Equilibrium and flying: Illusions of flight

Flying safely while in instrument conditions requires a solid understanding of the ordinary illusions that can be expected to occur. These are the illusions created by the vestibular system while it attempts to map the motion of the aircraft to some recognizable three-dimensional attitude. While several common illusions are regularly taught in ground schools, seldom are they expounded upon, nor are the more rare illusions typically mentioned. Although it is unlikely that some of these illusions can be adequately demonstrated outside of a three-dimensional environment and while in motion, a thorough discussion of them here may enhance your awareness and provide techniques to avoid them.

Probably one of the most familiar illusions is the sensation of "opposite turn after spinning." We demonstrate this illusion on the ground using a Barany chair (a smooth turning chair that makes it almost impossible to determine motion with the eyes shut). The sensation is created when a prolonged turn is made and then decelerated back to a stop with no visual reference to the actual motion. In the prolonged turn, the endolymphatic fluid in the inner ear will no longer experience acceleration, which originally indicated the direction of turn. However, the inertia of movement is still present so when the turn is decelerated, a sensation of turn (in the opposite direction) again occurs and, if in flight, the pilot may have a tendency to roll back into the right turn. A particularly dangerous situation develops if a spin should occur during instrument flight. During recovery, when attempting to stop the turn, not only is the illusion of opposite turn created, but a sensation of falling into the original direction of turn can occur. This will lead a pilot to believe that the aircraft has started spinning to the opposite direction when, in fact, it is in a straight downhill glide. The normal reaction will be to adjust controls to the to stop the spin. Of course the original direction spin will be quickly reestablished if this is done.

Another well-known illusion is referred to as "the leans." This illusion results primarily because our bodies are not able to adequately detect gradual changes in motion. While our instrument scan typically precludes experiencing this illusion, if attention is diverted from the control and performance instruments long enough, a small change in motion may take place that is not detected. The sequence of motion changes is important in determining if the leans will occur. For instance, if the aircraft gently takes on a five-degree bank while you are looking away, and then corrects itself before you look at the instruments, no illusion is likely to be experienced. Even if you look back to the instruments to find the bank error and then make a gentle correction, there is little chance for an illusion. However, if a gust of wind sharply banks the aircraft so that you know the motion has taken place, but the aircraft gently returns to wings level before you look back to the instruments, you will most likely experience the leans. Your sensation will be that the aircraft is still tilted in the direction it originally banked. You can accomplish the same effect by rolling into a bank slowly and then having a gust sharply return you to wings-level. Minimizing sharp changes of bank angle is key to avoiding the leans. In addition, if you have to fly in conditions where wings-level control is difficult to hold, try to maintain an undistracted scan of the instruments.

Closely related to the leans is the concept of undetected motion illusions. In the same way that our bodies do not perceive gentle changes in motion, they also do not perceive slight accelerations. With inattention to or mistrust of the instruments, a high rate of bank, climbing or descending can build up with little accompanying sensation of the change in motion.

Why do we make turns at standard rate when flying solely by instruments? Well, have you ever heard of the illusion called "underestimating the degree of bank?" Essentially, this illusion again deals with sub-threshold motion (motion our vestibular system can’t detect). This illusion occurs because we normally make turns by rolling into a bank at a rate that is below the threshold of detection. As a result, a sensation of not having rolled enough is created and the inattentive pilot may then bank too steeply. The same illusion occurs during rollout and creates a tendency to overcorrect while recovering from the turn. Solution? Simple! Rely on the instruments and don’t look away from the bank instruments until the desired bank has been accomplished.

So far we have not addressed the effect of moving the head around while changing the motion of the aircraft. As it turns out, just about any head movement at right angles to the plane of rotation (or movement) is bad news for the pilot. The Coriolis Reaction will rear its ugly head in these situations. I firmly believe that this is the illusion that most pilots experience when they say they had a momentary uncontrollable loss of equilibrium. This sensation can cause complete incapacitation while flying so it is important to keep you head still during changes in attitude. The reason it occurs is well understood. Acceleration is taking place in one plane of motion, while your head is in another. This means the body will sense motion that may not even be possible or is grossly distorted from what is actually occurring.

The sensation of climbing while turning or diving during recovery from a turn are both illusions created by a change in the position of gravity and resulting load factor (or g-force) that occurs during the turn. The body more closely associates a g-force loading with the climb manoeuvre as opposed to a turn. Therefore, as a turn is made with perceptible bank angle, perceptible g-force will occur and subsequently give the sensation of an accelerating pitch up attitude. As a result, expect to feel the need to descend when you feel this sensation, but please rely on the instruments and fly accordingly. The opposite sensation occurs during recovery from a turn since the g-force is being relieved as the wings are leveled.

The last illusion is that of the sensation of "opposite tilt when in skidding flight." When we skid or slip an aircraft, the forces acting on the aircraft (and on the pilot) create improperly compensated centrifugal force. This is the sensation of your "bottom" not feeling square in the chair. It’s the same sensation you experience when trying to round a corner in a car at too high of a speed (that would be a skidding sensation). The direction in which your body tilts in the aircraft (depending on whether you skid or slip) determines the direction in which a tilt is perceived and any subsequent attempt to fix the incorrect sensation will result in increasing either the skid or slip. The easy solution to these problems is to keep the ball centred when making turns.

If you can find a Barany chair, I highly suggest you try out some of these illusions with a trained aeromedical doctor or physiologist. I firmly believe that if you can identify how your body reacts to certain events that are expected to occur in flight, prior to getting airborne, that you should take that opportunity. Some individuals have peculiar reactions to certain motions, which may not fall in line with conventional physiological doctrine so it is imperative to experience the sensation under controlled conditions. Take what you learn and apply it to your daily flying activities. If you do, you’ll feel more confident about your flying abilities and that may make you an all-around safer pilot.