Graveyard spiral
Graveyard spin Correct path
Figure 3-7. Graveyard spiral.
Coriolis Illusion
The coriolis illusion occurs when a pilot has been in a turn
long enough for the fluid in the ear canal to move at the same
speed as the canal. A movement of the head in a different
plane, such as looking at something in a different part of the
flight deck, may set the fluid moving and create the illusion
of turning or accelerating on an entirely different axis.
This action causes the pilot to think the aircraft is doing a
maneuver that it is not. The disoriented pilot may maneuver
the aircraft into a dangerous attitude in an attempt to correct
the aircraft’s perceived attitude.
For this reason, it is important that pilots develop an
instrument cross-check or scan that involves minimal
head movement. Take care when retrieving charts and
other objects in the flight deck—if something is dropped,
retrieve it with minimal head movement and be alert for
the coriolis illusion.
Graveyard Spiral
As in other illusions, a pilot in a prolonged coordinated,
constant-rate turn, will have the illusion of not turning. During
the recovery to level flight, the pilot experiences the sensation
of turning in the opposite direction. The disoriented pilot may
return the aircraft to its original turn. Because an aircraft tends
to lose altitude in turns unless the pilot compensates for the
loss in lift, the pilot may notice a loss of altitude. The absence
of any sensation of turning creates the illusion of being in a
level descent. The pilot may pull back on the controls in an
attempt to climb or stop the descent. This action tightens the
spiral and increases the loss of altitude; hence, this illusion is
referred to as a graveyard spiral. [Figure 3-7] At some point,
this could lead to a loss of control by the pilot.
Somatogravic Illusion
A rapid acceleration, such as experienced during takeoff,
stimulates the otolith organs in the same way as tilting the
head backwards. This action creates the somatogravic illusion
of being in a nose-up attitude, especially in situations without
good visual references. The disoriented pilot may push the
aircraft into a nose-low or dive attitude. A rapid deceleration
by quick reduction of the throttle(s) can have the opposite
effect with the disoriented pilot pulling the aircraft into a
nose-up or stall attitude.
Inversion Illusion
An abrupt change from climb to straight-and-level flight can
stimulate the otolith organs enough to create the illusion of
tumbling backwards or inversion illusion. The disoriented
pilot may push the aircraft abruptly into a nose-low attitude,
possibly intensifying this illusion.
Elevator Illusion
An abrupt upward vertical acceleration, as can occur in
an updraft, can stimulate the otolith organs to create the
illusion of being in a climb. This is called elevator illusion.
The disoriented pilot may push the aircraft into a nose-low
Skid, slip, and uncoordinated turns feel alike.
Pilots feel they are being forced sideways in their seat.
Level Coordinated turn Pull out
Level skid Forward slip Uncoordinated turn
Figure 3-8. Sensations from centrifugal force.
attitude. An abrupt downward vertical acceleration, usually
in a downdraft, has the opposite effect with the disoriented
pilot pulling the aircraft into a nose-up attitude.
Visual Illusions
Visual illusions are especially hazardous because pilots rely
on their eyes for correct information. Two illusions that lead
to spatial disorientation, false horizon and autokinesis, are
concerned with only the visual system.
False Horizon
A sloping cloud formation, an obscured horizon, an aurora
borealis, a dark scene spread with ground lights and stars,
and certain geometric patterns of ground lights can provide
inaccurate visual information, or false horizon, for aligning
the aircraft correctly with the actual horizon. The disoriented
pilot may place the aircraft in a dangerous attitude.
Autokinesis
In the dark, a stationary light will appear to move about when
stared at for many seconds. The disoriented pilot could lose
control of the aircraft in attempting to align it with the false
movements of this light called autokinesis.
Postural Considerations
The postural system sends signals from the skin, joints, and
muscles to the brain that are interpreted in relation to the
Earth’s gravitational pull. These signals determine posture.
Inputs from each movement update the body’s position to
the brain on a constant basis. “Seat of the pants” flying is
largely dependent upon these signals. Used in conjunction
with visual and vestibular clues, these sensations can be
fairly reliable. However, because of the forces acting upon
the body in certain flight situations, many false sensations
can occur due to acceleration forces overpowering gravity.
[Figure 3-8] These situations include uncoordinated turns,
climbing turns, and turbulence.
Demonstration of Spatial Disorientation
There are a number of controlled aircraft maneuvers a pilot
can perform to experiment with spatial disorientation. While
each maneuver normally creates a specific illusion, any false
sensation is an effective demonstration of disorientation. Thus,
even if there is no sensation during any of these maneuvers,
the absence of sensation is still an effective demonstration
in that it shows the inability to detect bank or roll. There are
several objectives in demonstrating these various maneuvers.
1. They teach pilots to understand the susceptibility of
the human system to spatial disorientation.
2. They demonstrate that judgments of aircraft attitude
based on bodily sensations are frequently false.
3. They help lessen the occurrence and degree of
disorientation through a better understanding of the
relationship between aircraft motion, head movements,
and resulting disorientation.
4. They help instill a greater confidence in relying on
flight instruments for assessing true aircraft attitude.
A pilot should not attempt any of these maneuvers at
low altitudes or in the absence of an instructor pilot or an
appropriate safety pilot.
Climbing While Accelerating
With the pilot’s eyes closed, the instructor pilot maintains
approach airspeed in a straight-and-level attitude for several
seconds, and then accelerates while maintaining straight-
and-level attitude. The usual illusion during this maneuver,
without visual references, is that the aircraft is climbing.
Climbing While Turning
With the pilot’s eyes still closed and the aircraft in a straight-
and-level attitude, the instructor pilot now executes, with a
relatively slow entry, a well-coordinated turn of about 1.5
positive G (approximately 50° bank) for 90°. While in the
turn, without outside visual references and under the effect of
the slight positive G, the usual illusion produced is that of a
climb. Upon sensing the climb, the pilot should immediately
open the eyes and see that a slowly established, coordinated
turn produces the same feeling as a climb.
Diving While Turning
Repeating the previous procedure, with the exception that
the pilot’s eyes should be kept closed until recovery from
the turn is approximately one-half completed can create this
sensation. With the eyes closed, the usual illusion is that the
aircraft is diving.
Tilting to Right or Left
While in a straight-and-level attitude, with the pilot’s eyes
closed, the instructor pilot executes a moderate or slight
skid to the left with wings level. This creates the illusion of
the body being tilted to the right. The same illusion can be
sensed with a skid to the right with wings level, except the
body feels it is being tilted to the left.
Reversal of Motion
This illusion can be demonstrated in any of the three planes of
motion. While straight and level, with the pilot’s eyes closed,
the instructor pilot smoothly and positively rolls the aircraft
to approximately a 45° bank attitude. This creates the illusion
of a strong sense of rotation in the opposite direction. After
this illusion is noted, the pilot should open his or her eyes
and observe that the aircraft is in a banked attitude.
Diving or Rolling Beyond the Vertical Plane
This maneuver may produce extreme disorientation. While
in straight-and-level flight, the pilot should sit normally,
either with eyes closed or gaze lowered to the floor. The
instructor pilot starts a positive, coordinated roll toward a
30° or 40° angle of bank. As this is in progress, the pilot
tilts his or her head forward, looks to the right or left, then
immediately returns his or her head to an upright position.
The instructor pilot should time the maneuver so the roll is
stopped as the pilot returns his or her head upright. An intense
disorientation is usually produced by this maneuver, and the
pilot experiences the sensation of falling downward into the
direction of the roll.
In the descriptions of these maneuvers, the instructor pilot is
doing the flying, but having the pilot do the flying can also
be a very effective demonstration. The pilot should close his
or her eyes and tilt their head to one side. The instructor pilot
tells the pilot what control inputs to perform. The pilot then
attempts to establish the correct attitude or control input with
eyes closed and head tilted. While it is clear the pilot has no
idea of the actual attitude, he or she will react to what the
senses are saying. After a short time, the pilot will become
disoriented, and the instructor pilot then tells the pilot to
look up and recover. The benefit of this exercise is the pilot
experiences the disorientation while flying the aircraft.
Coping with Spatial Disorientation
To prevent illusions and their potentially disastrous
consequences, pilots can:
1. Understand the causes of these illusions and remain
constantly alert for them. Take the opportunity to
understand and then experience spatial disorientation
illusions in a device, such as a Barany chair, a
Vertigon, or a Virtual Reality Spatial Disorientation
Demonstrator.
2. Always obtain and understand preflight weather
briefings.
3. Before flying in marginal visibility (less than 3 miles)
or where a visible horizon is not evident such as flight
over open water during the night, obtain training and
maintain proficiency in airplane control by reference
to instruments.
4. Do not continue flight into adverse weather conditions
or into dusk or darkness unless proficient in the use of
flight instruments. If intending to fly at night, maintain
night-flight currency and proficiency. Include cross-
country and local operations at various airfields.
5. Ensure that when outside visual references are used,
they are reliable, fixed points on the Earth’s surface.
6. Avoid sudden head movement, particularly during
takeoffs, turns, and approaches to landing.
7. Be physically tuned for flight into reduced visibility.
Ensure proper rest, adequate diet, and, if flying at
night, allow for night adaptation. Remember that
illness, medication, alcohol, fatigue, sleep loss, and
Water Refraction
Rain on the windscreen can create an illusion of being at a
higher altitude due to the horizon appearing lower than it is.
This can result in the pilot flying a lower approach.
Haze
Atmospheric haze can create an illusion of being at a greater
distance and height from the runway. As a result, the pilot
has a tendency to be low on the approach. Conversely,
extremely clear air (clear bright conditions of a high attitude
airport) can give the pilot the illusion of being closer than
he or she actually is, resulting in a high approach that may
cause an overshoot or go around. The diffusion of light due
to water particles on the windshield can adversely affect
depth perception. The lights and terrain features normally
used to gauge height during landing become less effective
for the pilot.
Fog
Flying into fog can create an illusion of pitching up. Pilots
who do not recognize this illusion often steepen the approach
quite abruptly.
Ground Lighting Illusions
Lights along a straight path, such as a road or lights on
moving trains, can be mistaken for runway and approach
lights. Bright runway and approach lighting systems,
especially where few lights illuminate the surrounding
terrain, may create the illusion of less distance to the
runway. The pilot who does not recognize this illusion will
often fly a higher approach.
How To Prevent Landing Errors Due to
Optical Illusions
To prevent these illusions and their potentially hazardous
consequences, pilots can:
1. Anticipate the possibility of visual illusions during
approaches to unfamiliar airports, particularly at night
or in adverse weather conditions. Consult airport
diagrams and the Airport/Facility Directory (A/FD)
for information on runway slope, terrain, and lighting.
2. Make frequent reference to the altimeter, especially
during all approaches, day and night.
3. If possible, conduct aerial visual inspection of
unfamiliar airports before landing.
4. Use Visual Approach Slope Indicator (VASI) or
Precision Approach Path Indicator (PAPI) systems
for a visual reference or an electronic glideslope,
whenever they are available.
mild hypoxia are likely to increase susceptibility to
spatial disorientation.
8. Most importantly, become proficient in the use of
flight instruments and rely upon them. Trust the
instruments and disregard your sensory perceptions.
The sensations that lead to illusions during instrument
flight conditions are normal perceptions experienced by
pilots. These undesirable sensations cannot be completely
prevented, but through training and awareness, pilots can
ignore or suppress them by developing absolute reliance
on the flight instruments. As pilots gain proficiency in
instrument flying, they become less susceptible to these
illusions and their effects.
Optical Illusions
Of the senses, vision is the most important for safe
flight. However, various terrain features and atmospheric
conditions can create optical illusions. These illusions
are primarily associated with landing. Since pilots
must transition from reliance on instruments to visual
cues outside the flight deck for landing at the end of an
instrument approach, it is imperative they be aware of the
potential problems associated with these illusions and take
appropriate corrective action. The major illusions leading
to landing errors are described below.
Runway Width Illusion
A narrower-than-usual runway can create an illusion the
aircraft is at a higher altitude than it actually is, especially
when runway length-to-width relationships are comparable.
[Figure 3-9A] The pilot who does not recognize this illusion
will fly a lower approach with the risk of striking objects
along the approach path or landing short. A wider-than-usual
runway can have the opposite effect with the risk of leveling
out high and landing hard or overshooting the runway.
Runway and Terrain Slopes Illusion
An upsloping runway, upsloping terrain, or both can create
an illusion the aircraft is at a higher altitude than it actually
is. [Figure 3-9B] The pilot who does not recognize this
illusion will fly a lower approach. Downsloping runways and
downsloping approach terrain can have the opposite effect.
Featureless Terrain Illusion
An absence of surrounding ground features, as in an
overwater approach, over darkened areas, or terrain made
featureless by snow, can create an illusion the aircraft is at
a higher altitude than it actually is. This illusion, sometimes
referred to as the “black hole approach,” causes pilots to fly
a lower approach than is desired.
Normal Approach
25 Normal Approach
Normal Approach
Normal Approach
Downsloping runway Upsloping runway
Upsloping runway Downsloping runway
Wider runway Narrower runway
Wider runwayNarrower runway
Normal approach Approach due to illusion
Figure 3-9A
Runway width illusion
• A narrower-than-usual runway can
created an illusion that the aircraft
is higher than it actually is, leading
to a lower approach.
• A wider-than-usual runway can
create an illusion that the aircraft is
lower than it actually is, leading to
a higher approach.
Figure 3-9B
Runway slope illusion
• A downsloping runway can create
the illusion that the aircraft is lower
than it actually is, leading to a
higher approach.
• An upsloping runway can create
the illusion that the aircraft is higher
than it actually is, leading to a lower
approach.
Figure 3-9. Runway width and slope illusions.
5. Utilize the visual descent point (VDP) found on many
nonprecision instrument approach procedure charts.
6. Recognize that the chances of being involved in an
approach accident increase when some emergency or
other activity distracts from usual procedures.
7. Maintain optimum proficiency in landing procedures.