1. Understanding Visual Illusions
OGHFA
Operator’s Guide to
Human Factors in Aviation
Human Performance &
Limitations
Understanding Visual Illusions
And Disorientation

2. 5. Preventing spatial disorientation
OGHFA
Operator’s Guide to
Human Factors in Aviation
Human Performance
Understanding Visual Illusions and Disorientation
1. Introduction
2. Visual illusions
3. Turning illusions
4. False climb illusions
5. Preventing spatial disorientation

3. Disorientation and illusions - Introduction
Human senses
Visual illusions
Disorientation
Turns
Pitch
How to avoid / minimise
How to recover
Visual illusions and spatial disorientation have contributed to many aircraft accidents; their effects are most pronounced at night and in instrument meteorological conditions (IMC).
A Visual illusion exists when looking at a misleading visual scene, this distortion of sensed information can have strong effects on situation assessment and decision making.
Spatial disorientation occurs when there are difficulties in orientation, or there is a mismatch between the real world and what is sensed.

4. Human Senses - Orientation and Balance
Eyes
Inner Ear (vestibular)
Tactile (motion or position)
Horizontal and Vertical
Distance and Height
Movement
Rate
Rotation
Acceleration
Feel
Force
Gravity
The body has five senses, vision, hearing, touch, smell, and taste.
The sensory inputs that provide orientation, gaze stabilization and balance are visual (sight), vestibular (balance and rotation), and proprioceptive (feedback from skin and muscle sensors for detection of body and limb positions), all acting simultaneously.
Orientation is achieved through the perception of these combined sensory inputs.
Failures in orientation.
a. Vertigo: A sensation of spinning or dizziness. Typically caused by a malfunctioning of the vestibular system, caused by either unnatural movements such as continuous spinning followed by a sudden stopping or through clinical circumstances by a disease of the inner ear. The term “vertigo” often is misused by aircrew members as a generic term to represent all forms of spatial disorientation that they may experience.
b. Sensory illusion: A false perception of reality caused by the conflict of orientation information from one or more mechanisms of the equilibrium. Sensory illusions are a major cause of spatial disorientation.
c. Spatial disorientation: Spatial disorientation is a subset of loss of situational awareness. It is the inability to determine position, attitude, or motion relative to the surface of the earth or other significant objects (i.e., airfields, runways, or buildings).
d. Orientation or balance: Orientation or balance involves the accurate perception of position, attitude, and motion relative to the earth.
Ref: US Army School of Aviation Medicine
They work together
but
They are all a source of error

5. Visual Illusions
Visual illusions may occur when visual cues are reduced by clouds, night, and/or other obscurities to vision. When there is no horizon, visual cues arising can easily be misinterpreted and lead to disorientation. Types of illusion include:
False horizon: The false horizon illusion occurs when the pilot confuses cloud formations with the horizon or the ground. A sloping cloud layer may be difficult to perceive as anything but horizontal if it extends for any great distance in the pilot’s peripheral vision. A cloudbank below may be perceived to be horizontal although it may not be parallel to the ground, resulting in the pilot perceiving a banked attitude. This condition is often insidious and goes undetected until the aviator recognizes it and transitions to the instruments and corrects appropriately.
This illusion can also occur if an aviator looks outside after having given prolonged attention to a task inside the cockpit. The confusion may result in the aviator placing the aircraft parallel to the cloudbank.
Flicker vertigo is technically not an illusion, however, as most people are aware from personal experience, viewing a flickering light can be both distracting and annoying.
(a) Flicker vertigo may be created by helicopter rotors blades or airplane propellers interrupting direct sunlight at a rate of 4 to 20 cycles per second.
(b) Other sources include such things as anti-collision strobe lights flashing, especially while in the clouds.
Confusion with ground lights: Occurs when an aviator mistakes ground lights for stars. This illusion prompts the aviator to place the aircraft in an unusual attitude to keep the misperceived ground lights above. Isolated ground lights can appear as stars and this could lead to the illusion that the aircraft is in a nose high or one wing low attitude. When no stars are visible due to overcast conditions, unlighted areas of terrain or sea can blend with the dark overcast to create the illusion that the unlighted terrain is part of the sky. This illusion can be avoided by referencing the flight instruments and establishing of a true horizon and attitude.
Ref: US Army School of Aviation Medicine

6. Visual Illusion - False Horizon
Rely on the flight instruments,
never on your perception;
ignore your internal instruments
Visual illusions may occur when visual cues are reduced by clouds, night, and/or other obscurities to vision. When there is no horizon, visual cues arising can easily be misinterpreted and lead to disorientation. Types of illusion include:
False horizon: The false horizon illusion occurs when the pilot confuses cloud formations with the horizon or the ground. A sloping cloud layer may be difficult to perceive as anything but horizontal if it extends for any great distance in the pilot’s peripheral vision. A cloudbank below may be perceived to be horizontal although it may not be parallel to the ground, resulting in the pilot perceiving a banked attitude. This condition is often insidious and goes undetected until the aviator recognizes it and transitions to the instruments and corrects appropriately.
This illusion can also occur if an aviator looks outside after having given prolonged attention to a task inside the cockpit. The confusion may result in the aviator placing the aircraft parallel to the cloudbank.
Flicker vertigo is technically not an illusion, however, as most people are aware from personal experience, viewing a flickering light can be both distracting and annoying.
(a) Flicker vertigo may be created by helicopter rotors blades or airplane propellers interrupting direct sunlight at a rate of 4 to 20 cycles per second.
(b) Other sources include such things as anti-collision strobe lights flashing, especially while in the clouds.
Confusion with ground lights: Occurs when an aviator mistakes ground lights for stars. This illusion prompts the aviator to place the aircraft in an unusual attitude to keep the misperceived ground lights above them. Isolated ground lights can appear as stars and this could lead to the illusion that the aircraft is in a nose high or one wing low attitude. When no stars are visible due to overcast conditions, unlighted areas of terrain or sea can blend with the dark overcast to create the illusion that the unlighted terrain is part of the sky. This illusion can be avoided by referencing the flight instruments and establishing of a true horizon and attitude.
Ref: US Army School of Aviation Medicine

7. Visual Illusion ‘Black Hole’ Approach
Incident 1. Night VFR Wide body
Photo: With permission
Copyright, Martin Aves
Copyright Martin Aves

8. Visual Illusion - An Incident
Modern ‘Heavy’ EFIS / FMS equipped aircraft (VNAV)
VOR/DME, (Non-Precision Approach), Night VFR
FAF 5 DME ft
VOR / DME
TERRAIN PULL UP
MDA 339 ft
The aircraft was flying a night visual flight rules (VFR) approach into a airport in a remote location. It started an early descent with a moderate rate, but reduced the vertical rate as it approached the runway. A “terrain” warning was given at 250 ft, which was only 124 ft above the threshold elevation – the runway was on a hill.
The final approach fix (FAF) for this approach was at 5nm, but to fly a 3-deg flight path the descent should be delayed until 4.3nm.
An early descent from the FAF creates a shallow approach, and if the standard descent rate is used the aircraft will descend below the ideal flight path.
Note that the VOR/DME is 0.4 nm before the runway threshold, thus some effort is required to cross check altitude against range to monitor the approach.
The approach chart used variable range scaling that indicated the DME displacement at approximately 1.5nm instead of 0.4nm, this might have encourage an early descent. The chart did not have an altitude – range table for the non-precision approach.
The analysis considered incorrect FMS programming, an early 3 degree approach, and a deliberate ‘dive and drive’ procedure.
However, none of the scenarios matched the recorded flight path.
250 ft agl, 124 ft aal
1.5 nm
300 ft/min *

9. Visual Illusion - ‘Black Hole’ Visual approach*
A long, straight-in approach
Up sloping terrain before a runway
An airport located on the near side of a small city, with an irregular matrix of lights spread over hillsides behind the airport.
Visual Illusion
However, none of the scenarios matched the recorded flight path.
The flight-path had a noticeable bow shape, consisting of an initial steep descent that slowly flattened out. This is typical of an approach where a pilot follows a false optical cue – the classic ‘black hole approach’. The preconditions for this were present:
A night visual approach.
A long straight-in final.
A runway in a remote location, few lights in the local area, but with a town in the distance beyond the airport or to the side.
Up sloping terrain before the runway, a runway on a hill.
Black hole conditions induce a visual illusion of the aircraft being high on the approach, which usually results in the aircraft being flown below the optimum glidepath and, in extreme cases, landing short of the runway.
The lessons to be learnt are that the failure to monitor the flight path allowed the aircraft to descend early, aided by the lack of an altitude-range chart and both crewmembers suffering the same illusion. This illustrates the need to use all resources, such as an EFIS display of runway position, vertical profile, and use of charted procedures (altitude – range table), even when VFR.
Use all available navigation aids – even when VFR
Cross check altitude vs. range even on visual approaches

10. Visual Illusions - ‘Black Hole’ Visual Approach
Check altitude against range Monitor vertical speed
Use all navigation aids
Beware of night visual approaches
Black hole approach
A Black-Hole Approach Illusion can happen during a final approach at night, with no stars or moonlight, over water or unlighted terrain to a lighted runway beyond which the horizon is not visible. In the example, when peripheral visual cues are not available to help you orient yourself relative to the earth, you may perceive the runway to be tilted left and up sloping. However, with the horizon visible, you can easily orient yourself correctly using your central vision.
Height-depth perception illusion: Due to a lack of sufficient visual cues, the pilot will experience the illusion that they are higher above the terrain than they actually are.
(a) Flying over an area devoid of visual references, such as desert, snow, or water will deprive the pilot of his perception of height.
(b) Flight in an area where visibility is restricted by fog, smoke, or haze produces the same illusion.
Fascination or fixation: Occurs when aircrew members ignore orientation cues and focus their attention on their object or goal.
Fixation: Target fixation, commonly referred to as target hypnosis, occurs when an aircrew member ignores orientation cues and focuses their attention on their object or goal; this can occur during approach and landing where the focus of attention is on the runway at the expense of other cues.
Fascination: may occur during the accomplishment of simple tasks within the cockpit. Crewmembers may become so engrossed with a problem or task that they fail to properly scan outside the aircraft. Other types of fascination are associated with wheels-up landings and over-concentration on one flight instrument.
Black hole approaches encourage pilots to develop a mental glideslope based on the orientation of the lights to a standard of lights in their previous history.  This causes a whole variety of approach paths based on many previous experiences.  The main problem is in judging distance to go, but close into the runway a lack of visual cues may encourage various flare techniques with undesirable landing outcomes.
Graphic credit – Airbus

11. Visual Illusion - runway perspective
Slope
Terrain
Aerial perspective illusions may make you change (increase or decrease) the slope of your final approach. They are caused by runways with different widths, upsloping or downsloping runways, and upsloping or downsloping final approach terrain. Pilots learn to recognize a normal final approach by developing and recalling a mental image of the expected relationship between the length and the width of an average runway.
A final approach over a flat terrain with an upsloping runway may produce the illusion of a high-altitude final approach (steep). If you believe this illusion, you may respond by reducing power to decrease the altitude, which, if performed too close to the ground, may result in an accident.
A final approach over a flat terrain with a downsloping runway may produce the illusion of a low-altitude final approach (shallow). If you believe this illusion, you may reduce your rate of descent with a nose-high attitude which, at that altitude, may result in a low-altitude stall.
A final approach over an upsloping terrain with a flat runway may produce the illusion of a low-altitude final approach.
A final approach over a downsloping terrain with a flat runway may produce the illusion of a high-altitude final approach.
A final approach to an unusually narrow runway or an unusually long runway may produce the illusion of a high-altitude final approach.
A final approach to an unusually wide or short runway may produce the illusion of a low-altitude final approach.
Graphic credit - FAA
Length Width
Low High

12. An Incident – up sloping, long, narrow runway; down slope terrain

13. An Incident – up sloping, long, narrow runway; down slope terrain
Beware of night visual approaches
Check QNH (altimeter setting)
Always Pull Up for EGPWS warnings

14. Turning illusions Suppose the aircraft makes a sustained turn.
After about 30 seconds of turning, the ear canals stop responding and the brain has no sense of turning any more.
If the turn is stopped and the aircraft leveled, the brain senses a turn in the opposite direction due to the angular deceleration and the pilot perceives a turn in the opposite direction
He may erroneously correct for this illusory spin, and reenter the original turn to compensate and return to what he perceives is stable flight.
Additionally, his gaze may be disturbed by the nystagmus of his eyes (eyes flickering sideways), that disables clear reading of the instruments.
With thanks to Prof Floris L Wuyts, Head of the Vestibular Function Lab, University of Antwerp, Belgium
Graphic credit Airbus

15. Turning Illusion - Leans (slow roll rate, steady turn)
Rely on the flight instruments -
never on your perception
Ignore your internal instruments
A common form of disorientation:
the pilot fails to sense angular motion during a slow rate of roll.
Somatogyral illusions are caused when angular accelerations and decelerations stimulate the semicircular canals. Those that may be encountered in flight are “the leans,” and Coriolis illusions.
The most common form of spatial disorientation is “the leans.” This illusion occurs when the pilot fails to perceive angular motion. During continuous straight-and-level flight, the pilot will correctly perceive that he is straight and Level. However, a pilot rolling into or out of a bank may experience perceptions that disagree with the reading on the attitude indicator.
With a slow rate of roll, the pilot may fail to perceive that the aircraft is no longer vertical. He may feel that his aircraft is still flying straight and level although the attitude indicator shows that the aircraft is in a bank.
Once the pilot detects the slow roll, he makes a quick recovery. He rolls out of the bank and resumes straight-and-level flight. The pilot may now perceive that the aircraft is banking in the opposite direction. However, the attitude indicator shows the aircraft flying straight and level. The pilot may then feel the need to turn the aircraft so that it aligns with the falsely perceived vertical position. Instead, the pilot should maintain straight-and-level flight as shown by the attitude indicator. To counter the falsely perceived vertical position, the pilot will lean his body in the original direction of the roll until the false sensation leaves.

16. Turning Illusion Coriolis (steady turn, head movement)
Coriolis Illusion occurs in a prolonged turn. If the pilot initiates a head movement in a different geometrical plane, the semicircular canals sense a turn in all three canals creating a new perception of motion in three different planes of rotation at once: yaw, pitch, and roll. The pilot experiences an overwhelming head-over-heels tumbling sensation.
Regardless of the type of aircraft flown, the Coriolis illusion is the most dangerous of all vestibular illusions. It causes overwhelming disorientation.
This illusion occurs in a prolonged turn and the pilot makes a head motion in a different geometrical plane. This involves the simultaneous stimulation of two semicircular canals and is associated with a sudden tilting (forward or backwards) of the pilot’s head while the aircraft is turning. This can occur when you tilt you head down (to look at an approach chart or to write a note on your knee pad), or tilt it up (to look at an overhead instrument or switch) or look sideways. This produces an almost unbearable sensation that the aircraft is rolling, pitching, and yawing all at the same time. The pilot experiences an overwhelming head-over-heels tumbling sensation. This illusion can make the pilot quickly become disoriented and lose control of the aircraft.
An almost unbearable sensation that the aircraft is rolling, pitching, and yawing all at the same time

17. Turning illusion (long turn, then roll exit)
i.e.. banking during holding pattern
After approx 30 seconds, the brain has no sense of turning any more.
If the pilot perceives a turn in the opposite direction, he may bank to re-enter the original turn and overbank to compensate.
Suppose the aircraft makes a sustained turn.
After about 30 seconds of turning, the ear canals stop responding and the brain has no sense of turning any more.
If the turn is stopped and the aircraft leveled, the brain senses a turn in the opposite direction due to the angular deceleration and the pilot perceives a turn in the opposite direction
He may erroneously correct for this illusory spin, and reenter the original turn to compensate and return to what he perceives is stable flight.
Additionally, his gaze may be disturbed by the nystagmus of his eyes (eyes flickering sideways), that disables clear reading of the instruments.
With thanks to Prof Floris L Wuyts, Head of the Vestibular Function Lab, University of Antwerp, Belgium
Graphic credit Airbus

18. Turning illusion (long turn, then roll exit)
The aircraft makes a sustained turn
After approx 30 seconds, the brain has no more sense of turning
The pilot thinks the aircraft is wings level
With thanks to Prof Floris L Wuyts, Head of the Vestibular Function Lab, University of Antwerp, Belgium

19. Turning illusion (long turn, then roll exit)
If the aircraft is now straightened, the brain senses a turn in the opposite direction.
The pilot thinks the aircraft is turning in the opposite direction.
He may erroneously correct for this illusory turn by reentering the original turn and over-banking to compensate, so that he perceives stable flight.

20. Turning illusions - Defenses
Rely on the flight instruments -
never on your perception
Ignore your internal instruments
Rely on the flight instruments – never on your perception (or your internal instruments)
Make the instruments read right !
Remember that sustained rotations are misperceived by the equilibrium system.
If your orientation is disturbed – look at and concentrate on a nearby
fixed point on the instrument panel. Check the aircraft roll attitude.
Sustained turns can be perceived as level flight or a false turn.

21. False Climb illusion
The pilot thinks the aircraft is climbing, but the aircraft pitch attitude is level or at a lower attitude than perceived.
Gravity (1g)
Acceleration
Vertical as sensed by gravity
False vertical cues due to acceleration; give apparent climb

22. False Climb illusion (during acceleration)
During accelerations the pilot thinks the aircraft is climbing, but the aircraft pitch attitude is at a lower attitude than sensed.
Acceleration leads to false climb attitude
Acceleration
Avoid the tendency to push forward.
Concentrate on pitch attitude.

23. False Climb illusion - Takeoff and Go Around
Accelerating from 170 to 200 knots over 10 seconds
+ 0.16g longitudinal acceleration
~ 9 degrees ‘nose up’ attitude change
Use the INSTRUMENTS, follow SOPs
Do not push the nose down
The false climb illusion of a ‘nose-up’ sensation during an acceleration may occur during go around or after takeoff; any erroneous correction by the pilot to push the controls forward could lead to a hazardous situation.
Rely on the flight instruments -
never on your perception
Ignore your internal instruments

24. False Attitude Illusion on Approach - Deceleration
Deceleration due to lowering the flaps or use of airbrake is perceived as a nose-down sensation
On the runway, before the nose wheel touches down, the deceleration from spoilers may be perceived falsely as a too-low pitch attitude.
False attitude due to deceleration; gives apparent nose down pitch
Vertical as sensed by gravity
Deceleration
Gravity (1g)

25. Simulators Cannot Imitate All Illusions
Simulators can provoke some illusions:
but ‘g’ never exceeds 1g
Simulators cannot imitate the false climb illusion
Simulators have tilt, but no acceleration.

26. Prevent / Recover from Disorientation
Believe the instruments, regardless of your sensation
Don't trust your senses, particularly in low-visibility conditions
In moments of stress, base decisions on the instruments; don’t use your ‘instinct’ or feelings
Human senses can be confused and degrade without warning
Aircraft sensors and displays are more reliable than humans
Aircraft sensors and displays have built-in warning and alerting systems; humans do not
RECOVERY FROM SPATIAL DISORIENTATION
Spatial disorientation can easily occur in the aviation environment. If disorientation occurs, aviators should—
Refer to the instruments and develop a good cross-check.
Delay intuitive actions long enough to check both visual references and instruments.
Transfer control to the other pilot if two pilots are in the aircraft, rarely will both experience disorientation at the same time.
Debrief on your erroneous perception and realize that it is a perfectly ‘human’ and ‘normal’ sensation. Humans can’t help it.
BUT, it is not suitable for flying.
FAA Instrument Flying Handbook, Advisory Circular 61-27C (AC) (Section II, "Instrument Flying: Coping
with Illusions in Flight"),
FSF Human Factors and Aviation Medicine Nov-Dec 1997
US Army

27. Preventing Spatial Disorientation
Confidence and currency in your instrument flying
Use an instrument scan - practice
Prioritize workload
Golden Rule #1: First fly the aircraft
Experience
Practice go-arounds
Always cross-check instruments when visual,
and especially with autopilot engaged
PREVENTION OF SPATIAL DISORIENTATION
Spatial disorientation cannot be totally eliminated. However, pilots need to remember that misleading sensations from sensory systems are predictable. These sensations can happen to anyone because they are due to the normal functions and limitations of the senses. It is ‘normal’ that the vestibular system gives misleading input to the brain under special conditions of accelerations and turns. However, training is essential to redirect the pilots attention to the instruments. Training, instrument proficiency, good health and aircraft design minimize spatial disorientation. A high level of attention will contribute to limit spatial disorientation because the brain immediately will focus and interpret the right instruments. Fatigue will enhance spatial disorientation because of the prevailing basic instinct to rely on the senses and the lack of focus on the instruments. Spatial disorientation becomes dangerous when pilots become incapable of making their instruments read right. All pilots, regardless of experience level, can experience spatial disorientation. For that reason, they should be aware of the potential hazards, understand their significance, and learn to overcome them.
To prevent disorientation, aviators should never fly without visual reference points, either the actual horizon or the artificial horizon provided by the instruments.
Trust the instruments.
Avoid fatigue, smoking, hypoglycaemia, hypoxia, and anxiety, which all heighten illusions.
Never try to fly VMC and IMC at the same time.
21% of Approach and landing accidents involved disorientation/visual illusion.

28. Scan your flight instruments
Scan all instruments and believe them
Your mental attitude controls your ‘attitude’.
Do not make control inputs based on your ‘feelings’.

29. Believe your flight instruments
Disorientation and Illusion
Believe your flight instruments
Graphics DJG