Sensory illusions in aviation

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.

Vestibular system

The vestibular system consists of the otolith organs and the semicircular canals. Illusions in aviation are caused when the brain cannot reconcile the vestibular and visual inputs. The semicircular canals, of which there are three recognizing accelerations in pitch, yaw, and roll, are stimulated by angular accelerations; the otolith organs, the saccule and utricle, are stimulated by linear accelerations. Stimulation of the semicircular canals occurs when movement of the endolymph inside the canals causes movement of the crista ampullaris and the hair cells within them. Stimulation of the otolith organs occurs when gravitational forces or linear accelerations cause movement of the otolith membrane, the otoliths, and the hair cells of the macula.^[1]

Somatogyral illusions occur as a result of angular accelerations stimulating the semicircular canals. Somatogravic illusions, on the other hand, occur as a result of linear accelerations stimulating the otolith organs.^[2]

Vestibular/somatogyral illusions

Illusions involving the semicircular and somatogyral canals of the vestibular system of the ear 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.

The leans

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 1 degree per second squared 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 level attitude.

Graveyard spin

The graveyard spin is an illusion that can occur to a pilot who 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 would see the turn needle indicating a left turn while he 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 not taken, impact with terrain will occur.

Graveyard spiral

The graveyard spiral is more common than the graveyard spin, and it is associated with a return to level flight following a 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 (for more than about 20 seconds), 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 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 would simply tighten the turn. 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 hits the ground (example picture).

Coriolis illusion

This involves the simultaneous stimulation of two semicircular canals and is associated with a sudden tilting (forward or backwards) of the pilot's head while the aircraft is turning. This can occur when tilting the head down (to look at an approach chart or to write on the knee pad), or up (to look at an overhead instrument or switch) or sideways. This can produce an overpowering sensation that the aircraft is rolling, pitching, and yawing all at the same time, which can be compared with the sensation of rolling down a hillside. This illusion can make the pilot quickly become disoriented and lose control of the aircraft.

Vestibular/somatogravic illusions

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.

Inversion illusion

An abrupt change from climb to straight-and-level flight can stimulate the otolith organs enough to create the illusion of tumbling backwards, or inversion illusion. The disoriented pilot may push the aircraft abruptly into a nose-low attitude, possibly intensifying this illusion.

Head-up illusion

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.

Head-down illusion

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.

Visual illusions

Visual illusions are familiar to most of us. Even under conditions of good visibility, one can experience visual illusions.

Linear perspective illusions

This illusion may make a pilot change (increase or decrease) the slope of their 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. An example would be a pilot used to small general aviation fields visiting a large international airport. The much wider runway would give the pilot the mental picture of the point where they would usually begin the flare, when they are much higher than they should be. A pilot flying an aircraft where the cockpit height relative to the ground is vastly higher or lower than they are used to can cause a similar illusion in the last part of the approach.

Upsloping terrain or narrow or long runway

A final approach over an upsloping terrain with a flat runway, or to an unusually narrow or long runway may produce the visual illusion of being too high on final approach. The pilot may then increase their rate of descent, positioning the aircraft unusually low on the approach path.

Downsloping terrain or wide runway

A final approach over a downsloping terrain with a flat runway, or to an unusually wide runway may produce the visual illusion of being too low on final approach. The pilot may then pitch the aircraft's nose up to increase the altitude, which can result in a low-altitude stall or a missed approach.

Other visual illusions in aviation

Black-hole approach illusion

A black-hole approach illusion can happen during a final approach at night (with no stars or moonlight) over water or unlit terrain to a lighted runway, in which the horizon is not visible.^[3] As the name suggests, it involves an approach to landing during the night where there is nothing to see between the aircraft and the intended runway, there is just a visual, “black-hole”.^[4] Pilots too often confidently proceed with a visual approach instead of relying on instruments during nighttime landings. As a result, this can lead to the pilot experiencing glide path overestimation (GPO) because of the lack of peripheral visual cues, especially, below the aircraft.^[5] In addition, with no peripheral visual cues allowing for an orientation relative to the earth there can be an illusion of the pilot being upright and the runway being tilted and sloping. As a result, they initiate an aggressive descent and wrongly adjust to an unsafe glide path below the desired three-degree glide path.

Autokinetic illusion

The autokinetic illusion occurs at night or in conditions with poor visual cues. This illusion gives the pilot 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. The reason why this visual illusion occurs is because of very small movements of the eyes. In conditions with poor visual cues accompanied by a single source of light, these eye movements are interpreted by the brain as movement of the object being viewed.^[3] This illusion can cause a misperception that such a light is on a collision course with the aircraft.

Planets or stars in the night sky can often cause the illusion to occur. Often these bright stars or planets have been mistaken for landing lights of oncoming aircraft, satellites, or even UFOs. An example of a star that commonly causes this illusion is Sirius, which is the brightest star in the northern hemisphere and in winter appears over the entire continental United States at one to three fists widths above the horizon. At dusk, the planet Venus can cause this illusion to occur and many pilots have mistaken it as lights coming from other aircraft ^[6]

False visual reference illusions

False visual reference illusions may cause the pilot to orient the aircraft in relation to a false horizon; these illusions can be 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.

Vection illusion

This is when the brain perceives peripheral motion, without sufficient other cues, as applying to itself. Consider the example of being in a car in lanes of traffic, when cars in the adjacent lane start creeping slowly forward. This can produce the perception of actually moving backwards, particularly if the wheels of the other cars are not visible. A similar illusion can happen while taxiing an aircraft.

Repeating pattern illusion

This is when an aircraft is moving at very low altitude over a surface that has a regular repeating pattern, for example ripples on water. The pilot's eyes can misinterpret the altitude if each eye lines up different parts of the pattern rather than both eyes lining up on the same part. This leads to a large error in altitude perception, and any descent can result in impact with the surface. This illusion is of particular danger to helicopter pilots operating at a few metres altitude over calm water.

References

^ Saladin, Kenneth (2012). Anatomy & Physiology, The Unity of Form and Function. New York, NY: McGraw-hill. pp. 605–8. ISBN 978-0-07-337825-1.
^ Woodrow, Andrew; Webb, James (2011). Handbook of Aerospace and Operational Physiology. Air Force Research Library. pp. 7–37, 7–42.
^ ^a ^b Antunano, M. J. (2013). FAA Pilot Safety Brochures: Spatial Disorientation. Federal Aviation Administration.
^ Newman, D. G. (2007). An overview of spatial disorientation as a factor in aviation accidents and incidents (No. B2007/0063). Australian Transport Safety Bureau.
^ Gibb, R. W. (2007). Visual spatial disorientation: revisiting the black hole illusion. Aviation, space, and environmental medicine, 78(8), 801-808.
^ Rossier, R. N. (2004). The Lessons We Forget-Distraction, disorientation and illusions. Business and Commercial Aviation, 95(3), 50-55.
^ http://www.itv.com/news/anglia/update/2015-10-08/co-pilot-was-not-very-happy-about-taking-off-in-fog-ahead-of-fatal-helicopter-crash/^{[dead link]}

[1] Saladin, Kenneth (2012). Anatomy & Physiology, The Unity of Form and Function. New York, NY: McGraw-hill. pp. 605–8. ISBN 978-0-07-337825-1.

[2] Woodrow, Andrew; Webb, James (2011). Handbook of Aerospace and Operational Physiology. Air Force Research Library. pp. 7–37, 7–42.

[auto-3] Antunano, M. J. (2013). FAA Pilot Safety Brochures: Spatial Disorientation. Federal Aviation Administration.

[4] Newman, D. G. (2007). An overview of spatial disorientation as a factor in aviation accidents and incidents (No. B2007/0063). Australian Transport Safety Bureau.

[5] Gibb, R. W. (2007). Visual spatial disorientation: revisiting the black hole illusion. Aviation, space, and environmental medicine, 78(8), 801-808.

[6] Rossier, R. N. (2004). The Lessons We Forget-Distraction, disorientation and illusions. Business and Commercial Aviation, 95(3), 50-55.

[7] ttp://www.itv.com/news/anglia/update/2015-10-08/co-pilot-was-not-very-happy-about-taking-off-in-fog-ahead-of-fatal-helicopter-crash/^{[dead link]}

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