Figure 17-4. Human sensation of angular acceleration.
No turning
No sensation.
Start of turn
Sensation of turning
as moving fluid deflects
hairs.
Constant rate turn
No sensation after fluid
accelerates to same
speed as tube wall.
Turn stopped
Sensation of turning in
opposite direction as moving
fluid deflects hairs in opposite
direction.
Endolymph
Cupola
Tube
Figure 17-5. Graveyard spiral.
Graveyard spin
Graveyard spiral
Vestibular Illusions
The Leans
A condition called the leans, is the most common illusion
during flight and is caused by a sudden return to level flight
following a gradual and prolonged turn that went unnoticed by
the pilot. The reason a pilot can be unaware of such a gradual
turn is that human exposure to a rotational acceleration of 2
degrees per second or lower is below the detection threshold
of the semicircular canals. [Figure 17-4] Leveling the wings
after such a turn may cause an illusion that the aircraft is
banking in the opposite direction. In response to such an
illusion, a pilot may lean in the direction of the original turn
in a corrective attempt to regain the perception of a correct
vertical posture.
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, creating the illusion
of turning or accelerating on an entirely different axis. This
action causes the pilot to think the aircraft is performing a
maneuver 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 may experience the illusion of not
turning. During the recovery to level flight, the pilot will
then experience the sensation of turning in the opposite
direction causing the disoriented pilot to 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; this illusion is referred to
as a “graveyard spiral.” [Figure 17-5] This may lead to a
loss of aircraft control.
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 may create what is known as
the “somatogravic illusion” of being in a nose-up attitude,
especially in conditions with poor 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.
Figure 17-6. Sensations from centrifugal force.
Skid, slip, and uncoordinated turns feel similar.
Pilots feel they are being forced sideways in their seat.
Level Coordinated turn Pull out
Level skid Forward slip Uncoordinated turn
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, known as “inversion illusion.” The
disoriented pilot may push the aircraft abruptly into a nose-
low attitude, which may intensify 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 known as “elevator illusion.”
The disoriented pilot may push the aircraft into a nose-low
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, affect
the visual system only.
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,” when
attempting to align the aircraft with the actual horizon.
The disoriented pilots as a result may place the aircraft in a
dangerous attitude.
Autokinesis
When flying in the dark, a stationary light may appear to
move if it is stared at for a prolonged period of time. As
a result, a pilot may attempt to align the aircraft with the
perceived moving light potentially causing him/her to lose
control of the aircraft. This illusion is known as “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 17-6] 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 because it illustrates the inability to detect
bank or roll.
aircraft to approximately 45° bank attitude while maintaining
heading and pitch 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 the 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 will tell the pilot to look
up and recover. This exercise allows the pilot to experience
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
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 aircraft control by reference
to instruments.
4. Do not fly into adverse weather conditions or into
dusk or darkness unless proficient in the use of flight
instruments. If intending to fly at night, maintain
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 decrease 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, 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 to see that a slowly established, coordinated
turn produces the same sensation as a climb.
Diving While Turning
Repeating the previous procedure, but with the pilot’s
eyes should be kept closed until recovery from the turn is
approximately one-half completed, can create the illusion of
diving while turning.
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.
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
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
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 that 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 that the
aircraft is at a higher altitude than it actually is, especially
when runway length-to-width relationships are comparable.
[Figure 17-7] 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
the pilot leveling out the aircraft high and landing hard or
overshooting the runway.
Runway and Terrain Slopes Illusion
An upsloping runway, upsloping terrain, or both can create an
illusion that the aircraft is at a higher altitude than it actually
is. [Figure 17-7] 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 that 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.
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
result in 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
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
Figure 17-7. Runway illusions.
25
Normal Approach
25 Normal Approach
2525
25
Normal Approach
25
Normal Approach
Downsloping runway Upsloping runway
Upsloping runway Downsloping runway
Wider runway Narrower runway
Wider runwayNarrower runway
Normal approach Approach due to illusion
Runway width illusion
• A narrower-than-usual runway can
create 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.
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.
diagrams and the Chart Supplement U.S. (formerly
Airport/Facility Directory) 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 an 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.
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 an emergency or
other activity distracts from usual procedures.
7. Maintain optimum proficiency in landing procedures.
In addition to the sensory illusions due to misleading inputs to
the vestibular system, a pilot may also encounter various visual
illusions during flight. Illusions rank among the most common
factors cited as contributing to fatal aviation accidents.
Sloping cloud formations, an obscured horizon, a dark scene
spread with ground lights and stars, and certain geometric
patterns of ground light can create illusions of not being
aligned correctly with the actual horizon. Various surface
features and atmospheric conditions encountered in landing
can create illusions of being on the wrong approach path.
Landing errors due to these illusions can be prevented by
anticipating them during approaches, inspecting unfamiliar
airports before landing, using electronic glideslope or VASI
systems when available, and maintaining proficiency in
landing procedures.
Motion Sickness
Motion sickness, or airsickness, is caused by the brain
receiving conflicting messages about the state of the body. A
pilot may experience motion sickness during initial flights, but
it generally goes away within the first few lessons. Anxiety
and stress, which may be experienced at the beginning of
flight training, can contribute to motion sickness. Symptoms
of motion sickness include general discomfort, nausea,
dizziness, paleness, sweating, and vomiting.
It is important to remember that experiencing airsickness is
no reflection on one’s ability as a pilot. If prone to motion
sickness, let the flight instructor know, there are techniques
that can be used to overcome this problem. For example,
avoid lessons in turbulent conditions until becoming more
comfortable in the aircraft or start with shorter flights and
graduate to longer instruction periods. If symptoms of motion
sickness are experienced during a lesson, opening fresh air
vents, focusing on objects outside the airplane, and avoiding
unnecessary head movements may help alleviate some of the
discomfort. Although medications like Dramamine can prevent
airsickness in passengers, they are not recommended while
flying since they can cause drowsiness and other problems.
Carbon Monoxide (CO) Poisoning
CO is a colorless and odorless gas produced by all internal
combustion engines. Attaching itself to the hemoglobin in
the blood about 200 times more easily than oxygen, CO
prevents the hemoglobin from carrying oxygen to the cells,
resulting in hypemic hypoxia. The body requires up to 48
hours to dispose of CO. If severe enough, the CO poisoning
can result in death. Aircraft heater vents and defrost vents
may provide CO a passageway into the cabin, particularly if
the engine exhaust system has a leak or is damaged. If a strong
odor of exhaust gases is detected, assume that CO is present.
However, CO may be present in dangerous amounts even
if no exhaust odor is detected. Disposable, inexpensive CO
detectors are widely available. In the presence of CO, these
detectors change color to alert the pilot of the presence of CO.
Some effects of CO poisoning are headache, blurred vision,
dizziness, drowsiness, and/or loss of muscle power. Any time
a pilot smells exhaust odor, or any time these symptoms are
experienced, immediate corrective action should be taken
including turning off the heater, opening fresh air vents and
windows, and using supplemental oxygen, if available.
Tobacco smoke also causes CO poisoning. Smoking at
sea level can raise the CO concentration in the blood and
result in physiological effects similar to flying at 8,000 feet.
Besides hypoxia, tobacco causes diseases and physiological
debilitation that can be medically disqualifying for pilots.
Stress
Stress is the body’s response to physical and psychological
demands placed upon it. The body’s reaction to stress includes
releasing chemical hormones (such as adrenaline) into the
blood and increasing metabolism to provide more energy
to the muscles. Blood sugar, heart rate, respiration, blood
pressure, and perspiration all increase. The term “stressor”
is used to describe an element that causes an individual to
experience stress. Examples of stressors include physical
stress (noise or vibration), physiological stress (fatigue), and
psychological stress (difficult work or personal situations).
Stress falls into two broad categories: acute (short term) and
chronic (long term). Acute stress involves an immediate
threat that is perceived as danger. This is the type of stress that
triggers a “fight or flight” response in an individual, whether
the threat is real or imagined. Normally, a healthy person can
cope with acute stress and prevent stress overload. However,
ongoing acute stress can develop into chronic stress.
Chronic stress can be defined as a level of stress that presents
an intolerable burden, exceeds the ability of an individual
to cope, and causes individual performance to fall sharply.
Unrelenting psychological pressures, such as loneliness,
financial worries, and relationship or work problems can
produce a cumulative level of stress that exceeds a person’s
ability to cope with the situation. When stress reaches these
levels, performance falls off rapidly. Pilots experiencing
this level of stress are not safe and should not exercise their
airman privileges. Pilots who suspect they are suffering from
chronic stress should consult a physician.