Spatial disorientation
From Biocrawler, the free encyclopedia.
During flight most of the senses are 'fooled' by centrifugal force, and indicate to the brain that 'down' is at the bottom of the cockpit no matter what the actual attitude of the aircraft. Only the inner ear and the visual sense provide data to the contrary. The inner ear contains rotational 'accelerometers,' known as the semicircular canals, which provide information to the lower brain on rotational accelerations in the pitch, roll and yaw axes. This system is imperfect, and errors develop in the brain's estimate of rate and direction of turn in each axis. Normally these errors are corrected using information from the visual sense, in particular an external visual horizon.
Once an aircraft enters conditions under which the pilot cannot see a distinct visual horizon, the drift in the inner ear continues uncorrected. Errors in the perceived rate of turn about any axis can build up at about 0.2 to 0.3 degrees per second per second. If the pilot is not trained for or is not proficient in the use of gyroscopic flight instruments these errors will build up to a point that control of the aircraft is lost, usually in a steep, diving turn known as a graveyard spiral. During the entire time leading up to and well into the maneuver the pilot remains unaware that he is turning, believing that he is maintaining straight flight.
The graveyard spiral usually terminates when (1) the g-forces on the aircraft build up to and exceed the structural strength of the airframe, resulting in catastrophic failure, or (2) the aircraft contacts the ground. In a 1954 study, the Air Safety Foundation found that out of 20 non-instrument-rated subject pilots, 19 of the 20 entered a graveyard spiral soon after entering simulated instrument conditions. The 20th pilot also lost control of his aircraft, but in another maneuver. The average time between onset of instrument conditions and loss of control was 178 seconds.
Spatial disorientation can also affect instrument-rated pilots in certain conditions. A powerful tumbling sensation (vertigo) can be set up if the pilot moves his head too much during instrument flight. This is called the Coriolis illusion. Pilots are also susceptible to spatial disorientation during night flight over featureless terrain.
This phenomenon was extensively reported in the press in 1999, after John F. Kennedy, Jr.'s plane went down during a night flight over water near Martha's Vineyard. Subsequent investigation indeed pointed to spatial disorientation as the likely cause.
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External links
Information from the following government documents is in the public domain.
- http://www.cami.jccbi.gov/AAM-400A/Brochures/spatialD2/spatialD2_home.htm
- http://www.cami.jccbi.gov/AAM-400A/Brochures/DISORIEN.HTM
- http://www.cami.jccbi.gov/AAM-400A/Brochures/SpatialD.htm
- http://www.cami.jccbi.gov/AAM-400A/Brochures/SpadialD_page2.htm
Spatial Orientation
Defines our natural ability to maintain our body orientation and/or posture in relation to the surrounding environment (physical space) at rest and during motion. Genetically speaking, humans are designed to maintain spatial orientation on the ground. The three-dimensional environment of flight is unfamiliar to the human body, creating sensory conflicts and illusions that make spatial orientation difficult, and sometimes impossible to achieve. Statistics show that between 5 to 10% of all general aviation accidents can be attributed to spatial disorientation, 90% of which are fatal. It is important to know the difference between spatial orientation and airsickness.
Spatial Orientation on the Ground
Good spatial orientation on the ground relies on the effective perception, integration, and interpretation of visual, vestibular (organs of equilibrium located in the inner ear), and proprioceptive (receptors located in the skin, muscles, tendons, and joints) sensory information. Changes in linear acceleration, angular acceleration, and gravity are detected by the vestibular system and the proprioceptive receptors, and then compared in the brain with visual information (Figure 1, at right).
Spatial Orientation in Flight
Spatial orientation in flight is difficult to achieve because numerous sensory stimuli (visual, vestibular, and proprioceptive) vary in magnitude, direction, and frequency. Any differences or discrepancies between visual, vestibular, and proprioceptive sensory inputs result in a sensory mismatch that can produce illusions and lead to spatial disorientation. Good spatial orientation relies on the effective perception, integration and interpretation of visual, vestibular (organs of equilibrium located in the inner ear) and proprioceptive (receptors located in the skin, muscles, tendons, and joints) sensory information.
Vestibular Aspects of Spatial Orientation
The inner ear contains the vestibular system, which is also known as the organ of equilibrium. About the size of an pencil eraser, the vestibular system contains two distinct structures: the semicircular canals, which detect changes in angular acceleration, and the otolith organs (the utricule and the saccule), which detect changes in linear acceleration and gravity. Both the semicircular canals and the otolith organs provide information to the brain regarding our body’s position and movement. A connection between the vestibular system and the eyes helps to maintain balance and keep the eyes focused on an object while the head is moving or while the body is rotating.
The inner ear
Most problems related to disorientation can be traced to the inner ear, a sensory organ about the size of an eraser on a pencil. It may well be the most well-protected organ in the human body, and for good reason. It's the key to our ability to balance when on the ground, or to remain oriented in space when we fly.
The inner ear is similar to a three-axis gyro. It detects movement in the roll, pitch, and yaw axes that pilots know so well. When the sensory outputs of the inner ear are integrated with appropriate visual references and spatial orientation cues from our bodies, there is little chance to experience disorientation.
Vision and the inner ear
The problem occurs when the outside visual input is obscured, and the seat-of-the-pants input is ambiguous. Then, you're down to just the output from the inner ear—and that's when trouble can start.
Fluid in the inner ear reacts only to rate of change, not a sustained change. For example, when 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.
Sensory illusions
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. That's because vision provides the predominant and coordinating sense we rely upon for stability.
Perhaps the most treacherous thing under such conditions is that the signals the inner ear produces—incorrect though they may be—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.
The Semicircular Canals
The semicircular canals are three half-circular, interconnected tubes located inside each ear that are the equivalent of three gyroscopes located in three planes perpendicular (at right angles) to each other. Each plane corresponds to the rolling, pitching, or yawing motions of an aircraft.Each canal is filled with a fluid called endolymph and contains a motion sensor with little hairs whose ends are embedded in a gelatinous structure called the cupula. The cupula and the hairs move as the fluid moves inside the canal in response to an angular acceleration.
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The movement of the hairs is similar to the movement of seaweed caused by ocean currents or that of wheat fields moved by wind gusts. When the head is still and the airplane is straight and level, the fluid in the canals does not move and the hairs stand straight up, indicating to the brain that there is no rotational acceleration (a turn). If you turn either your aircraft or your head, the canal moves with your head, but the fluid inside does not move because of its inertia. As the canal moves, the hairs inside also move with it and are bent in the opposite direction of the acceleration by the stationary fluid (A). This hair movement sends a signal to the brain to indicate that the head has turned. The problem starts when you continue turning your aircraft at a constant rate (as in a coordinated turn) for more than 20 seconds. In this kind of turn, the fluid inside the canal starts moving initially, then friction causes it to catch up with the walls of the rotating canal (B). When this happens, the hairs inside the canal will return to their straight up position, sending an erroneous signal to the brain that the turn has stopped–when, in fact, the turn continues. If you then start rolling out of the turn to go back to level flight, the fluid inside the canal will continue to move (because of its inertia), and the hairs will now move in the opposite direction (C), sending an erroneous signal to the brain indicating that you are turning in the opposite direction, when in fact, you are actually slowing down from the original turn.
Vestibular Illusions
(Somatogyral - Semicircular Canals)
Illusions involving the semicircular 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.
 | 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 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. |
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 [edit] The Graveyard Spiralis 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. |
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The Otolith Organs
Two otolith organs, the saccule and utricle, are located in each ear and are set at right angles to each other. The utricle detects changes in linear acceleration in the horizontal plane, while the saccule detects gravity changes in the vertical plane. However, the inertial forces resulting from linear accelerations cannot be distinguished from the force of gravity; therefore, gravity can also produce stimulation of the utricle and saccule. These organs are located at the base (vestibule) of the semicircular canals, and their structure consists of small sacs (maculas) covered by hair cell filaments that project into an overlying gelatinous membrane (cupula) tipped by tiny, chalk-like calcium stones called otoconia.
Change in Gravity
When the head is tilted, the weight of the otoconia of the saccule pulls the cupula, which in turn bends the hairs that send a signal to the brain indicating that the head has changed position. A similar response will occur during a vertical take-off in a helicopter or following the sudden opening of a parachute after a free fall.
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Change in Linear Acceleration
The inertial forces resulting from a forward linear acceleration (take-off, increased acceleration during level flight, vertical climb) produce a backward displacement of the otoconia of the utricle that pulls the cupula, which in turn bends the haircell filaments that send a signal to the brain, indicating that the head and body have suddenly been moved forward. Exposure to a backward linear acceleration, or to a forward linear decceleration has the opposite effect.
Vestibular Illusions
(Somatogravic - Utricle and Saccule) Illusions involving the utricle and the saccule of the vestibular system are most likely under conditions with unreliable or unavailable external visual references. These illusions include: the Inversion Illusion, Head-Up Illusion, and Head-Down Illusion.
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 [edit] The Head-Up Illusioninvolves 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 yolk 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.[edit] The Head-Down Illusioninvolves 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. |
The Proprioceptive Receptors
The proprioceptive receptors (proprioceptors) are special sensors located in the skin, muscles, tendons, and joints that play a very small role in maintaining spatial orientation in normal individuals. Proprioceptors do give some indication of posture by sensing the relative position of our body parts in relation to each other, and by sensing points of physical contact between body parts and the surrounding environment (floor, wall, seat, arm rest, etc.). For example, proprioceptors make it possible for you to know that you are seated while flying; however, they alone will not let you differentiate between flying straight and level and performing a coordinated turn.
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Vision and Spatial Orientation
Visual references provide the most important sensory information to maintain spatial orientation on the ground and during flight, especially when the body and/or the environment are in motion. Even birds, reputable flyers, are unable to maintain spatial orientation and fly safely when deprived of vision (due to clouds or fog). Only bats have developed the ability to fly without vision but have replaced their vision with auditory echolocation. So, it should not be any surprise to us that, when we fly under conditions of limited visibility, we have problems maintaining spatial orientation.
Central Vision
Central vision, also known as foveal vision is involved with the identification of objects and the perception of colors. During instrument flight rules (IFR) flights, central vision allows pilots to acquire information from the flight instruments that is processed by the brain to provide orientational information. During visual flight rules (VFR) flights, central vision allows pilots to acquire external information (monocular and binocular) to make judgments of distance, speed, and depth.
Peripheral Vision
Peripheral vision, also known as ambient vision, is involved with the perception of movement (self and surrounding environment) and provides peripheral reference cues to maintain spatial orientation. This capability enables orientation independent from central vision and that is why we can walk while reading. With peripheral vision, motion of the surrounding environment produces a perception of self-motion even if we are standing or sitting still.
Visual References
- Visual references that provide information about distance, speed, and depth of visualized objects include:
- Comparative size of known objects at different distances.
- Comparative form or shape of known objects at different distances.
- Relative velocity of images moving across the retina. Nearby objects are perceived as moving faster than distant objects.
- Interposition of known objects. One object placed in front of another is perceived as being closer to the observer.
- Varying texture or contrast of known objects at different distances. Object detail and contrast are lost with distance.
- Differences in illumination perspective of objects due to light and shadows.
- Differences in aerial perspective of visualized objects. More distant objects are seen as bluish and blurry.
The flight attitude of an airplane is generally determined by the pilot's visual reference to the natural horizon. When the natural horizon is obscured, attitude can sometimes be maintained by visual reference to the surface below. If neither horizon nor surface visual references exist, the airplane's attitude can only be determined by artificial means such as an attitude indicator or other flight instruments. Surface references or the natural horizon may at times become obscured by smoke, fog, smog, haze, dust, ice particles, or other phenomena, although visibility may be above VFR minimums. This is especially true at airports located adjacent to large bodies of water or sparsely populated areas, where few, if any, surface references are available. Lack of horizon or surface reference is common on over-water flights, at night, or in low visibility conditions.
Visual Illusions
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 including:
Aerial Perspective Illusions 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 (Figure 2, below—click for larger image).
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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 (Figure 3, below).
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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 (Figure 4, below).
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A final approach over an upsloping 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 (Figure 5, below).
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A final approach over a downsloping 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 (Figure 6, below).
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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 (Figure 7, below).
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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 (Figure 8, below).
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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 (Figure 9, below), 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.
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However, with the horizon visible (Figure 10, below), you can easily orient yourself correctly using your central vision.
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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 (Figure 11, below).
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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 (Figure 12, below; click on image for larger view).
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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 (Figure 13, below).
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Vection Illusion: 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.
"Seat of the pants" flying
Does "seat of the pants" flying work in IFR weather? Judge for yourself: Anyone sitting in an airplane that is making a coordinated turn, no matter how steep, will have little or no sensation of being tilted in the air unless the horizon is visible. Similarly, it is possible to gradually climb or descend without a noticeable change in pressure against the seat. In some airplanes, it is possible to execute a loop without pulling negative "G's," so that without visual reference, you could be upside down without being aware of it. That's because a gradual change in any direction of movement may not be strong enough to activate the fluid in the semicircular canals, so you may not realize that the aircraft is accelerating, decelerating, or banking.
How to Prevent Spatial Disorientation
The following are basic steps that should help prevent spatial disorientation:
| • Take the opportunity to experience spatial disorientation illusions in a Barany chair, a Vertigon, a GYRO, or a Virtual Reality Spatial Disorientation Demonstrator. |
| • Before flying with less than 3 miles visibility, obtain training and maintain proficiency in airplane control by reference to instruments. |
| • When flying at night or in reduced visibility, use the flight instruments. |
| • If intending to fly at night, maintain night-flight currency. Include cross-country and local operations at different airports. |
| • If only Visual Flight Rules-qualified, do not attempt visual flight when there is a possibility of getting trapped in deteriorating weather. |
| • If you experience a vestibular illusion during flight, trust your instruments and disregard your sensory perceptions. |
