Advanced Flight Technologies

Illusion Confusion

By Patrick Sherman | [email protected]

As seen in the January 2023 issue of Model Aviation.

STRAIGHT AWAY, I need to acknowledge the fact that the photos that accompany this article should be downright terrifying for any drone pilot or model airplane enthusiast—and with good reason. They depict small UAS flying dangerously close to crewed aircraft. As the television has been warning you for decades: Do not try this at home. These images were captured by the FAA under carefully controlled circumstances, in part to illustrate the very hazards that I will be discussing in this article.

Figure 1: In this image, is the drone above or below the crewed aircraft? By how much? Image courtesy of FAA.

Drones and model airplanes must always yield the right of way to other aircraft. The problem is that, given the limitations of human perception, it isn’t always clear what maneuver will actually result in increased separation between your UAS and a crewed aircraft in the immediate vicinity. To help us understand—and solve—this conundrum, we’ll begin by learning a little bit more about how we perceive the world around us.

Before we do that, however, take a moment to look at the three photos on these pages and ask yourself whether the drone is above or below the airplane in each of them. By how much? Should the drone pilot ascend or descend to create more separation from the other aircraft?

Depth Misperception

I’m writing about visual perception, using our standard-issue, Mark 1 human eyeballs. No doubt you have learned that because we have two of them, we are able to perceive the world around us in three dimensions. It seems like this would be an enormous help in determining the distance between two objects in the environment, but it turns out that we can easily be fooled.

This ability is commonly referred to as depth perception, and here’s how it works. When you look at an object in your immediate vicinity—say, a vase of flowers sitting on a nearby table—you go just a tiny little bit cross-eyed because both of your eyes are pointing at the same object. This creates a triangle defined by the sight line between each of your eyes and the object, as well as the fixed distance between your eyes themselves.

Your eyes are controlled by incredibly precise muscles that carefully position them to allow you to focus on objects such as that vase of flowers. As a side benefit, they also effectively detect the angle of each eye in the ocular cavity. As Pythagoras told us 2,500 years ago, if we know the length of one limb of a triangle, as well as two of its angles, we can determine the length of the other two limbs.

Of course, your ocular muscles have never heard of the Pythagorean theorem nor its corollaries that actually make this possible, but throughout the years, your brain has learned to associate a certain load on these muscles with the distance to objects in the environment.

With that stated, there is a problem, according to Adam Hendrickson, the lead investigator with the FAA’s New Entrants Section (AFS-410C). "Human depth perception really only works at very short ranges," according to Adam, when there is sufficient difference between the angle of your eyes for your brain to detect.

"At long ranges, which is what we’re often dealing with while operating drones, it’s all inferred by the brain. What that means is that we’re not making an actual measurement with our eyes; we’re deciding through relative motion, perspective, shadows, or some of these other mechanisms, which object is farther away and which one is closer. The limitation of the human vision system to perceive depth can create optical illusions at long range."

Figure 2: This image was captured by the FAA under carefully controlled conditions to test your ability to answer a critical question. Is the drone above or below the oncoming crewed aircraft? By how much? Image courtesy of FAA.

Figure 3: In this image, is the drone above or below the crewed aircraft? By how much? Image courtesy of FAA.

Down Is Up

"These optical illusions can be pretty powerful, and in many cases, they can tempt the remote pilot in command to actually maneuver their drone closer to an approaching crewed aircraft rather than away from it," Adam explained.

Our ability to determine the range to various objects, such as a drone and an oncoming airplane, is further degraded when we are making observations against a uniform background, such as the sky. Of course, the sky is precisely where drones and airplanes are most likely to be found, at least in the circumstances relevant to this discussion.

Specifically, absent of any clear knowledge of which object is closer and which one is further away, our brain is likely to substitute another variable, such as the visual angle, to judge the relative altitude of the two objects. If the drone appears to be above the airplane in our field of view, as it does in Figure 1, our brain assumes that the drone is at a higher altitude than the airplane.

Without meaningful depth perception, the brain relies on alternative cues, such as visual angle, to determine the relative altitude of objects. In the example illustrated here, the remote pilot perceives that the drone is at a higher altitude than the crewed aircraft, but the opposite is actually true.

"You can end up in a situation where it appears that the drone is above the manned aircraft, when it is actually below it," stated Adam. "So, there might be a temptation as the manned aircraft approaches the drone for the remote pilot in command to want to command the drone to fly higher to increase separation, but that would be the wrong approach."

The same situation can occur in a lateral direction as well, causing a drone that is actually east of an approaching crewed aircraft to appear to be west of that aircraft and, again, tempting the well-meaning remote pilot to steer toward the oncoming traffic. Ironically, this problem can be further compounded by the use of a visual observer, who is added to the air crew with the intention of improving the remote pilot’s situational awareness.

In the preflight briefing, the remote pilot in command should ensure that the visual observer clearly understands these types of illusions and that the two agree on clear and concise language to describe what each is seeing and what might actually be happening in the sky overhead.

To address this issue, Adam suggests that the best choice is the most conservative. "Often, the safest maneuver is simply to exit the airspace or go to a protected area. Command the drone to hover next to a tree or in some other location where it’s clearly out of the way. When you’re operating your drone at long ranges, that’s when these optical illusions are at their strongest."

Now take one last look at the three photos on the preceding pages with what you have learned and estimate the relative altitude of the drone and the airplane. Here are the results, according to the FAA:

In Figure 1, the drone is 100 feet below the approaching aircraft. In Figure 2, the drone is 200 feet below the approaching aircraft. In Figure 3, the drone is 100 feet below the approaching aircraft.

By Patrick Sherman | [email protected]