Because human senses are adapted for use on the ground, navigating by sensory input alone during flight

Aviation

Aviation is the design, development, production, operation, and use of aircraft, especially heavier-than-air aircraft. Aviation is derived from avis, the Latin word for bird.-History:...

 can be dangerous: sensory input does not always accurately reflect the movement of the aircraft

Aircraft

An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...

, causing sensory illusions. These illusions can be extremely dangerous for pilots

Aviator

An aviator is a person who flies an aircraft. The first recorded use of the term was in 1887, as a variation of 'aviation', from the Latin avis , coined in 1863 by G. de la Landelle in Aviation Ou Navigation Aérienne...

.

Vestibular system

Fluid in the inner ear

Inner ear

The inner ear is the innermost part of the vertebrate ear. In mammals, it consists of the bony labyrinth, a hollow cavity in the temporal bone of the skull with a system of passages comprising two main functional parts:...

 reacts only to rate of change

Rate of change

Rate of change may refer to:* Derivative, rate of change in a mathematical function* Difference quotient, the difference between two output values divided by the difference between the corresponding input values...

, 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

Vestibular system

The vestibular system, which contributes to balance in most mammals and to the sense of spatial orientation, is the sensory system that provides the leading contribution about movement and sense of balance. Together with the cochlea, a part of the auditory system, it constitutes the labyrinth of...

 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

Graveyard spiral

In aviation, a graveyard spiral is a dangerous spiral dive entered into accidentally by a pilot who is not trained or not proficient in instrument flight when flying in instrument meteorological conditions...

, 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 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

Rudder

A rudder is a device used to steer a ship, boat, submarine, hovercraft, aircraft or other conveyance that moves through a medium . On an aircraft the rudder is used primarily to counter adverse yaw and p-factor and is not the primary control used to turn the airplane...

 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 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

Saccule

The saccule is a bed of sensory cells situated in the inner ear. The saccule translates head movements into neural impulses which the brain can interpret. The saccule is sensitive to linear translations of the head, specifically movements up and down...

 of the vestibular system

Vestibular system

The vestibular system, which contributes to balance in most mammals and to the sense of spatial orientation, is the sensory system that provides the leading contribution about movement and sense of balance. Together with the cochlea, a part of the auditory system, it constitutes the labyrinth of...

 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

Yoke (aircraft)

A yoke, alternatively known as control column, is a device used for piloting in most fixed-wing aircraft.- Principle :The aviator uses the yoke to control the attitude of the plane, usually in both pitch and roll. Rotating the control wheel controls the ailerons and the roll axis...

 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

Aircraft catapult

An aircraft catapult is a device used to launch aircraft from ships—in particular aircraft carriers—as a form of assisted take off. It consists of a track built into the flight deck, below which is a large piston or shuttle that is attached through the track to the nose gear of the aircraft, or in...

 take-off from an aircraft carrier

Aircraft carrier

An aircraft carrier is a warship designed with a primary mission of deploying and recovering aircraft, acting as a seagoing airbase. Aircraft carriers thus allow a naval force to project air power worldwide without having to depend on local bases for staging aircraft operations...

 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

Air brake (aircraft)

In aeronautics, air brakes or speedbrakes are a type of flight control surface used on an aircraft to increase drag or increase the angle of approach during landing....

, lowering flaps

Flap (aircraft)

Flaps are normally hinged surfaces mounted on the trailing edges of the wings of a fixed-wing aircraft to reduce the speed an aircraft can be safely flown at and to increase the angle of descent for landing without increasing air speed. They shorten takeoff and landing distances as well as...

, 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

Stall (flight)

In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded...

 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.

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

Autokinetic effect

The autokinetic effect is a phenomenon of human visual perception in which a stationary, small point of light in an otherwise dark or featureless environment appears to move. It was first recorded by a Russian officer keeping watch who observed illusory movement of a star near the horizon...

 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

Collision course

A collision course, also known as a kamikaze run, is the deliberate maneuver by the operator of a moving object to collide with another object...

 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
  Pilot error
  Pilot error is a term used to describe the cause of an accident involving an airworthy aircraft where the pilot is considered to be principally or partially responsible...
  
  
• Brownout
  Brownout (aviation)
  In aviation, a brownout is an in-flight visibility restriction due to dust or sand in the air.In a brownout, the pilot cannot see nearby objects which provide the outside visual references necessary to control the aircraft near the ground. This can cause spatial disorientation and loss of...
  
  
• Spatial disorientation
  Spatial disorientation
  Spatial disorientation is the inability to correctly interpret aircraft attitude, altitude or airspeed, in relation to the Earth or point of reference. Spatial disorientation is a condition in which an aircraft pilot's perception of direction does not agree with reality...
  
  
• John F. Kennedy, Jr.
• Bárány chair
  Barany chair
  The Barany chair or Bárány chair, named for the Austro-Hungarian physiologist Robert Bárány, is a device used for aerospace physiology training, particularly for student pilots. The subject is placed in the chair, blindfolded, then spun about the vertical axis while keeping his head upright or...
  
  
• Kopp-Etchells effect

Examples

• Air India Flight 855
  Air India Flight 855
  Air India Flight 855 was a scheduled passenger flight that crashed during the evening of 1 January 1978 about off the coast of Bandra, Bombay , India. All 213 lives on board were lost...
  
  , graveyard spiral accident
• Flash Airlines Flight 604
  Flash Airlines flight 604
  Flash Airlines Flight 604 was a charter flight operated by Egyptian charter company Flash Airlines. On 3 January 2004, the Boeing 737-300 crashed into the Red Sea shortly after takeoff from Sharm el-Sheikh International Airport, killing all 142 passengers, many of them French tourists, and all six...
  
  , graveyard spiral accident (controversial)
• Air New Zealand Flight 901
  Air New Zealand Flight 901
  Air New Zealand Flight 901 was a scheduled Air New Zealand Antarctic sightseeing flight that operated between 1977 and 1979, from Auckland Airport to Antarctica and return via Christchurch...
  
  , false visual reference illusion accident
• Alitalia Flight 4128
  Alitalia Flight 4128
  Alitalia Flight 4128 was a scheduled flight from Leonardo da Vinci Airport, in Rome, Italy, to Palermo International Airport in Palermo, Italy, with 129 on board. On December 23, 1978, it crashed into the Tyrrhenian Sea about 3 km north of Palermo while on approach...
  
  , black-hole approach illusion
• VASP Flight 168
  VASP Flight 168
  VASP Flight 168, a Boeing 727-212A, serial number 21347, registered PP-SRK, was a scheduled passenger flight from Rio de Janeiro, Brazil to Fortaleza on June 8, 1982, which crashed into terrain while descending into Fortaleza, killing all 137 people on board....
  
  , black-hole approach illusion
• Adam Air Flight 574
  Adam Air Flight 574
  Adam Air Flight 574 was a scheduled domestic passenger flight operated by Adam Air between the Indonesian cities of Surabaya and Manado that disappeared near Polewali in Sulawesi on 1 January 2007. The plane, a Boeing 737-4Q8, was ultimately determined to have crashed into the ocean, from...