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

Fluid in the inner ear reacts only to rate of change, not a sustained change. For example, if a pilot initiates a banking left turn, the inner ear will detect the roll into the turn, but if the turn is held constant, the inner ear will compensate and rather quickly, although inaccurately, report to the brain that it has returned to level flight.

As a result, when the pilot finally levels the wings, that new change will cause the inner ear to produce signals that produce the perception of banking to the right. This is the crux of the problem experienced by pilots 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 external 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.

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

Main article: 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 2 degrees 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 correct vertical posture.

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

Main article: 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 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 hits the ground.

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. As children, we learned that railroad tracks — contrary to what our eyes might tell us — don't come to a point at the horizon. 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 his 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 he/she 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 pitch the aircraft's nose down to decrease the altitude, potentially resulting in dropping short of the runway at high speed.

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

Black-hole approach illusion

A black-hole approach illusion can happen during a final approach at night (no stars or moonlight) over water or unlit terrain to a lighted runway beyond which the horizon is not visible. If the pilot has no peripheral visual cues to be oriented relative to the earth, there may be the illusion of being upright and the runway itself to be tilted and sloping.

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 being too high on final approach, resulting in pitching the aircraft nose down to decrease the perceived approach angle.

Autokinetic illusion

The autokinetic 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. This illusion can cause a misperception that such a light is on a collision course with the aircraft.

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.

See also

• Pilot error
• Brownout
• Spatial disorientation
• Roger Peterson (pilot)
• John F. Kennedy, Jr.
• Bárány chair
• Kopp-Etchells effect

Examples

• Air India Flight 855, graveyard spiral accident
• Flash Airlines Flight 604, graveyard spiral accident (controversial)
• Air New Zealand Flight 901, false visual reference illusion accident
• Alitalia Flight 4128, black-hole approach illusion
• VASP Flight 168, black-hole approach illusion
• Adam Air Flight 574

References

• FAA Pilot Safety Brochures - Spatial Disorientation (pdf)
• FAA Pilot Safety Brochures - Spatial Disorientation - Visual Illusions (pdf)