United States Army School of Aviation Medicine
Fort Rucker, Alabama
JUNE 1997
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STUDENT HANDOUT
TITLE: SPATIAL DISORIENTATION AND SENSORY ILLUSIONS OF FLIGHT
FILE NUMBER: 2/5/9/9E/UEA/UEC/UEE/4505-3
PROPONENT FOR THIS LESSON PLAN IS:
United States Army School of Aviation Medicine
ATTN: MCCS-HAF
Fort Rucker, Alabama 36362-5000
FOREIGN DISCLOSURE RESTRICTIONS: The materials contained in this lesson plan have been
reviewed by the instructor/training developer and determined to be public domain
materials. This product is releasable to military students from all requesting foreign
countries without restrictions.
June 1997
SPATIAL DISORIENTATION AND SENSORY ILLUSIONS OF FLIGHT
TERMINAL LEARNING OBJECTIVE (TLO):
ACTION: Manage spatial disorientation.
CONDITION: While performing as an aircrew member.
STANDARD: In accordance with (IAW) FM 1-301 and FM 8-2.
A. ENABLING LEARNING OBJECTIVE (ELO) #1:
ACTION: Identify spatial disorientation terminology.
CONDITION: While serving as an aircrew member.
STANDARD: In accordance with (IAW) FM 1-301, FM 8-2.
a. Vertigo—spinning sensation caused by a physiologic abnormality in the middle ear. Often misused by aircrew members as a generic term to represent all forms of spatial disorientation. That they may experience.
b. Sensory illusion—is a false perception of reality caused by the conflict of orientation information from two or more mechanisms of equilibrium. Sensory illusion is a major cause of spatial disorientation.
c. Spatial disorientation—the inability to determine one’s position, attitude, and motion relative to the surface of the earth or significant objects (i.e., trees, poles, or building during hover).
B. ENABLING LEARNING OBJECTIVE (ELO) # 2:
ACTION: Identify the modes of vision used for orientation.
CONDITION: While serving as an aircrew member.
STANDARD: In accordance with (IAW) FM 8-2, FM 1-301.
a. Focal vision. The focal or central vision allows us to perceive images clearly.
(1) Its purpose is to recognize objects and identify them. For instance, when we read a book or our flight instruments we are utilizing our focal vision.
(2) Focal vision provides us information necessary to determine distance and depth perception. The images we perceive throughout our lifetime are stored in memory so that we may compare their size to our own position relative to them. (For example, a pine tree is of a known size, therefore, we can judge our distance from it, depending on its retinal projection.)
b. Peripheral vision or ambient vision is responsible for orienting us within our environment. Peripheral vision depends totally on a horizon for orientation.
(1) Peripheral vision detects motion, either our own motion or motion of other objects around us. (For example, when we view a movie on a wide screen such as flying through the grand canyon, we perceive ourselves in motion.)
(2) Peripheral vision provides adequate orientation information in absence of the vestibular apparatus. An individual can still maintain balance through the use of the peripheral mechanism of orientation.
c. Role of vision in spatial disorientation. The visual mechanism is the most reliable and important mechanism for equilibrium and spatial orientation on the ground as well as in flight. Eighty percent of our orientation information comes from the visual system.
d. Conditions for spatial disorientation. The conditions most likely to cause spatial disorientation occur during an unexpected transition from visual meteorological conditions to instrument meteorological conditions. In aviation this is called inadvertent IMC.
C. ENABLING LEARNING OBJECTIVE (ELO) # 3:
ACTION: Identify the visual illusions.
CONDITION: While serving as an aircrew member.
STANDARD: In accordance with (IAW) FM 1-301, FM 8-2.
a. Role of visual cues. Orientation by vision requires visual cues. In other words, a person must determine their position by understanding where other objects are in relation to themselves. The more cues (things to see) the more accurate the orientation information is.
b. Visual illusions may occur when the cues are reduced by clouds, night, or other obscurities to vision.
(1) Relative motion illusion.
(a) Definition. Relative motion illusion is falsely perceived self-motion in relation to the real motion of another object.
(b) The relative motion illusion can also occur to helicopter pilots hovering over tall grass. The rotor wash creates a continual waving motion, which makes it difficult to maintain stationary hovering.
(c) Preventive techniques. A good outside scanning procedure helps to reduce this illusion. Do not stare or become fixated on tasks inside the cockpit.
(2) False horizons. False horizons (false vertical and horizontal cues) illusions can occur when the pilot unconsciously (and foolishly) chooses the wrong reference point (i.e., clouds) for orientation information.
(a) The pilot falsely perceives the cloud bank below to be horizontal. The cloud bank may not be parallel to the ground. The pilot then flies the aircraft in a banked condition.
(b) To prevent this illusion, cross check with the attitude indicator.
(3) Depth perception illusion. This illusion is due to a lack of sufficient visual cues. The aviator will experience the illusion of being higher than he actually is.
(a) Flying over an area devoid of visual references, such as desert, snow or water will deprive the pilot of his perception of height. Flight in an area where visibility is restricted by fog, smoke, or haze produces the same illusion.
(b) This illusion can be overcome by dropping a smoke grenade or similar object in the area of intended flight. If operating at night, chemical lights or flares would be useful. If on an approach to an unfamiliar runway, use instrument landing systems to obtain height information.
(4) Structural illusions. Structural illusions are the phenomenon in which objects become distorted when visual obscurants are present.
(a) A straight line may appear curved when viewed through a heat wave.
(b) Heavy rain against aircraft windshields may cause a pilot on half-mile final to see the runway as 200 feet lower than it actually is.
(c) Prevention. The aviator must be aware of this illusion and overcome its effects by proper scanning techniques. If on approach to an airfield, use instrument landing systems to obtain height information.
(5) Fascination (fixation) occurs when aircrew members ignore orientation cues and focus their attention on their object or goal.
(a) Target fixation. In combat, the pilot becomes so intent on hitting the target that he neglects to pull up in time to prevent impact with the ground. Other types of fascination are associated with wheels-up landings, rigid fixation on the lead aircraft during formation flight, and over concentration on one instrument during instrument flight.
(b) Task saturation. Fascination may also occur during accomplishment of simple tasks within the cockpit. Crew members may become so engrossed with a problem or task that they fail to properly scan outside the aircraft. This often occurs at night when the crew is tired, on night vision goggles, and are masking and unmasking.
(c) Prevention. To overcome this illusion, the pilot must continually scan and "fly" the aircraft.
(6) Size-distance illusion. The size-distance illusion is the false perception of distance from an object or the ground, created when a pilot misinterprets an unfamiliar object’s size to be the same as an object he/she is normally accustomed to viewing.
(a) An aircraft hovering close by with its dim position lights on, may appear to be farther away than when viewed at the same distance with its lights on bright.
(b) This illusion also occurs if the visual cues, such as trees, are of a different size than expected. For example, the small trees of the mid-west have the same shape and contrast as the tall trees of the east coast. The pilot may fly his aircraft dangerously low, thinking that he is further away from the ground.
(c) A pilot may falsely perceive an unfamiliar LZ to be the same size as to which he is used to landing. For example, a pilot who is used to landing at an airfield with a large runway 200 feet wide and 5,000 feet long, may fly too low if making the same approach to a small airstrip of 100 feet wide, 2,000 feet long.
(7) Altered planes of reference. Altered planes of reference is the inaccurate sense of altitude, attitude, or flight path position in relation to an object very great in size so that the object becomes the new plane of reference rather than the correct plane of reference; the horizon.
(a) A pilot approaching a line of mountains may experience the false perception of being too low. This is because the horizon, which maintains orientation, is subconsciously moved to the top of the ridgeline. Without an adequate horizon the brain attempts to fix a new horizon.
(b) Conversely, an aircraft entering a valley which contains a slowly increasing upslope condition may become trapped because the slope may quickly increase and exceed the aircraft’s ability to climb above the hill, causing the aircraft to crash into the surrounding hills.
(c) When flying next to large cloud formation, the eyes may interpret the cloud formations as a horizon. The tendency would be to tilt away from the clouds. Transition to instruments if this illusion occurs.
(d) Prevention. To avoid this illusion, the pilot must conduct pre-mission planning and verify the height of the terrain before flight.
(8) Autokinesis. The autokinetic illusion results when a static light appears to move when it is stared at for several seconds. Uncontrolled eye movement may possibly cause the illusion of movement as the eye attempts to find some other visual reference points.
(9) Reversible perspective. At night, an aircraft may appear to be going away when it is actually approaching. To determine the direction of flight, the aircrew member should observe the position of the aircraft lights.
(10) Crater illusion. When landing at night, the position of the landing light may be too far under the nose of the aircraft. This will cause the illusion of landing into a hole(crater).
(11) Confusion with ground lights. Pilots have put their aircraft in unusual attitudes to keep ground lights above them; having mistook the ground lights for stars. By establishing a true horizon and attitude through instruments this can be avoided.
(12)Flicker vertigo. Flicker vertigo can be caused by the sunlight flickering through the rotor blades or propellers or by rotating beacons reflecting against an overcast sky.
D. ENABLING LEARNING OBJECTIVE (ELO) # 4:
ACTION: Identify the components of the vestibular system.
CONDITION: While serving as an aircrew member.
STANDARD: In accordance with (IAW) FM 1-301, FM 8-2.
a. Location.
(1) The vestibular apparatus is located in the temporal bone on each side of the head.
(2) The vestibular apparatus consists of the semicircular canals and the otolith organs that reside above the cochlea.
b. Function. The vestibular apparatus has three distinct functions.
(1) Visual tracking. First and most important, the vestibular system provides input to the brain to trigger reflex mechanisms in the eyes so that when the head is turned, they can track an object accurately to prevent a blurred image on the retina. This reflex mechanism is responsible for nystagmus; an involuntary rapid movement of the eyeballs in a horizontal, vertical, or rotary direction due to the over stimulation of the vestibular system. Nystagmus is created when the body is subjected to a prolonged angular acceleration. Nystagmus will cause an inability to focus on the instruments.
(a) The brain requires the eyes to present a clear picture. The clear picture is easily accomplished when the head and body are still. Imagine what would happen to the picture if you were to turn to the right while still maintaining the image in sight. The images would pass across the retina, as the head is turned, which would create a blurred image.
(b) The vestibular apparatus tells the brain to send the eyes in an opposite direction of the turn. So, if you were to turn your head to the right, your eyes would deflect left, thereby, keeping the image in focus.
(c) To demonstrate the vestibular function, hold your hand at eye level to that the palm is facing you. Now, turn your head rapidly from side to side. The image of your palm should remain clear. Now, move your hand back and forth in front of your eyes; the image becomes blurred. This image is what you would see if you did not have a vestibular apparatus.
(2) Reflex information. Secondly, the vestibular apparatus provides the brain with information on the body’s activities so that reflex actions can be made.
(a) The cat has the incredible ability to right itself when dropped upside down. It performs this action subconsciously.
(b) The vestibular apparatus sense the vertical motion and the boy enacts the reflex muscles that govern the cat’s legs.
(c) If we were to fall forward, our arms would present themselves to break the fall.
(d) This same reflexive and subconscious action will be difficult to overcome during flight.
(3) Orientation without vision. Thirdly, the vestibular apparatus provides the brain the necessary orientation information when in the absence of vision.
(a) If we did not have the vestibular mechanism providing additional orientation information, we would not be able to walk in complete darkness. We would always lose our balance and fall.
(b) The body receives information during turns and rolls even when the eyes aren’t providing this information.
c. Components of the vestibular system.
(1) Semicircular canals.
(a) The semicircular canals are arranged at right angles to one another so that motion in any of the three planes; yaw, pitch, and roll can be detected.
(b) The semicircular canals contain fluid called endolymph. The fluid is allowed to travel freely throughout the vestibular cavities. When the body turns, the semicircular canals must also turn. However, the fluid lags behind the moving canals. Just like when a glass of water is rotated. The water lags behind the direction of rotation until friction between the glass and the water pulls the water to the same speed of the turning glass.
(c) The motion of the lagging endolymph fluid, causes a gelatinous structure (the cupula) to bend. This structure is located at the enlarged base of the canal called the ampulla. When no acceleration takes place, the cupula remains upright.
(d) The cupula, when bent, stimulates hair cells (cilia) located beneath it. This, in turn, sends nervous impulses to the brain that are interpreted as rotation of the head. If in a right turn or clockwise rotation, the cupula is deflected right.
(e) If the turn is prolonged and remains constant, the endolymph catches up with the canal wall.
(f) The cupula is no longer deviated and the brain receives the completely false impression that the turn has stopped.
(g) If the turn is slowed down, or the rate of acceleration is changed, the endolymph’s inertia will continue in the direction of the turn while the canal wall is slowed. This results in the deflection of the cupula in the opposite direction.
(h) A false impression of an opposite turn is received by the brain.
(2) The otolith organs are responsive to linear motion (fore and aft and up and down). They are totally dependent on gravity or gravito-inertial forces for activation.
(a) The otolith organs (the utricle and the saccule) are small fluid filled sacs that lie below the semicircular canals in the vestibule.
(b) When the head is in an upright position, the utricle lies in a horizontal plane, and the saccule lies in the vertical plane.
(c) The otolith organs consist of plate-like congregations of sensory cells (cilia), which project up into a gelatinous layer (otolithic membrane) filled with small calcium carbonate crystals. The calcium carbonate plate is very dense. It shifts when the head is subjected to linear acceleration.
(d) When the cilia are bent by the fore and aft shift of this membrane, they send nervous impulses to the brain. These impulses are interpreted as movement of the body or head: such as head tilt forward or falling forward.
(e) For example, when the head is tilted backward, the gelatinous cube is shifted, thereby bending the cilia. The brain receives signals that the head is tilted backward.
(f) The otolith organs can be fooled by false gravitational (G-force) forces caused by aircraft maneuvers.
E. ENABLING LEARNING OBJECTIVE (ELO) # 5:
ACTION: Identify the components of the vestibular illusions.
CONDITION: While serving as an aircrew member
STANDARD: In accordance with (IAW) FM 1-301, FM 8-2.
a. Somatogyral illusions are illusions created when the body is turning or when it is subjected to angular acceleration for prolonged periods.
(1) The leans. The most common form of spatial disorientation. Usually occurs when an aircrew member lacks visual cues for orientation.
(a) Characterized by a false sensation of bank when the aircraft is in level flight.
(b) The leans are caused when a pilot allows the aircraft to enter into an imperceptible turn for several seconds. The semicircular canals will not detect the roll if the rate of roll is less than 2 degrees per second (sub-threshold maneuver).
(c) Some time afterward, the pilot may become aware of the wing-low attitude from reference to the instruments and initiate a recovery to level flight.
(d) This maneuver will stimulate the semicircular canals such that, the pilot will perceive a banked attitude in the opposite direction of his initial turn. His vestibular system will tell him he is now turning.
(e) The false sensation of bank can persist for several minutes. The pilot feels compelled to align his body with the apparent vertical. Thus, he will lean in the direction of the original sub-threshold roll to correct for the inaccurate perception.
(2) Graveyard spiral.
(a) Usually occurs in a fixed-wing aircraft but can occur in rotary-wing aircraft.
(b) The pilot unknowingly enters a spin or a coordinated steep descending spiral which falsely stimulates the semicircular canals.
(c) If a pilot unknowingly enters a turn of less than 2 degrees per second, he/she will have the false perception of straight and level flight. However, the aircraft will be descending in a turn.
(d) Upon recovery from the prolonged spin too straight and level flight, the pilot will experience a sensation of turning in the opposite direction. This is because the fluid remains in motion, which deflects the cupula. This makes him/her feel as though he/she were turning in the opposite direction.
(e) If the pilot is inexperienced, he/she may correct for the false impression by entering back into the original spiral direction.
(f) The rates of descent in a spiral are extremely high, therefore quick action is necessary to overcome this type of illusion.
(3) Coriolis illusion.
(a) This is the most dangerous vestibular illusion because of its overwhelming and incapacitating effects.
(b) The Coriolis illusion occurs whenever a prolonged turn is initiated and the pilot makes a head motion in a different geometric plane.
(c) 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 the cupula.
(d) If the pilot initiates a head movement in another geometric plane other than that of the turn, the yaw axis semicircular canal is moved from the plane of rotation to a new plane of non-rotation. The fluid then slows down in that canal, resulting in a sensation of a turn opposite that of the original turn.
(e) Simultaneously, the two other canals are brought within a plane of rotation. The fluid then stimulates the two other cupulaes.
(f) The combined effect of the coupler deflection in all three canals, creates the perception of motion in three different planes of rotation: yaw, pitch, and roll. The pilot will experience a very strong tumbling sensation.
b. Somatogravic illusions are produced when the body is subjected to gravito-inertial (simulated gravity produced by acceleration) forces whereby the pilot falsely perceives a nose high or nose low attitude during changes in linear acceleration (up and down, fore and aft).
(1) Oculoagravic illusion (movement of the eyes during weightlessness).
(a) The oculoagravic illusion results from a rapid downward motion of the aircraft. Usually, occurring during a down draft condition or during rapid descent from a hover condition.
(b) The vertical stimulation of the otolith organ produces a shift of gaze in the eyes upward. The eyes then sense a movement of the aircraft instrument panel in a downward motion.
(c) This results in a sensation that the aircraft is in a nose-low attitude (diving).
(d) The pilot will then erroneously correct for the perceived condition by pulling aft cyclic.
(2) The elevator illusion.
(a) Exact opposite of the oculoagravic illusion. It occurs during upward acceleration.
(b) The upward vertical stimulation of the otolith organ produces a shift of gaze in the eyes downward. The eyes then detect movement of the aircraft instrument panel.
(c) This results in a sensation that the aircraft is nose high.
(d) Most commonly felt on take-off where the aircraft is climbing but is also felt in turbulent updrafts.
(e) Normal pilot response is to push forward on the cyclic to reduce perceived nose up attitude.
(3) Oculogravic illusion (eye affected by gravito-inertial forces).
(a) When an aircraft accelerates forward, as in a takeoff or added power condition, a gravito-inertial force is applied to the head in a rearward motion.
(b) The otolith organ (utricle) is shifted to the rear just as if the head were tilted backward. This movement of the membrane is the same movement that would occur if the head were tilted backward.
© The otolith organ sends erroneous signals to the brain that the head is tilting backwards. This signal results in the pilot sensing a nose high attitude.
(d) The normal reaction is to push forward on the cyclic and dive the aircraft.
F. ENABLING LEARNING OBJECTIVE (ELO) # 6:
ACTION: Identify the proprioceptive mechanism of equilibrium.
CONDITION: While serving as an aircrew member.
STANDARD: In accordance with (IAW) FM 1-301, FM 8-2.
a. There are a variety of sensory nerve endings contained within the muscles, tendons, and joints.
(1) Cues. These nerve endings provide orientation cues to the brain while on the ground as well as in flight.
(2) System validation. The nerve endings respond to pressure created by gravity or inertial forces. For example, when we sit, pressure on the buttocks sends signals to the brain, reinforcing the orientation information provided by the two other mechanisms of equilibrium.
b. Seat of the pant’s flying.
(1) Most aircrew members are familiar with seat of the pants flying. It is easy to maintain altitude just by "feeling" the aircraft in a climb or descent condition.
(2) However, in the absence of visual cues, a pilot will be unable to maintain a level attitude because the proprioceptive system detects gravito-inertial forces that are often inaccurate.
(a) During a turn, the downward forces of gravity coupled with the outward centrifugal forces, creates a resultant force. The resultant force is midway between the two vectors of gravity and centrifugal force.
(b) This resultant force pulls the aircrew member toward his seat regardless of the speed or direction of the turn.
(3) The proprioceptive mechanism is unreliable in the absence of vision while in flight.
(a) The proprioceptive mechanism tends to agree with the vestibular mechanism. This reinforces the inaccurate orientation information being provided to the brain.
(b) An aviator must not try to fly by what he/she "feels" is right when deprived of his visual sense.
G. ENABLING LEARNING OBJECTIVE (ELO) # 7:
ACTION: Identify the classifications of spatial disorientation.
CONDITION: While serving as an aircrew member.
STANDARD: In accordance with (IAW) FM 8-2, FM 1-301.
a. Type I (unrecognized): pilot does not perceive any indications of spatial disorientation. In other words, he has no apparent orientation problems. Type I disorientation is the most dangerous type of disorientation, because the pilot is unaware of a problem and fails to correct the disorienting situation. This type of disorientation usually results in aircraft mishaps.
(1) The pilot may see the instruments as functioning properly. There is no suspicion of a malfunctioning instrument.
(2) There may be no indications of aircraft control malfunction. The aircraft is performing normally.
(3) An example of this type of SD would be the height/depth perception illusion where the pilot inadvertently flies the aircraft into the ground due to lack of visual cues.
b. Type II (recognized): pilot perceives a problem (resulting from spatial disorientation) but may not recognize it as spatial disorientation.
(1) The pilot may feel that a control malfunction is occurring.
(2) 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 on the attitude indicator because his false vestibular indications of straight and level are so strong.
c. Type III (incapacitating): pilot experiences an overwhelming sensation of movement, such that he/she cannot properly orient himself/herself by using the aircraft instruments.
H. ENABLING LEARNING OBJECTIVE (ELO) # 8:
ACTION: Identify the measures that help prevent spatial disorientation.
CONDITION: While serving as an aircrew member.
STANDARD: In accordance with (IAW) FM 1-301, FM 8-2.
a. Develop the aviator through:
(1) Training. Training is the most important measure to reduce the possibility of spatial disorientation. Through training, an aircrew member learns the "hows" and "whys" of spatial disorientation. An aircrew member must understand the limitations of the sensory mechanisms, the particular flight maneuvers which can lead to spatial disorientation, and the conditions where errors in perception are most likely to occur.
(2) Instrument proficiency. Instrument training must be performed on a regular basis in order to maintain proficiency. It also reinforces the skills necessary for a good instrument cross check.
b. Fly the aircraft.
(1) Never try to fly both VMC and IMC at the same time. If you lose sight of the ground or significant objects, transition to the instruments and perform the emergency IMC procedures.
(2) Never fly without visual reference points (either an actual horizon or an artificial horizon).
(3) Utilize continuous scanning techniques during the day and during night operations. Never stare (either at lights or objects).
c. Instrumentation.
(1) Trust your instruments.
(2) Cockpit design-position new equipment within the cockpit in areas that reduce the necessity for head movements. Ideally, instruments should be as easy to interpret as external cues.
d. Avoid self-imposed stressors. They irritate sensory illusions.
I. ENABLING LEARNING OBJECTIVE (ELO) # 9:
ACTION: Identify the corrective actions to treat spatial disorientation.
CONDITION: While serving as an aircrew member.
STANDARD: In accordance with (IAW) FM 1-301, FM 8-2.
a. Transfer control of the aircraft if there are two pilots (seldom will both pilots experience disorientation at the same time).
b. Delay intuitive reactions.
c. Refer to the instruments immediately upon losing the horizon as reference.
d. Develop and maintain an instrument cross-checks.
e. MAKE THE INSTRUMENTS READ RIGHT!