Somatogravic Illusions Operator’s Guide to Human Factors in Aviation Human Performance and Limitations Managing Visual Somatogravic Illusions

Managing Visual Somatogravic Illusions Operator’s Guide to Human Factors in Aviation Human Performance and Limitations Managing Visual Somatogravic Illusions 1. Introduction to the vestibular system 2. Somatogyral illusions 3. Somatogravic illusions 4. Conclusion To be used with: Briefing Note Vestibular System and Illusions 1.HP_11_ Vestibular & Postural Control

1. Introduction to the vestibular system Labyrinths Visual input Proprioceptive input Gaze stabilisation Orientation in space Balance 1.HP_11_ Vestibular & Postural Control

Location of the vestibular system 1.HP_11_ Vestibular & Postural Control

1.HP_11_ Vestibular & Postural Control Six degrees of freedom 1.HP_11_ Vestibular & Postural Control

1.HP_11_ Vestibular & Postural Control The human inner ear 1.HP_11_ Vestibular & Postural Control

1.HP_11_ Vestibular & Postural Control Posterior canal Anterior canal Horizontal canal Cochlea N. cochlearis N. vestibularis Sacculus Utriculus 1.HP_11_ Vestibular & Postural Control

Mechanism of rotation detection 1.HP_11_ Vestibular & Postural Control

1.HP_11_ Vestibular & Postural Control The driving stimulus for the semicircular canal sensory cells is angular acceleration The canal dynamics, however, have an integrating function and convert acceleration into angular rate Under conditions of sustained rotation, the elastic properties of the cupula (the membrane with the detectors) drive it back to its zero position after ± 7 seconds Despite the existence of a velocity storage mechanism in the brain, after 20 to 30 seconds there is no accurate detection of movement 1.HP_11_ Vestibular & Postural Control

1.HP_11_ Vestibular & Postural Control Transient rotations, typically for head movements, are perfectly detected 1.HP_11_ Vestibular & Postural Control

Sustained rotations are not appropriately detected 1.HP_11_ Vestibular & Postural Control

1.HP_11_ Vestibular & Postural Control Somatogyral illusion A somatogyral illusion is A false sensation of rotation or absence of rotation Any discrepancy between actual and perceived rate of self-rotation It originates in the inability of the semicircular canals to register accurately prolonged rotation (> 30 s), e.g. banking during a holding pattern The operation window of the canals corresponds to ‘physiological’ frequencies, i.e. 0.1 – 5 Hz 1.HP_11_ Vestibular & Postural Control

Somatogyral illusion example: the graveyard spin Suppose the aircraft makes a sustained turn. After ± 30s, the canals stop responding, and the brain has no sense of turning any more. If the trajectory of the aircraft is now straightened, the brain senses a turn in the opposite direction due to the angular deceleration. The pilot perceives a turn in the opposite direction He may erroneously correct for this illusory spin and re-enter the original turn to compensate, so that he perceives stable flight. Additionally, his gaze may be disturbed by the nystagmus of his eyes, that disables clear reading of the solely reliable instruments. 1.HP_11_ Vestibular & Postural Control

Solution to somatogyral illusions Rely on the flight instruments – never on your perception ( your internal instruments) Make the instruments read right ! When nystagmus disturbs your vision – fixate on a nearby fixed point on the instrument panel Converging the eyes also diminishes nystagmus Continuously remember that sustained rotations are, by definition, misperceived by the equilibrium system Visual information is of a higher order than vestibular information 1.HP_11_ Vestibular & Postural Control

1.HP_11_ Vestibular & Postural Control Is this right? 1.HP_11_ Vestibular & Postural Control

1.HP_11_ Vestibular & Postural Control Make the instruments read right 1.HP_11_ Vestibular & Postural Control

Acceleration detectors Saccule Utricle 1.HP_11_ Vestibular & Postural Control

Principle of otolith organ function The otoliths consist of calcium carbonate ‘stones’ embedded in a gelatinous substance. When the head moves, the inertia or weight of the stones bends the hair cells and thus activates nerve cells, sending a signal to the brain proportional to the amount of head movement. Driving stimulus equals linear accelerations, change of orientation with respect to gravity 1.HP_11_ Vestibular & Postural Control

The otolith membrane in the inner ear 1.HP_11_ Vestibular & Postural Control

Ambiguity of the otolithic membrane action Backward Tilt = Forward acceleration 1.HP_11_ Vestibular & Postural Control

Ambiguity of the otolithic membrane action Forward Tilt = Deceleration 1.HP_11_ Vestibular & Postural Control

Gravito-inertial acceleration The gravito-inertial acceleration (GIA) is the vector sum of the vector of gravitational acceleration (upward) and all other linear accelerations acting on the head 1.HP_11_ Vestibular & Postural Control

Somatogravic illusion A somatogravic illusion is a false sensation of body tilt that results from perceiving as vertical the direction of non-vertical gravito-inertial acceleration or force 1.HP_11_ Vestibular & Postural Control

Somatogravic illusion during takeoff The somatogravic illusion of ‘nose-up’ sensation after takeoff and the erroneous correction of the pilot to push the yoke forward has caused more than a dozen airline crashes An aircraft accelerating from 170 to 200 knots over a period of 10 seconds just after takeoff, generates +0.16 G on the pilot The GIA is only 1.01 G The corresponding sensation is 9 degrees ‘nose up’ When no visual cues are present and the instruments are ignored, an unwary pilot might push the nose down and crash 1.HP_11_ Vestibular & Postural Control

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Somatogravic illusion during final approach An inexperienced pilot may perceive deceleration due to lowering the flaps as a steep nose-down sensation On the runway, before the nose wheel touches down, the deceleration may be perceived as a too-low vertical attitude. An erroneous correction to bring the nose up may cause damage 1.HP_11_ Vestibular & Postural Control

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1.HP_11_ Vestibular & Postural Control Caution 21 percent of approach-and-landing accidents involved disorientation or visual illusion Flying in the simulator can provoke some of these illusions, but the GIA never exceeds 1 G and can not mimic the somatogravic illusion of false nose-up sensation due to acceleration or nose- down one due to deceleration 1.HP_11_ Vestibular & Postural Control

1.HP_11_ Vestibular & Postural Control Conclusion Would the best pilots be those who have no ‘misleading’ vestibular organ? No, because they would not be able to stabilize their gaze to read the instruments However, being aware of the misleading information of the vestibular organ is crucial; humans are not designed to fly Debrief on your erroneous perceptions and realize that it is a perfectly human and normal sensation (we can’t help it). But, it is not suitable for flying 1.HP_11_ Vestibular & Postural Control

1.HP_11_ Vestibular & Postural Control Conclusion cont. Confidence, competence and currency in instrument flying greatly reduces the risk of disorientation Prioritize the workload; first fly the aircraft, then do everything else Build up experience controlling the aircraft in an environment of conflicting orientation cues 1.HP_11_ Vestibular & Postural Control

1.HP_11_ Vestibular & Postural Control Conclusion cont. Avoid disorientation by making frequent instrument cross-checks, even when the autopilot is on Match the instrument readings with your internal mental representation of the flight path Recover from disorientation by: Making the instruments read right, regardless of your sensation 1.HP_11_ Vestibular & Postural Control

1.HP_11_ Vestibular & Postural Control Conclusion cont. Don’t trust your built-in equilibrium organs, particularly in low-visibility conditions In moments of stress, make decisions based on the instruments; don’t fall back on your instinct or perceptions Garbage in leads to garbage out. The human equilibrium system is designed to function on land, to chase animals… not to fly aircraft. 1.HP_11_ Vestibular & Postural Control

Short calculation 1 knot = 0.514 m/s Acceleration after takeoff: 30 kts/10s = 1.54 m/s2 1 G = 9.81 m/s2  acceleration = 0.16 G GIA = sqrt(12 + 0.162) = 1.01 G Inclination = Arc Tan(0.16/1) = 9 degrees Nose-up impression of 9 degrees