ranging from diving, turning, climbing, etc. (Lima et al. 69). Another study found that when flying directly towards an aircraft, birds usually attempted to climb to avoid a collision, but when flying away from an aircraft, birds usually attempted to descend (Fernández-Juricic et al. 2). Given this seemingly hopeless case in the terrain flight environment, your best bet is to keep your altitude stable while simultaneously decelerating and turning away from the bird. Attempting to climb or descend in this situation is not advisable, since the unpredictable maneuvering of birds at low altitudes may lead to you and the bird inadvertently climbing or descending in harmony, only to greet each other at a different altitude. These guidelines are, of course, predicated on whether or not you see the bird(s) in the first place

Conclusions

At the end of the day, our options are limited. Neither we, nor birds are going away, so we’ll always be forced to contend with one another in our shared skies. Changing how and when we fly to accommodate bird behavior is also likely not going to happen. However, what we can do is at least attempt to minimize the frequency and overall damage of bird strikes by recognizing some of the aforementioned limits birds have when flying in close quarters with an aircraft. Keeping in mind that while it is impossible to cover every variable that plays into a BASH event, hopefully now you have some idea of how a bird may react, thus allowing you to maneuver to avoid, or at the very least minimize the damage, of a bird strike. In parting, BASH reports are a crucial piece to this issue and continuously go underreported across all of Naval Aviation. With that said, the more reports submitted the more effectively biologists are able to identify, track, and correct problematic species and areas to keep us safe. So, with all this said, pay heed all who take flight and know that the threat of a bird strike is omnipresent. We’re outnumbered, out of our element and lack the home-field advantage. Besides, we haven’t even discussed insects, bats, or pterodactyls yet.

References

1.Lima, Steven L., Blackwell, Bradley F., DeVault, Travis L. and Fernández-Juricic, Esteban (2015). Animal reactions to oncoming vehicles: a conceptual review. Biological Review 90, 60-76.

2.Pfeiffer, Morgan B., Blackwell, Bradely F., DeVault, Travis. L. (2018). Quantification of avian hazards to military aircraft and implications for wildlife management. PLOS ONE 13, 11

3.Fernández-Juricic E, Brand, J, Blackwell, Bradely F., Seamans, Thomas W., DeVault, Travis L. (2018). Species With Greater Aerial Maneuverability Have Higher Frequency of Collisions With Aircaft: A Comparative Study. Frontiers in Ecology and Evolution 6, 17

4.Alexander, David, E. On The Wing: Insects, Pterosaurs, Birds, Bats and the Evolution of Animal Flight. Oxford University Press, 2015

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