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Because human senses are adapted for use on the ground, navigating by sensory input alone during flight can be dangerous: sensory input does not always accurately reflect the movement of the aircraft, causing sensory illusions. These illusions can be extremely dangerous for pilots.
Fluid in the inner ear reacts only to rate of change, not a sustained change. For example, if you are a pilot and you initiate a banking left turn, your inner ear will detect the roll into the turn, but if you hold the turn constant, your inner ear will compensate and rather quickly, although inaccurately, sense that it has returned to level flight.
As a result, when you finally level the wings, that new change will cause your inner ear to produce signals that make you believe you're banking to the right. This is the crux of the problem you have when flying without instruments in low-visibility weather. Even the best pilots will quickly become disoriented if they attempt to fly without instruments when there are no outside visual references, because vision provides the predominant and coordinating sense that humans rely upon for stability. Perhaps the most treacherous thing under such conditions is that the signals the inner ear produces are incorrect though they may feel right.
These sensory illusions occur because flight is an unnatural environment; our senses are not capable of providing reliable signals that we can interpret and relate to our position in three dimensions without visual reference.
Illusions involving the semicircular and Somatogyral canals of the vestibular system occur primarily under conditions of unreliable or unavailable external visual references and result in false sensations of rotation. These include The Leans, the Graveyard Spin and Spiral, and the Coriolis Illusion.
This is the most common illusion during flight, and is caused by a sudden return to level flight following a gradual and prolonged turn that went unnoticed by the pilot. The reason a pilot can be unaware of such a gradual turn is that human exposure to a rotational acceleration of 2 degrees per second or lower is below the detection threshold of the semicircular canals. Leveling the wings after such a turn may cause an illusion that the aircraft is banking in the opposite direction. In response to such an illusion, a pilot may lean in the direction of the original turn in a corrective attempt to regain the perception of a correct vertical posture.
The graveyard spin is an illusion that can occur to a pilot who intentionally or unintentionally enters a spin. For example, a pilot who enters a spin to the left will initially have a sensation of spinning in the same direction. However, if the left spin continues the pilot will have the sensation that the spin is progressively decreasing. At this point, if the pilot applies right rudder to stop the left spin, the pilot will suddenly sense a spin in the opposite direction (to the right).
If the pilot believes that the airplane is spinning to the right, the response will be to apply left rudder to counteract the sensation of a right spin. However, by applying left rudder the pilot will unknowingly re-enter the original left spin. If the pilot cross checks the turn indicator, he/she would see the turn needle indicating a left turn while he/she senses a right turn. This creates a sensory conflict between what the pilot sees on the instruments and what the pilot feels. If the pilot believes the body sensations instead of trusting the instruments, the left spin will continue. If enough altitude is lost before this illusion is recognized and corrective action is taken, impact with terrain is inevitable.
The graveyard spiral is more common than the Graveyard Spin, and it is associated with a return to level flight following an intentional or unintentional prolonged bank turn. For example, a pilot who enters a banking turn to the left will initially have a sensation of a turn in the same direction. If the left turn continues (~20 seconds or more), the pilot will experience the sensation that the airplane is no longer turning to the left. At this point, if the pilot attempts to level the wings this action will produce a sensation that the airplane is turning and banking in the opposite direction (to the right). If the pilot believes the illusion of a right turn (which can be very compelling), he/she will re-enter the original left turn in an attempt to counteract the sensation of a right turn.
Unfortunately, while this is happening, the airplane is still turning to the left and losing altitude. Pulling the control yoke/stick and applying power while turning would not be a good idea because it would only make the left turn tighter. If the pilot fails to recognize the illusion and does not level the wings, the airplane will continue turning left and losing altitude until it impacts the ground.
This involves the simultaneous stimulation of two semicircular canals and is associated with a sudden tilting (forward or backwards) of the pilots head while the aircraft is turning. This can occur when you tilt your head down (to look at an approach chart or to write a note on your knee pad), or tilt it up (to look at an overhead instrument or switch) or tilt it sideways. This produces an almost unbearable sensation that the aircraft is rolling, pitching, and yawing all at the same time, which can be compared with the sensation of rolling down on a hillside. This illusion can make the pilot quickly become disoriented and lose control of the aircraft.
Somatogravic illusions are caused by linear accelerations. These illusions involving the utricle and the saccule of the vestibular system are most likely under conditions with unreliable or unavailable external visual references.
The inversion illusion involves a steep ascent (forward linear acceleration) in a high-performance aircraft, followed by a sudden return to level flight. When the pilot levels off, the aircraft speed is relatively higher. This combination of accelerations produces an illusion that the aircraft is in inverted flight. The pilots response to this illusion is to lower the nose of the aircraft.
The head-up illusion involves a sudden forward linear acceleration during level flight where the pilot perceives the illusion that the nose of the aircraft is pitching up. The pilot's response to this illusion would be to push the yoke or the stick forward to pitch the nose of the aircraft down. A night take-off from a well-lit airport into a totally dark sky (black hole) or a catapult take-off from an aircraft carrier can also lead to this illusion, and could result in a crash.
The head-down illusion involves a sudden linear deceleration (air braking, lowering flaps, decreasing engine power) during level flight where the pilot perceives the illusion that the nose of the aircraft is pitching down. The pilot's response to this illusion would be to pitch the nose of the aircraft up. If this illusion occurs during a low-speed final approach, the pilot could stall the aircraft.
Aerial Perspective IllusionsEdit
Visual illusions are familiar to most of us. As children, we learned that railroad tracks -contrary to what our eyes showed us- don't come to a point at the horizon. Even under conditions of good visibility, you can experience visual illusions.
This illusion may make you change (increase or decrease) the slope of your final approach. They are caused by runways with different widths, upsloping or downsloping runways, and upsloping or downsloping final approach terrain. Pilots learn to recognize a normal final approach by developing and recalling a mental image of the expected relationship between the length and the width of an average runway.
A final approach over a flat terrain with an upsloping runway may produce the visual illusion of a high-altitude final approach. If you believe this illusion, you may respond by pitching the aircraft nose down to decrease the altitude, which, if performed too close to the ground, may result in an accident.
A final approach over a flat terrain with a downsloping runway may produce the visual illusion of a low-altitude final approach. If you believe this illusion, you may respond by pitching the aircraft nose up to increase the altitude, which may result in a low-altitude stall or a missed approach.
A final approach over an upsloping terrain with a flat runway may produce the visual illusion of a high-altitude final approach. If you believe this illusion, you may respond by pitching the aircraft nose down to decrease the altitude, which, if performed too close to the ground, may result in an accident.
A final approach over a downsloping terrain with a flat runway may produce the visual illusion of a low-altitude final approach. If you believe this illusion, you may respond by pitching the aircraft nose up to increase the altitude, which may result in a low-altitude stall or a missed approach.
Narrow or long runwayEdit
A final approach to an unusually narrow runway or an unusually long runway may produce the visual illusion of a high-altitude final approach. If you believe this illusion, you may respond by pitching the aircraft nose down to decrease the altitude, which, if performed too close to the ground may result in an accident.
A final approach to an unusually wide runway may produce the visual illusion of a low-altitude final approach. If you believe this illusion, you may respond by pitching the aircraft nose up to increase the altitude, which may result in a low-altitude stall or a missed approach.
Black-Hole approach illusionEdit
A Black-Hole Approach Illusion can happen during a final approach at night (no stars or moonlight) over water or unlighted terrain to a lighted runway beyond which the horizon is not visible. In the example when peripheral visual cues are not available to help you orient yourself relative to the earth, you may have the illusion of being upright and may perceive the runway to be tilted left and upsloping. However, with the horizon visible you can easily orient yourself correctly using your central vision.
A particularly hazardous black-hole illusion involves approaching a runway under conditions with no lights before the runway and with city lights or rising terrain beyond the runway. These conditions may produce the visual illusion of a high-altitude final approach. If you believe this illusion, you may respond by lowering your approach slope.
The Autokinetic illusion gives you the impression that a stationary object is moving in front of the airplane's path; it is caused by staring at a fixed single point of light (ground light or a star) in a totally dark and featureless background. This illusion can cause a misperception that such a light is on a collision course with your aircraft.
False visual reference illusionsEdit
False Visual Reference Illusions may cause you to orient your aircraft in relation to a false horizon; these illusions are caused by flying over a banked cloud, night flying over featureless terrain with ground lights that are indistinguishable from a dark sky with stars, or night flying over a featureless terrain with a clearly defined pattern of ground lights and a dark, starless sky.
A common example is when you are stopped at a traffic light in your car and the car next to you edges forward. Your brain interprets this peripheral visual information as though you are moving backwards and makes you apply additional pressure to the brakes. A similar illusion can happen while taxiing an aircraft.