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

2. 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

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

4. Location of the vestibular system
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Six degrees of freedom
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The human inner ear
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Posterior canal
Anterior canal
Horizontal canal
Cochlea
N. cochlearis
N. vestibularis
Sacculus
Utriculus
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8. Mechanism of rotation detection
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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
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Transient rotations, typically for head movements, are perfectly detected
1.HP_11_ Vestibular & Postural Control

11. Sustained rotations are not appropriately detected
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12. 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

13. 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

14. 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
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Is this right?
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Make the instruments read right
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17. Acceleration detectors
Saccule
Utricle
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18. 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
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19. The otolith membrane in the inner ear
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20. Ambiguity of the otolithic membrane action
Backward Tilt =
Forward acceleration
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21. Ambiguity of the otolithic membrane action
Forward Tilt =
Deceleration
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22. 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
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23. 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
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24. 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 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
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26. 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
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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
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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
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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
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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
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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.
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33. 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( ) = 1.01 G
Inclination = Arc Tan(0.16/1) = 9 degrees
Nose-up impression of 9 degrees