Understanding spatial disorientation

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Operators Guide to Human Factors in Aviation

Human Performance and Limitations
Managing Visual Somatogravic Illusions
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Operators 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
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1. Introduction to the vestibular system
Labyrinths
Visual input
Proprioceptive input
Balance
Orientation in space
Gaze stabilisation
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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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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
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Sustained rotations are not appropriately detected
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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 (gt 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

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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.

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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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Acceleration detectors
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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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The otolith membrane in the inner ear
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Ambiguity of the otolithic membrane action
Backward Tilt
Forward acceleration

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Ambiguity of the otolithic membrane action
Forward Tilt
Deceleration

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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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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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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

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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

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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 cant 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.

• Dont trust your built-in equilibrium organs,
  particularly in low-visibility conditions
• In moments of stress, make decisions based on the
  instruments dont 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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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