human 6 Spatial Disorientation and performance Sensory Illusions and L 6.1 Introduction to Spatial imitations Disorientation The brain processes information from the (VFR) flights into instrument meteorological eyes, the vestibular system, the ears and conditions (IMC). These “VFR flight into the proprioceptors (sensory receptors in the IMC” accidents accounted for approximately muscles, tendons and joints) to determine 11% of the fatalities in that 8-year period. the direction of gravity (‘the vertical’) and 75% of all VFR flight into IMC accidents the position of the body in space. That is in that time period were fatal (Goh & what we call spatial orientation. Wiegmann, 2000). The most important information for The aviation accident records of other orientation must come from well-defined countries (e.g., United Kingdom and New external visual cues or visual references. If Zealand) also show similar trends, indicating these are not or not sufficiently present we that VFR flight into IMC is a major hazard in have a problem: we can become spatially general aviation. disorientated. Tests have proved that without adequate While on earth we may use our hands visual clues, directional control of an aircraft or ears to compensate for the loss of can be lost within 60 seconds during straight visual cues, the pilot can’t. In addition, and level flight and even quicker during a the vestibular and somatosensory senses turn. have become fully unreliable. Many fatal accidents have resulted and still result from The lessons learned from these accidents spatial disorientation in the air under bad and tests may already be clear: visual conditions. • VFR-pilots who have little or no experience in IFR-flying should never An analysis of the USA’s National decide to go into adverse meteorological Transportation Safety Board’s (NTSB) conditions because they stand a fair aviation accident database indicates that chance of becoming spatially disorientated between 1990 and 1997, 2.5% of more than • Pilots flying under IMC must RELY SOLELY 14 000 general aviation (GA) accidents were ON THEIR INSTRUMENTS instead of any classified as involving visual flight rules perception resulting from their vestibular cues. Spatial Disorientation and Sensory Illusions 6-1 human Why are cues from our vestibular apparatus canals, resulting in the correct perception of so unreliable under conditions of few visual being straight and level. performance reference? Because in 3-dimensional space our vestibular apparatus generates illusions, However, any movement of the head as a which are false perceptions of reality that result of aircraft roll, pitch or yaw will cause we can’t avoid. We can, however, become the hairs in the fluid of the appropriate and aware of them and that is the theme of semi-circular canal(s) to move. Continued L imitations section 6.2 Vestibular Illusions. motion at a steady rate of turn will, after 10- 15 seconds, allow the fluid to ‘catch up’ with When flying, even our eyes and visual the canal causing the hairs to revert to their perceptions can deceive us by illusions. upright position. This lack of vestibular input But the same is true here; we can learn to leads to a false perception that the aircraft is be alert and aware of visual illusions. straight and level, when in fact it is not. See These Visual Illusions are the theme of fig. HP 6.1 left and middle. section 6.3. 6.2.2 The Illusion of Banking (Rotating) 6.2 Vestibular Illusions While Flying Straight-and-Level (Somatogyral Illusion) An illusion is a false interpretation of A somatogyral illusion is a false sense of sensory information by the brain. rotation that results from misperceiving the magnitude or direction of actual rotation. Vestibular illusions are illusions created It may, for example, result from the situation in general by a conflict of information described in section 6.2.1. when from a received by external visual clues and by the prolonged turn we roll the aircraft back to vestibular system. straight and level flight. Also the proprioreceptors, which are In fig. HP 6.1, middle, the aircraft has a receptors in the muscles and tendons of the constant rate of turn and the sensory hairs body and which gives us information about are erect because the fluid in the roll canal our posture, may play a role in the creation has come to a rest. The pilot perceives a of spatial disorientation. straight and level position. The following are examples of vestibular When the wings are levelled, the fluid in the illusions, which may be encountered in semi-circular canals moves in the opposite flight. direction, but when the wings are level and the canal itself has come to rest the fluid 6.2.1 The Illusion of Straight-and-Level continues to move due to inertia. This leads Flight While Banking and Turning to a sensation of turning in the opposite When in straight and level flight there is direction and the pilot will tend to bank the no movement of fluid in the semi-circular aircraft away from the falsely perceived turn. 6-2 Spatial Disorientation and Sensory Illusions human Prolonged right turn performance 3°/s Angular velocity (deg/s) 0°/s and L Output signal imitations of semi-circular canal Time 0 sec 15 sec 0 sec 15 sec Deflection of cupula Actual airplane attitude Perception Correct False Back to horizontal Correct of attitude perception of perception of but false perception of by pilot turning right straight & level perception of straight & level turning left Fig. HP 6.1 Correct and false perceptions when rolling 6.2.3 The Leans being straight and level. When he becomes The false sense of rolling when levelling suddenly aware of the inaccurate the wings described in 6.2.2 may lead to a roll attitude and abruptly rolls the aircraft phenomenon we call the “Leans”. It means back to wings level he applies a “supra- that the pilot perceives the conflict between threshold” roll movement sufficient to make the rolling sensation and the level wings and him aware of a rolling sensation. As a tries to “solve” this conflict by leaning to one result of the false perception of a roll to the side of the cockpit (the side of the original opposite side he will “lean” his body to a turn). The process is depicted in fig. HP 6.2. “false vertical”. The leans could happen when the pilot 6.2.4. Coriolis Illusion inadvertently enters a turn gradually and This illusion is caused by the interaction of smoothly without exceeding the angular angular motion in more than one plane. detection threshold of the semicircular For example, if you are in a turn, drop your canals (“sub-threshold movement”). While pencil and bend over to pick it up, the softly banking he keeps the illusion of motion of the head you make adds to the Spatial Disorientation and Sensory Illusions 6-3 human performance A and L imitations B 1 2 3 4 A - Perceived attitude B - True attitude 1. Pilot allows wing to drop with a rate under the perceptual threshold 2. Pilot still thinks “wings level” 3. Pilot notices bank and corrects above perceptual threshold 4. Pilot perceives banking to other side, experiences perceptual conflict, and tries to solve this by leaning in the direction of the original bank Fig. HP 6.2 The leans, the most common form of spatial disorientation motion of turning. While the fluid in the 6.2.5 Vertigo canals had come to a rest, there is suddenly Vertigo is a false sense of turning usually in movement in another canal and in the one plane in which the individual or his/her utricule and saccule. This results in a strong surroundings appear to whirl dizzily about. sensation of tumbling backwards or forward. It may be accompanied by nausea or The body feels a new “subjective (but false) vomiting and can be caused by disease. vertical”, giving you a strong sensation of Vertigo occurring in flight is usually climbing or diving. You may tend to correct temporary, lasting for a few seconds. this illusion by control inputs, but these will certainly be wrong because you were only in a level turn. It is considered that when the 6.2.6 Graveyard Spin head is turned at a rate of 3º per second or When pulling out of a dive following a more, while in a turn, the coriolis effect may spin, the endolymph fluid in the semicircular be encountered. canals will still be moving under its own inertia and the pilot may feel that the The coriolis illusion may provoke nausea spinning manoeuvre has not yet been and is used in air sickness desensitisation recovered. He may respond to this illusion therapy to test the susceptibility for motion of still being in a spin by putting the aircraft sickness. into a spin in the opposite direction. 6-4 Spatial Disorientation and Sensory Illusions human 6.2.7 Somatogravic Illusion the brain, when there is insufficient visual information to correct the misinformation. The somatogravic illusion is a dangerous performance illusion that is believed to have caused a See fig. HP 6.3. large number of mishaps in civilian and military aviation over the years. When In the early days of launching Navy aircraft from aircraft carriers at sea, the somatogravic accelerating in straight and level flight, and illusion – which is quite intensely felt by you may falsely perceive that the aircraft L imitations is climbing. Similarly, when decelerating, the pilots during the launch - has led to a sensation of pitching down may be many accidents.