CFI Instrument Practical Test Standards, FAA-S-8081-9B, June 2001

II. Technical Subject Areas

B. Aeromedical Factors

4. Spatial disorientation
a. A state of confusion due to misleading information being sent to the brain from various sensory organs, resulting in a lack of awareness of the aircraft position in relation to a specific reference point
b. Sensory systems for orientation
1) Eyes
a) The major orientation source
b) Visual cues usually prevail over false sensations from other sensory systems
c) Instrument meteorological conditions (IMC)
i) False, disorienting sensations may arise from nonvisual sensory systems
ii) Use your eyes to determine aircraft attitude: look at and rely totally on the flight instruments
2) Ears
a) Semicircular canals
i) Detect angular acceleration of the body
ii) Consist of 3 tubes at right angles to each other, each in one of the axes: pitch, roll, or yaw
iii) Filled with endolymph fluid
iv) In center of each canal is gelatinous cupola resting upon sensory hairs at the end of the vestibular nerve
v) In a turn, the relative motion of the endolymph moves the cupola which stimulates the sensory hairs to provide the sensation of turning
vi) As the turn continues for approximately 20 seconds or more the endolymph fluid has accelerated and is moving at the same speed as the semicircular canal so the sensation of turning ceases
vii) When this turn stops, the fluid continues to move the cupola for a while after the canal has stopped moving, creating the sensation of turning in the opposite direction (even though the turn has stopped)
b) Otolith organs
i) Detect linear acceleration and gravity
ii) Consists of gelatinous membranes containing chalk-like crystals covering sensory hairs at the end of the vestibular nerve
iii) Tilting the head backward or forward cause movements of the crystals and membrane that are similar to those caused by accelerating or decelerating the head
iv) Forward acceleration may cause the illusion of the head tilting backward
3) Nerves
a) Forces created in turns can lead to false sensations of the true direction of gravity
b) Uncoordinated, especially climbing, turns can send misleading signals to the brain
c) Skids and slips can give the sensation of banking or tilting
d) Fatigue or illness can exacerbate these sensations and lead to subtle incapacitation
c. Illusions leading to spatial disorientation
1) Inner ear
a) The leans
i) Created by entering turn so slowly that semicircular tubes are not stimulated and then rolling out relatively abuptly to straight and level flight (SLF)
ii) Creates illusion, on roll out to SLF, of turn in opposite direction
iii) Disoriented pilot may bank aircraft back into the original turn, or, if SLF maintained, feel compelled to "lean" in that direction until the illusion subsides
b) Coriolis illusion
i) In long stabilized turn, move head relatively abruptly in different plane
ii) May create illusion of turning or accelerating on an entirely different axis
iii) To help avoid, develop instrument scan (and other actions) with minimal head movement (move eyes, not head)
c) Graveyard spiral or spin
i) Recover to SLF after prolonged coordinated, constant-rate turn
ii) Pilot experiences turning in the opposite direction (the leans) and may return aircraft to the original turn
iii) Not sensing the turn, but noting the loss of altitude that may result from the normal loss of lift seen in a turn, the disoriented pilot may pull back on the control wheel
iv) These actions may lead to a tightening turn or spiral with increasing loss of altitude
d) Somatographic illusion
i) Rapid acceleration, such as experienced during takeoff may create illusion of nose pitching up
ii) Disoriented pilot may push the nose low into a dive attitude
iii) Rapid deceleration may have opposite effect, with disoriented pilot pulling the nose up into a climb or even stall attitude
e) Inversion illusion
i) Abrupt change from climb to SLF may creat illusion of tumbling backwards
ii) Disoriented pilot may rapidly lower aircraft nose, possibly intensifying the illusion
f) Elevator illusion
i) Abrupt vertical acceleration, as in an updraft, may create illusion of climbing
ii) Disoriented pilot pushes aircraft nose low
iii) Abrupt downward acceleration, as in a downdraft, may have the opposite effect, with disoriented pilot pulling aircraft nose up
2) Visual
a) False horizon
i) Sloping cloud formation
ii) Obscured horizon
iii) Aurora borealis
iv) Dark scene spread with ground lights and stars
b) Autokinesis
i) In the dark, a stationary light appear to move about when stared at for many seconds
ii) Disoriented pilot may attempt to align aircraft with these false movements
3) Postural
a) Many false sensations can occur due to acceleration forces overpowering gravity
b) This may occur in uncoordinated turns, climbing turns and turbulence
d. Demonstrating spatial disorientation
1) Objectives
a) Teach pilots to understand their susceptibility to spatial disorientation
b) Show that bodily sensations cannot be reliably used to determine aircraft attitude
c) Lessen the occurrence and degree of disorientation through understanding the relationship between aircraft motion, head motion and resulting disorientation
d) Instill confidence in relying on the flight instruments to determine true aircraft attitude
2) Caution: never conduct these demonstrations at a low altitude or without an instructor or appropriate safety pilot
3) Climbing while accelerating
i) Accelerate in SLF (student pilot's eyes closed)
ii) Creates illusion that aircraft is climbing
4) Climbing while turning
i) Slowly enter coordinated 1.5 G (approximately 50° bank) turn for 90°
ii) Usual illusion is that of a climb
5) Diving while turning
i) Slowly enter coordinated 1.5 G turn for 90° and start recovery
ii) Usual illusion is that the aircraft is diving
6) Tilting to right or left
i) In SLF, execute moderate or slight skid with wings level
ii) Usual illusion is that body is being tilted opposite the skid
7) Reversal of motion
i) In SLF, smoothly roll to 45° bank while maintaining heading and pitch
ii) Usual illusion is strong sense of rotation in opposite direction
8) Diving or rolling beyond the vertical plane
i) May produce extreme disorientation
ii) Start coordinated roll to 30 or 40° of bank
iii) Student pilot should tilt head forward, look to right or left, then immediately return head to upright position
iv) Time maneuver so roll is stopped just as student returns head to upright
v) Usually produces intense illusion of falling downwards into the direction of roll
9) May also be beneficial for some students to do the flying, closing eyes, following instructor's directions, then attempt to establish correct attitude with eyes still closed, thus actually experiencing the disorientation while flying the aircraft
e. Coping with spatial disorientation
1) Understand the illusions and remain constantly alert for them
2) Always obtain preflight weather briefing
3) Do not continue flight into adverse weather, dusk or darkness until proficient in the use of the flight instruments
4) Use only those outside visual references that are reliable, fixed points on the Earth's surface
5) Avoid sudden head movements, especially during takeoffs, turns, approaches and landings
6) Remember that susceptibility to spatial disorientation is increased by illness, medication, alcohol, fatigue, sleep loss and mild h ypoxia
7) Become proficient in the use of and rely on the flight instruments
f. Optical illusions
1) Runway width illusion
i) Narrow runway can create illusion that aircraft is higher than actual, causing pilot to fly too low on approach
ii) Wide runway may look closer than actual, so pilot may approach too high, level out high and land hard or overshoot the runway
2) Runway and terrain slopes illusion
i) Upsloping runway and/or terrain may create illusion that aircaft is higher than actual, so pilot may fly too low
ii) Downsloping runway and/or terrain may have opposite effect
3) Featureless terrain illusion ("black hole approach")
i) Absence of ground features, as in an approach over water, darkened areas or snow may create illusion that aircraft is too high
ii) Pilot may thus approach too low iii) Don't get sucked into the black hole on approach
4) Water refraction
i) Rain on windscreen can create illusion that aircraft is higher than actual
ii) Too low approach may be flown
5) Haze
i) May create illusion of being at a greater than actual distance from runway
ii) Pilot will tend to be high on the approach
iii) Beware tendency to fly long, too low approach in extremely clear air when the runway may appear closer than actual
iv) Diffusion of light by water particles adversely affects depth perception
6) Fog
i) Penetration of fog can create illusion of pitching up
ii) Disoriented pilot may abruptly steepen approach
7) Ground lighting illusions
i) Line of lights along roads, trains may be mistaken for runway or approach lights
ii) Bright runway and approach lights may create illusion of being too close, leading pilot to fly a higher approach
g. How to prevent landing errors due to visual illusions
a) Anticipate possibility of visual illusions (consult airport diagrams and Airport/Facility Directory for runway information)
b) Refer to altimeter frequently, especially during approach, day and night
c) Conduct aerial visual inspection of unfamiliar airports before landing
d) Use VASI, PAPI or electronic glide slope whenever available
e) Use the visual descent point (VDP) found on many nonprecision instrument approach charts
f) Recognize that chance of approach error increases when distracted by an emergency or other activity
g) Maintain optimum prociency in landing procedures

Instrument Flying Handbook, FAA-H-8083-15, 1999
AIM 8-1-5

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