2.  **Spatial Disorientation in Aviation:**

Definition of spatial orientation and disorientation

Basic anatomy and physiology of the vestibular system (semicircular
canals, otoliths) and proprioceptive receptors

Interactive role of the visual, vestibular, and proprioceptive systems
in maintaining spatial orientation on the ground

Visual aspects of spatial orientation (central vision -monocular
references and binocular references- and peripheral vision)

Visual illusions (aerial perspective, autokinetic, false visual
references, black hole approach, vection)

Vestibular aspects of spatial orientation (perception of changes in
angular and linear acceleration)

Vestibular illusions (the leans, graveyard spin, Coriolis, inversion,
head up, head down)

Preventive methods and countermeasures

Spatial disorientation training using Barany chair, Vertigon, GYRO, and
virtual reality devices

When a pilot does not know inflight where his or her body is in relation
to the surface of the

Earth, the pilot has spatial disorientation (S.D.). This is a dangerous
condition accounting for

approximately 5-16% of accidents and 6-26% of all aircraft fatalities
(depending upon the

study); 90% of S.D. accidents result in fatalities. These statistics
have not changed much over

the past four decades in spite of great efforts to prevent S.D.
occurrences.

Pilots rely on three organ systems to maintain orientation: the
vestibular apparatus, vision,

and proprioception. Of the three, vision is our most important modality.
When the pilot

is deprived of visual references, e.g., because of weather conditions or
darkness, the vestibular

apparatus and organs of proprioception are most vulnerable to producing
illusions.

There are three types of S.D. Type I occurs when the pilot is
disoriented but does not

recognize it. This is the most common type and most dangerous. On the
other hand, with Type II,

the pilot recognizes he or she is disoriented and, therefore, can
attempt corrective action. Hence,

it is less dangerous than Type I. Type III S.D. is recognized by the
pilot but corrective action

cannot be taken because of severe, incapacitating nystagmus (rapid,
uncontrolled eye

movements), making it impossible to read instruments or the emergency
check list.

The nystagmus is caused by overstimulation of the vestibulo-ocular
reflex (VOR).

There are nerve connections between the vestibular apparatus and
extraocular muscles that

allow us to have a sharp retinal image with un-blurred vision even when
we are in motion,

such as walking or running. If this reflex is overstimulated, e.g., by
aircraft motion, it can cause extreme nystagmus.

To gain an understanding of the vestibular causes of S.D., it is
necessary to appreciate the

anatomy and physiology of the inner ear, in particular the vestibular
apparatus, a structure that is

very important in our sense of equilibrium. The vestibular apparatus is
located in the petrous

portion of the temporal bone and measures just 1.5 cm., a remarkably
small structure for its

function. Each has three semicircular canals (anterior, posterior, and
horizontal) and an otolith

organ (or utricle-saccule). Within the semicircular canals and otolith
organs are hair cells that

protrude in a liquid mileu called endolymph.

As will be described below, if the endolymph is set in motion, e.g., by
head movement or turning in a car or airplane, the hair cells will be
bent by this mechanical force eliciting a neuroelectrical impulse to the
midbrain via the vestibular nerve (cranial nerve VIII). Thus, with a
turn, even if the eyes are closed, we know where we are in relation to
the Earth and what direction we are taking because of the signal sent to
the brain by the hair cells.

As marvelous as this system is on Earth, it can be a liability in flight
because, under certain

circumstances, the signals transmitted by the vestibular apparatus can
be misinterpreted by the

pilot resulting in vestibular illusions.

Before proceeding to a discussion of vestibular illusions, it is first
necessary to understand

the meanings of speed, velocity, and acceleration. Speed describes how
fast one is moving, e.g.,

in knots or mph. Velocity also describes how fast one is moving
(synonymous with speed in this

sense) but it can also describe direction, e.g., in degrees. Therefore,
velocity has two possible

components, speed and/or direction. Acceleration defines the rate of
[change]{.underline} of velocity.

Therefore, one is accelerating if there is a [change]{.underline} of
speed say from 150 to 160 knots and/or

[change]{.underline} of direction say from 018 to 022 degrees. When a
pilot accelerates (or decelerates), there

is an increased risk for S.D.

There are three types of acceleration; linear, radial, and angular.
Linear acceleration occurs

when there is a change only of speed (and not direction), e.g., take-off
roll or landing. Radial

acceleration applies to only a change of direction (and not speed),
e.g., in a turn. Angular

acceleration occurs when there is a change in both speed and direction,
a common maneuver in

aviation operations. In the vestibular apparatus, linear acceleration
excites the hair cells of the

otolith organ, while radial and angular acceleration excite those of the
semicircular canals.

The following example illustrates how a pilot can experience a
vestibular illusion.

If you are in conditions whereby you cannot see the ground and you begin
a turn to the left, the body goes to the left, but the liquid endolymph
in the inner ear goes in the opposite direction, thereby bending the
hair cells to the right.

To further illustrate, if you fill a balloon with water and pull it
forcefully in one direction, the water will tend to flow in the
"opposite" direction. This action sends an electrical impulse that
direction has been changed (i.e., there is acceleration) but you will
know you are turning left even if the eyes are closed, or there are
weather/night conditions denying visual reference with the ground,
because of the bending hair cells by the agitated endolymph. However,
once you are in a steady left turn and holding a steady air speed, the
endolymph will rebound and "catch up" to the body with the hair cells
eventually restored to their normal upright position.

At this point, the vestibular apparatus signals the pilot that flight is
straight and level (because the hair cells are upright) when actually
the aircraft is in a left turn. Without visual references, it would be
very difficult to overcome this illusion. This example of a vestibular
illusion illustrates the fallibility of the vestibular system in flight.

Now, suppose the same pilot (now in a left turn) rolls to the right to a
true straight and level

attitude. Again, the endolymph goes in the opposite direction, bending
the hair cells to the left.

This signals the pilot that he/she is in a right turn---again, a
vestibular illusion (because in

reality the aircraft is flying straight and level). This is a classical
illustration of the imperfection

of the vestibular apparatus---it is reliable on the Earth's surface but
in flight it can provide false

information, leading in many cases to a crash. Although there are a
number of variations of this

example, understanding the above principle will suffice to understand
S.D. induced by other

vestibular illusions.

Turning to specific vestibular illusions, there are a number that have
been well described in

the literature that have caused accidents. Among the more common ones
are the somatogravic,

oculogravic, somatogyral, oculogyral, and Coriolis illusions.

The somatogravic illusion can occur just after takeoff on climb into
total darkness or bad

weather when there are no lights beyond the end of the runway. As the
aircraft rotates and

becomes airborne, the pilot, denied any ground reference lights either
from the airport or nearby

towns, might have a sensation of a steep upward pitch. (The USAF air
base in Okinawa is

notorious for this because the runway ends just short of a cliff beyond
which is only ocean.) In

order to "compensate", the pilot may push the yolk/stick forward
resulting in a descent

or dive, which, at very low altitude, could result in a crash. Many
aircraft accidents have

been documented to be caused by the somatogravic illusion.

The oculogravic illusion is a close relative of the somatogravic because
it occurs under

similar conditions. In this case, however, the instruments appear to be
rising giving the pilot the

sensation of a very steep climb. The pilot might react the same way by
pushing the yolk/stick

forward in order to reduce the falsely perceived excessive rate of
upward pitch.

Another serious vestibular illusion is somatogyral illusion of which
there are two types:

the graveyard spin and the graveyard spiral. To illustrate the former,
imagine being in a left spin.

The direction of the spin is detectable even in darkness or heavy clouds
because the perturbations

of the endolymph bend the hair cells accordingly to the right. To break
the spin, the right rudder

pedal must be pushed. Once the left spin is successfully broken, the
endolymph will abruptly

flow in the opposite direction and bend the hair cells (to the left) now
giving the pilot the illusion

of a spin to the right when in reality the aircraft is flying straight
and level. The pilot, thinking

the aircraft is in a right spin will push the left rudder pedal
reentering the original left spin. The pilot will then attempt to break
this left spin by again activating the right rudder pedal. This

sequence might be repeated and repeated with a rapid loss of altitude to
ground impact.

Graveyard spin is a fitting epithet for this illusion. The graveyard
spiral is somewhat similar to the graveyard spin and is just as
dangerous. This illusion can occur when the aircraft enters a turn, say
to the left. Again, the turn can be interpreted correctly by the pilot
even if the ground cannot be seen because of the flow of endolymph
bending the hair cells to the right. However, once the turn is held
steady and the air speed is held constant, the endolymph will no longer
be agitated and the hair cells will assume a neutral upright position
giving the pilot the sensation that flight is straight and level.

If the pilot then decelerates, the endolymph will be disturbed bending
the hair cells to the left which will give the pilot the sensation of a
right turn. To "compensate", the pilot will increase the left turn that
will lower the nose, prompting an increase in power. Like the graveyard
spin, this sequence can repeat itself each time tightening the turn with
loss of altitude until ground impact.

In both cases, graveyard spin and spiral, the basic illusion is caused
by agitation of the endolymph and bending of the hair cells in a
situation in which visual references are denied, giving a false sense of
motion.

The oculogyral illusion is a correlate of the somatogyral in that it
occurs under the same circumstances of flight. With this illusion,
instruments appear to be drifting to the right or left in the opposite
direction of the flight path. This can induce the pilot to make
unnecessary and possibly dangerous control inputs.

The most provocative and dangerous illusion in aviation is the Coriolis
illusion because it can

cause a severe sensation of rolling, pitching, or tumbling. It is caused
when two of the three

semicircular canals are simultaneously stimulated. Classically it occurs
when the pilot in a turn

(which stimulates one semicircular canal) bends the head downward and
backward (which

stimulates the 2^nd^ semicircular canal). This has been reported when
the pilot turning final looks

downward and backward for a switch to make a radio frequency change.

The sudden sensation of rolling, pitching, or tumbling can be extremely
dangerous, especially at low altitudes. The Coriolis illusion can be
safely demonstrated in a Barany chair wherein the subject is asked to
place the chin on his or her chest while the swivel chair is turned in a
left or right rotation.

In general, vestibular illusions occur under conditions whereby the
pilot is unable to see the

ground. The risk is particularly increased at night, in clouds, or in
bad weather. These and other

conditions conducive to S.D. are found in Table XII.

TABLE XII

CONDITIONS CONDUCIVE TO SPATIAL DISORIENTATION

-False horizons

-Prolonged turning at a constant rate

-Head movement in a turn

-Instrument/weather conditions

-Night flying

-Formation flying

-Alcohol

-Illness/stress

Pilots are also subject to visual illusions. Good vision, in general, is
essential for pilots and

is particularly critical in our appreciation of S.D. Nevertheless, even
a pilot with 20/20 vision can

be fooled by a number of visual illusions (Table XIII).

For example, landing on an upsloping runway can give the illusion of a
high approach that could cause the pilot to land short. The opposite
could occur on approach to a downsloping runway, resulting in landing
long. A particularly dangerous illusion is the black hole effect that is
well known to many pilots. It can occur when coming to an airport,
usually in a semiremote or remote area, on a dark evening with no lights
in the surrounding area other than those on the runway. For some reason,
these circumstances have induced pilots to land short of the runway, at
times with fatal consequences.

Autokinesis is another illusion that also occurs at night. If a pilot
stares at a single light, e.g., on the wing or even a star, it might
appear to move. This could induce the pilot to mistake a star for an
aircraft and take unnecessary "corrective" action. In formation flying,
the false sensation of a moving wing tip light could cause a midair
collision.

Aerial perspective affects a pilot's ability to estimate distances while
inflight. Under hazy

conditions or at dusk or dawn, the ability to judge distances can be
significantly distorted.

Vection is a visual illusion that most of us have experienced while
stopped at a traffic light next

to another car. If the 2^nd^ car moves slowly forward, the driver of the
1^st^ car may feel like his or

her car is rolling backwards. Upon stomping on the brakes, it is then
realized the car was at a

standstill. This illusion can occur during formation flying, causing a
pilot to make inappropriate

control inputs.

TABLE XIII

VISUAL ILLUSIONS

-Upsloping Runway

-Downsloping Runway

-Black Hole Effect

-Autokinesis

-Aerial Perspective

-Vection

Proprioceptive stimulation can also cause illusions. For example, as the
aircraft climbs, there

is pressure on the buttocks indicating upward acceleration. However, if
there are no visual

references and the aircraft, for example, is in a steady turn at a
steady air speed, pulling back on

the yolk or stick will give the impression of entering a climb. But in
reality, this would not be

true if the aircraft were banked far to the right or left. In this case,
the control input would cause

only tightening of the turn and not a real climb.

For over 70 years, considerable research has gone into ways and means of
preventing S.D.

Undoubtedly these efforts have prevented some S.D. accidents, although
there still occurs an

unacceptable number, particularly in military and general aviation. It
remains one of the major

challenges of aviation safety. Education and training has been one of
the cornerstones of

prevention by raising the aviator's level of awareness of the deadliness
of S.D.

Materials, such as in this section, have been taught to pilots in
beefed-up safety programs. Training has also been provided in simulators
of varying sophistication so pilots can safely experience the various
visual and vestibular illusions on the ground. The earliest and most
primitive is the Barany chair as described above. Other commercial
devices commonly used now for training such as the GYRO or Vertigon are
cockpit simulators with the capacity to induce common illusions.

There have been further efforts, particularly in the military, by
increasing instrument flying training with an instructor pilot and
improving cockpit layout of instruments. Also, the headup

display (HUD) used in most military aircraft has been helpful.

At the present time, the most effective countermeasures for S.D. are
increased pilot

awareness, competency in instrument flying, and trusting the instruments
when S.D. is experienced. The various illusions as described can be so
overwhelming that some pilots have

trusted their senses before their instruments. This can lead to
disaster.

When there is conflict between the senses and the instruments, the oft
repeated adage is to trust your instruments. Besides S.D., we now speak
more broadly of situational awareness (S.A.) of which S.D. is but one
facet. A loss of S.A. is applied to any set of circumstances that causes
the pilot to become unaware of his or her environment. In simpler terms,
it means the pilot is not paying attention to all aspects of the flight.

More formally stated, situational awareness is a person's perception of
the elements in the environment, the comprehension of their meaning, and
the projection of their status in the near future. Consequently, an
added threat to flying safety is posed whenever a pilot's situational
awareness is degraded. This can occur through a number of mechanisms,
S.D as described above being just one.

Task saturation can result in loss of S.A. This might occur during an
in-flight emergency

when the pilot becomes so overloaded in taking corrective action that he
or she becomes

unaware of aircraft altitude or attitude in relation to the surface of
the Earth.

Channelized attention can also disrupt one's situational awareness. For
example, the pilot may focus on only one aspect of flight,e.g., map
reading, and in so doing, disregard other important aspects of flight
such as air speed, attitude, direction etc.

Other examples leading to loss of S.A. might include distraction, poor
communications, fatigue, and taking unauthorized medication.

One unusual form of situational awareness is the breakoff phenomenon.
This is best described as a dream-like or fugue state. It most often
occurs in pilots who are flying alone at high altitude at a time when
there are few cockpit chores. The pilot may then daydream to such an
extent that there is a mental disengagement from flight. Although this
most commonly occurs in military reconnaissance pilots when flying at
very high altitude, it can occur in general aviation as well.

Even though there is enough altitude for recovery in most cases, the
breakoff phenomenon is a potentially dangerous form of loss of
situational awareness.

In any event, situational awareness in all phases of flight is critical
for safety in flight. Too

frequently, lost situational awareness, regardless of type, has resulted
in accidents due to

controlled flight into terrain (CFIT). There are many such accidents on
record of a pilot with a

well functioning aircraft flying into the ground because he or she lost
situational awareness,

resulting in an unappreciated loss of altitude, leading to ground
impact.