9-1. Spatial disorientation is an individual’s inability to determine his or her position, attitude, and motion relative to the surface of the earth or significant objects; for example, trees, poles, or buildings during hover. When it occurs, pilots are unable to see, believe, interpret, or prove the information derived from their flight instruments. Instead, they rely on the false information that their senses provide.

9-2. A sensory illusion is a false perception of reality caused by the conflict of orientation information from one or more mechanisms of equilibrium. Sensory illusions are a major cause of spatial disorientation.

9-3. Vertigo is a spinning sensation usually caused by a peripheral vestibular abnormality in the middle ear. Aircrew members often misuse the term vertigo, applying it generically to all forms of spatial disorientation or dizziness.

9-4. A disoriented aviator does not perceive any indication of spatial disorientation. In other words, he does not think anything is wrong. What he sees—or thinks he sees—is corroborated by his other senses. Type I disorientation is the most dangerous type of disorientation. The pilot—unaware of a problem—fails to recognize or correct the disorientation, usually resulting in a fatal aircraft mishap:

• The pilot may see the instruments functioning properly. There is no suspicion of an instrument malfunction.
• There may be no indication of aircraft-control malfunction. The aircraft is performing normally.
• An example of this type of SD would be the height-/depth-perception illusion when the pilot descends into the ground or some obstacle above the ground because of a lack of situational awareness.

9-5. In Type II spatial disorientation, the pilot perceives a problem (resulting from spatial disorientation). The pilot, however, may fail to recognize it as spatial disorientation:

• The pilot may feel that a control is malfunctioning.
• The pilot may perceive an instrument failure as in the graveyard spiral, a classic example of Type II disorientation. The pilot does not correct the aircraft roll, as indicated by the attitude indicator, because his vestibular indications of straight-and-level flight are so strong.

9-6. In Type III spatial disorientation, the pilot experiences such an overwhelming sensation of movement that he or she cannot orient himself or herself by using visual cues or the aircraft instruments. Type III spatial disorientation is not fatal if the copilot can gain control of the aircraft.

9-7. Three sensory systems—the visual, vestibular, and proprioceptive systems—are especially important in maintaining equilibrium and balance. Figure 9-1 shows these systems. Normally, the combined functioning of these senses maintains equilibrium and prevents spatial disorientation. During flight, the visual system is the most reliable. In the absence of the visual system, the vestibular and proprioceptive systems are unreliable in flight.

9-8. Of the three sensory systems, the visual system is the most important in maintaining equilibrium and orientation. To some extent, the eyes can help determine the speed and direction of flight by comparing the position of the aircraft relative to some fixed point of reference. Eighty percent of our orientation information comes from the visual system. (Chapter 8 contains information about the eye).

9-9. On flights under IMC, crew members lose fixed points of reference outside of the aircraft. Under IMC, the pilot must rely on visual sensory input from the instruments for spatial orientation. The decision to rely on the visual sense—and to believe the instruments rather than the input of the other senses—demands disciplined training.

9-10. The eyes allow the pilot to scan sensitive flight instruments that give accurate spatial-orientation information. These instruments indicate unusual aircraft attitudes resulting from turbulence, distraction, inattention, mechanical failure, or spatial disorientation.

9-11. The inner ear contains the vestibular system, which contains the motion- and gravity-detecting sense organs. This system is located in the temporal bone on each side of the head. Each vestibular apparatus consists of two distinct structures: the semicircular canals and the vestibule proper, which contain the otolith organs. Figure 9-2 depicts the vestibular system. Both the semicircular canals and the otolith organs sense changes in aircraft attitude. The semicircular canals of the inner ear sense changes in angular acceleration and deceleration.

9-12. The otolith organs are small sacs located in the vestibule. Sensory hairs project from each macula into the otolithic membrane, an overlaying gelatinous membrane that contains chalklike crystals, called otoliths. The otolith organs, shown in Figure 9-3, respond to gravity and linear accelerations/decelerations. Changes in the position of the head, relative to the gravitational force, cause the otolithic membrane to shift position on the macula. The sensory hairs bend, signaling a change in the head position.

9-13. When the head is upright, a "resting" frequency of nerve impulses is generated by the hair cells. Figure 9-4 shows the position of the hair cells when the head is upright.

9-14. When the head is tilted, the "resting" frequency is altered. The brain is informed of the new position. The positions of the hair cells when the head is tilted forward and backward are shown in Figure 9-5.

9-15. Linear accelerations/decelerations also stimulate the otolith organs. The body cannot physically distinguish between the inertial forces resulting from linear accelerations and the force of gravity. A forward acceleration results in backward displacement of the otolithic membranes. When an adequate visual reference is not available, aircrew members may experience an illusion of backward tilt. Figure 9-6 shows this false sensation of backward tilt.

9-16. The semicircular canals of the inner ear sense changes in angular acceleration. The canals will react to any changes in roll, pitch, or yaw attitude. Figure 9-7 shows where these changes are registered in the semicircular canals.

9-17. The semicircular canals are situated in three planes, perpendicular to each other. They are filled with a fluid called endolymph. The inertial torque resulting from angular acceleration in the plane of the canal puts this fluid into motion. The motion of the fluid bends the cupula, a gelatinous structure located in the ampulla of the canal. This, in turn, moves the hairs of the hair cells situated beneath the cupula. This movement stimulates the vestibular nerve. These nerve impulses are then transmitted to the brain, where they are interpreted as rotation of the head. Figure 9-8 shows a cutaway section of the semicircular canal.

9-18. When no acceleration takes place, the hair cells are upright. The body senses that no turn has occurred. The position of the hair cells and the actual sensation correspond, as shown in Figure 9-9.

9-19. When a semicircular canal is put into motion during clockwise acceleration, the fluid within the semicircular canal lags behind the accelerated canal walls. This lag creates a relative counterclockwise movement of the fluid within the canal. The canal wall and the cupula move in the opposite direction from the motion of the fluid. The brain interprets the movement of the hairs to be a turn in the same direction as the canal wall. The body correctly senses that a clockwise turn is being made. Figure 9-10 shows the position of the hair cells and the resulting true sensation during a clockwise turn.

9-20. If the clockwise turn then continues at a constant rate for several seconds or longer, the motion of the fluid in the canals catches up with the canal walls. The hairs are no longer bent, and the brain receives the false impression that turning has stopped. The position of the hair cells and the resulting false sensation during a prolonged, constant clockwise turn is shown in Figure 9-11. A prolonged constant turn in either direction will result in the false sensation of no turn.

9-21. When the clockwise rotation of the aircraft slows or stops, the fluid in the canal moves briefly in a clockwise direction. This sends a signal to the brain that is falsely interpreted as body movement in the opposite direction. In an attempt to correct the falsely perceived counterclockwise turn, the pilot may turn the aircraft in the original clockwise direction. Figure 9-12 shows the position of the hair cells—and the resulting false sensation when a clockwise turn is suddenly slowed or stopped.

9-22. This system reacts to the sensation resulting from pressures on joints, muscles, and skin and from slight changes in the position of internal organs. It is closely associated with the vestibular system and, to a lesser degree, the visual system. Forces act upon the seated pilot in flight. With training and experience, the pilot can easily distinguish the most distinct movements of the aircraft by the pressures of the aircraft seat against the body. The recognition of these movements has led to the term "seat-of-the-pants" flying.

9-23. Illusions give false impressions or misconceptions of actual conditions; therefore, aircrew members must understand the type of illusions that can occur and the resulting disorientation. Although the visual system is the most reliable of the senses, some illusions can result from misinterpreting what is seen; what is perceived is not always accurate. Even with the references outside the cockpit and the display of instruments inside, aircrew members must be on guard to interpret information correctly.

9-24. Relative motion is the falsely perceived self-motion in relation to the motion of another object. The most common example is when an individual in a car is stopped at a traffic light and another car pulls alongside. The individual that was stopped at the light perceives the forward motion of the second car as his own motion rearward. This results in the individual applying more pressure to the brakes unnecessarily. This illusion can be encountered during flight in situations such as formation flight, hover taxi, or hovering over water or tall grass.

9-25. Confusion with ground lights occurs when an aviator mistakes ground lights for stars. This illusion prompts the aviator to place the aircraft in an unusual attitude to keep the misperceived ground lights above them. Isolated ground lights can appear as stars and this could lead to the illusion that the aircraft is in a nose high or one wing low attitude (Part A of Figure 9-13). When no stars are visible because of overcast conditions, unlighted areas of terrain can blend with the dark overcast to create the illusion that the unlighted terrain is part of the sky (Part B of Figure 9-13). This illusion can be avoided by referencing the flight instruments and establishing a true horizon and attitude.

The false horizon illusion (Figure 9-14) occurs when the aviator confuses cloud formations with the horizon or the ground. This illusion occurs when an aviator subconsciously chooses the only reference point available for orientation. A sloping cloud deck may be difficult to perceive as anything but horizontal if it extends for any great distance in the pilot’s peripheral vision. An aviator may perceive the cloudbank below to be horizontal although it may not be horizontal to the ground; thus, the pilot may fly the aircraft in a banked attitude. This condition is often insidious and goes undetected until the aviator recognizes it and makes the transition to the instruments and corrects it. This illusion can also occur if an aviator looks outside after having given prolonged attention to a task inside the cockpit. The confusion may result in the aviator placing the aircraft parallel to the cloudbank.

2-27. The height-depth perception illusion is due to a lack of sufficient visual cues and causes an aircrew member to lose depth perception. Flying over an area devoid of visual references—such as desert, snow, or water—will deprive the aircrew member of his perception of height. The aviator, misjudging the aircraft’s true altitude, may fly the aircraft dangerously low in reference to the ground or other obstacles above the ground. Flight in an area where visibility is restricted by fog, smoke, or haze can produce the same illusion.

9-28. The crater illusion occurs when aircrew members land at night, under NVG conditions, and the IR searchlight is directed too far under the nose of the aircraft. This will cause the illusion of landing with up-sloping terrain in all directions. This misperceived up-sloping terrain will give the aviator the perception of landing into a crater. This illusionary depression lulls the pilot into continuing to lower the collective. This can result in the aircraft prematurely impacting the ground, causing damage to both aircraft and crew. If observing another aircraft during hover taxi, the aviator may perceive that the crater actually appears to move with the aircraft being observed.

9-29. Structural illusions are caused by the effects of heat waves, rain, snow, sleet, or other visual obscurants. A straight line may appear curved when it is viewed through the heat waves of the desert. A single wing-tip light may appear as a double light or in a different location when it is viewed during a rain shower. The curvature of the aircraft windscreen can also cause structural illusions, as illustrated in Figure 9-15. This illusion is due to the refraction of light rays as they pass through the windscreen. When encountering environments that contain these visual obscurants, the aviator must remain aware that these obscurants may present a false perception.

9-30. The size-distance illusion (Figure 9-16) is the false perception of distance from an object or the ground, created when a crew member misinterprets an unfamiliar object’s size to be the same as an object that he is accustomed to viewing. This illusion can occur if the visual cues, such as a runway or trees, are of a different size than expected. An aviator making an approach to a larger, wider runway may perceive that the aircraft is too low. Conversely, an aviator—making an approach to a smaller, narrower runway—may perceive that the aircraft is too high. A pilot making an approach 25 feet above the trees in the State of Washington, where the average tree is 100 feet tall, may fly the aircraft dangerously low if trying to make the same approach at Fort Rucker, Alabama, where the average tree height is 30 feet. This illusion may also occur when an individual is viewing the position lights of another aircraft at night. If the aircraft being observed suddenly flies into smoke or haze, the aircraft will appear to be farther away than before.

9-31. Fascination, or fixation, flying can be separated into two categories: task saturation and target fixation. Task saturation may occur during the accomplishment of simple tasks within the cockpit. Crew members may become so engrossed with a problem or task within the cockpit that they fail to properly scan outside the aircraft. Target fixation, commonly referred to as target hypnosis, occurs when an aircrew member ignores orientation cues and focuses his attention on his object or goal; for example, an attack pilot on a gunnery range becomes so intent on hitting the target that he forgets to fly the aircraft, resulting in the aircraft striking the ground, the target, or the shrapnel created by hitting the target.

9-32. At night, an aircraft may appear to be moving away when it is actually approaching. If the pilot of each aircraft has the same assumption, and the rate of closure is significant, by the time each pilot realizes the misassumption, it may be too late to avoid a mishap. This illusion is termed reversible perspective and is often experienced when an aircrew member observes an aircraft flying a parallel course. In this situation, aircrew coordination is paramount. To determine the direction of flight, the aircrew member should observe the other aircraft’s position lights. Remember the following: red on right returning; that is, if you see an aircraft with the red position light on the right and the green position light on the left, the observed aircraft is traveling in the opposite direction of your flight path.

9-33. In altered planes of reference (Figure 9-17), the pilot has an inaccurate sense of altitude, attitude, or flight-path position in relation to an object so great in size that the object becomes the new plane of reference rather than the correct plane of reference, the horizon. A pilot approaching a line of mountains may feel the need to climb although the altitude of the aircraft is adequate. This is because the horizon, which helps the pilot maintain orientation, is subconsciously moved to the top of the ridgeline. Without an adequate horizon, the brain attempts to fix a new horizon. Conversely, an aircraft entering a valley that contains a slowly increasing up-slope condition may become trapped because the slope may quickly increase and exceed the ability of the aircraft to climb above the hill, causing the aircraft to crash into the surrounding hills.

9-34. Autokinesis primarily occurs at night when ambient visual cues are minimal and a small, dim light is seen against a dark background. After about 6 to 12 seconds of visually fixating on the light, one perceives movement at up to 20 degrees in any particular direction or in several directions in succession, although there is no actual displacement of the object. This illusion may allow an aviator to mistake the object fixated as another aircraft. In addition, a pilot flying at night may perceive a relatively stable lead aircraft to be moving erratically, when in fact, it is not. The unnecessary and undesirable control inputs that the pilot makes to compensate for the illusory movement of the aircraft represent increased work and wasted motion, at best, and an operational hazard at worst.

9-35. Flicker vertigo (Figure 9-18) is technically not an illusion; however, as most people are aware from personal experience, viewing a flickering light can be both distracting and annoying. Flicker vertigo may be created by helicopter rotor blades or airplane propellers interrupting direct sunlight at a rate of 4 to 20 cycles per second. Flashing anticollision strobe lights, especially while the aircraft is in the clouds, can also produce this effect. One should also be aware that photic stimuli at certain frequencies could produce seizures in those rare individuals who are susceptible to flicker-induced epilepsy.

9-36. The vestibular system provides accurate information as long as an individual is on the ground. Once the individual is airborne, however, the system may function incorrectly and cause illusions. These illusions pose the greatest problem with spatial disorientation. Aircrew members must understand vestibular illusions and the conditions under which they occur. They must be able to distinguish between the inputs of the vestibular system that are accurate and those that cause illusion.

9-37. Somatogyral illusions are caused when angular accelerations and decelerations stimulate the semicircular canals. Those that may be encountered in flight are the leans, graveyard spin, and Coriolis illusions.

9-38. The most common form of spatial disorientation is the leans. This illusion occurs when the pilot fails to perceive angular motion. During continuous straight-and-level flight, the pilot will correctly perceive that he is straight and level (part A, Figure 9-19). However, a pilot rolling into or out of a bank may experience perceptions that disagree with the reading on the attitude indicator. In a slow roll, for instance, the pilot may fail to perceive that the aircraft is no longer vertical. He may feel that his aircraft is still flying straight and level although the attitude indicator shows that the aircraft is in a bank (part B, Figure 9-19). Once the pilot detects the slow roll, he makes a quick recovery. He rolls out of the bank and resumes straight-and-level flight. The pilot may now perceive that the aircraft is banking in the opposite direction. However, the attitude indicator shows the aircraft flying straight and level (part C, Figure 9-19). The pilot may then feel the need to turn the aircraft so that it aligns with the falsely perceived vertical position. Instead, the pilot should maintain straight-and-level flight as shown by the attitude indicator. To counter the falsely perceived vertical position, the pilot will lean his body in the original direction of the subthreshold roll until the false sensation leaves (part D, Figure 9-19).

9-39. This illusion, shown in Figure 9-20, usually occurs in fixed-wing aircraft. For example, a pilot enters a spin and remains in it for several seconds. The pilot’s semicircular canals reach equilibrium; no motion is perceived. Upon recovering from the spin, the pilot undergoes deceleration, which is sensed by the semicircular canals. The pilot has a strong sensation of being in a spin in the opposite direction even if the flight instruments contradict that perception. If deprived of external visual references, the pilot may disregard the instrumentation and make control corrections against the falsely perceived spin. The aircraft will then reenter a spin in the original direction.

9-40. To compound the action of the semicircular canals under these conditions, a pilot, noting a loss of altitude as the spin develops, may apply back pressure on the controls and add power in an attempt to gain altitude. This maneuver tightens the spin and may cause the pilot to lose control of the aircraft.

9-41. Regardless of the type of aircraft flown, the Coriolis illusion is the most dangerous of all vestibular illusions. It causes overwhelming disorientation.

9-42. This illusion occurs whenever a prolonged turn is initiated and the pilot makes a head motion in a different geometrical plane. When a pilot enters a turn and then remains in the turn, the semicircular canal corresponding to the yaw axis is equalized. The endolymph fluid no longer deviates, or bends, the cupula. Figure 9-21 shows the movement of the fluid in a semicircular canal when a pilot enters a turn.

9-43. If the pilot initiates a head movement in a geometrical plane other than that of the turn, the yaw axis semicircular canal is moved from the plane of rotation to a new plane of nonrotation. The fluid then slows in that canal, resulting in a sensation of a turn in the direction opposite that of the original turn.

9-44. Simultaneously, the two other canals are brought within a plane of rotation. The fluid stimulates the two other cupulas. The combined effect of the coupler deflection in all three canals creates the new perception of motion in three different planes of rotation: yaw, pitch, and roll. The pilot experiences an overwhelming head-over-heels tumbling sensation.

9-45. Somatogravic illusions are caused by changes in linear accelerations and decelerations or gravity that stimulate the otolith organs. The three types of somatogravic illusions that can be encountered in flight are oculogravic, elevator, and oculoagravic.

9-46. This type of illusion occurs when an aircraft accelerates and decelerates. Inertia from linear accelerations and decelerations cause the otolith organ to sense a nose-high or nose-low attitude. In a linear acceleration, the gelatinous layer, which contains the otolith organ, is shifted aft. The aviator falsely perceives that the aircraft is in a nose-high attitude. A pilot correcting for this illusion without cross-checking the instruments would most likely dive the aircraft. This illusion does not occur if adequate outside references are available. If making an instrument approach in inclement weather or in darkness, the pilot would be considerably more susceptible to the oculogravic illusion. An intuitive reaction to the sensed nose-high attitude could have catastrophic results

9-47. This illusion occurs during upward acceleration. Because of the inertia encountered, the pilot’s eyes will track downward as his body tries, through inputs supplied by the inner ear, to maintain visual fixation on the environment or instrument panel. With the eyes downward, the pilot will sense that the nose of the aircraft is rising. This illusion is common for aviators flying aircraft that encounter updrafts.

9-48. This illusion is the opposite of the elevator illusion and results from the downward movement of the aircraft. Because of the inertia encountered, the pilot’s eyes will track upward. The pilot’s senses then usually indicate that the aircraft is in a nose-low attitude. This illusion is commonly encountered as a helicopter enters autorotation. The pilot’s usual intuitive response is to add aft cyclic, which decreases airspeed below the desired level.

9-49. Proprioceptive illusions rarely occur alone. They are closely associated with the vestibular system and, to a lesser degree, with the visual system. The proprioceptive information input to the brain may also lead to a false perception of true vertical. During turns, banks, climbs, and descending maneuvers, proprioceptive information is fed into the central nervous system. A properly executed turn vectors gravity and centrifugal force through the vertical axis of the aircraft. Without visual reference, the body only senses being pressed firmly into the seat. Because this sensation is normally associated with climbs, the pilot may falsely interpret it as such. Recovering from turns lightens pressure on the seat and creates an illusion of descending. This false perception of descent may cause the pilot to pull back on the stick, which would reduce airspeed. Figure 9-22 shows proprioceptive illusions.

9-50. Spatial disorientation cannot be totally eliminated. However, aircrew members need to remember that misleading sensations from sensory systems are predictable. These sensations can happen to anyone because they are due to the normal functions and limitations of the senses. Training, instrument proficiency, good health, and aircraft design minimize spatial disorientation. Spatial disorientation becomes dangerous when pilots become incapable of making their instruments read right. All pilots, regardless of experience level, can experience spatial disorientation. For that reason, they should be aware of the potential hazards, understand their significance, and learn to overcome them. To prevent disorientation, aviators should—

• Never fly without visual reference points (either the actual horizon or the artificial horizon provided by the instruments).
• Trust the instruments.
• Avoid fatigue, smoking, hypoglycemia, hypoxia, and anxiety, which all heighten illusions.
• Never try to fly VMC and IMC at the same time.

9-51. Spatial disorientation can easily occur in the aviation environment. If disorientation occurs, aviators should—

• Refer to the instruments and develop a good cross-check.
• Delay intuitive actions long enough to check both visual references and instruments.
• Transfer control to the other pilot if two pilots are in the aircraft. Rarely will both experience disorientation at the same time.