retroyou nostal(G) 2002 - 2003 : Illusions in Flight
Illusions in FlightVisual Illusions, Vestibular Illusions An illusion is a false percept. An orientational illusion is a false percept of one's position or motion--either linear or angular--relative to the plane of the earth's surface. A great number of orientational illusions occur during flight: some named, others unnamed; some understood, others not understood. Those that are sufficiently impressive to cause pilots to report them, whether because of their repeatability or because of their emotional impact, have been described in the aeromedical literature and will be discussed here. The illusions in flight are categorized into those resulting from visual misperceptions and those involving vestibular errors.Visual IllusionsShape Constancy, Size Constancy Aerial Perspective, Absent Focal Cues, Absent Ambient Cues, Autokinesis Vection Illusions, False Horizons and Surface Planes, Other False Ambient CuesWe shall organize the visual illusions in flight according to whether they involve primarily the focal mode of visual processing or primarily the ambient mode. Although this categorization is somewhat arbitrary and may seem too coarse in some cases, it serves to emphasize the dichotomous nature of visual orientation information processing. We begin with illusions involving primarily focal vision. (Many of the visual illusions related in this section were first described by Pitts.23) Shape ConstancyTo appreciate how false shape constancy cueing can create orientational illusions in flight, consider the example provided by a runway that is constructed on other than level terrain. Figure 14a shows the pilot's view of the runway during an approach to landing and demonstrates the linear perspective and foreshortening of the runway that the pilot associates with a 3° approach slope. If the runway slopes upward 1° (a rise of only 35 m in a 2-km runway), the foreshortening of the runway for a pilot on a 3° approach slope is substantially less (the height of the retinal image of the runway is greater) than it would be if the runway were level. This can give the pilot the illusion of being too high on the approach. The pilot's natural response to such an illusion is to reshape the image of the runway by seeking a shallower approach slope (Fig. 14b). This response, of course, could be hazardous. The opposite situation results when the runway slopes downward. To perceive the accustomed runway shape under this condition, the pilot flies a steeper approach slope than usual (Fig. 14c).
Figure 14. Effect of runway slope on the pilot's image of runway during final approach (left) and potential effect on the approach slope angle flown (right). a. Flat runway -- normal approach. b. An up-sloping runway creates the illusion of being high on approach -- pilot flies the approach too low. c. A down-sloping runway has the opposite effect. Size ConstancySize constancy is very important in judging distance, and false cues are frequently responsible for aircraft mishaps due to illusions of focal visual origin. The runway width illusions are particularly instructive in this context. Figure 15a shows the accustomed runway width and a normal approach. A runway that is narrower than that to which a pilot is accustomed can create a hazardous illusion on the approach to landing. Size constancy causes the pilot to perceive the narrow runway to be farther away (i.e., the aircraft is higher) than is actually the case; hence, the pilot may flare too late and touch fly a steeper approach than if the approach terrain were level (Fig. 16a). If the approach terrain slopes up to the runway, on the other hand, the pilot tends to fly a less steep approach (Fig. 16b). Although the estimation of height above the approach terrain depends on both focal and ambient vision, the contribution of focal vision is particularly clear: consider the pilot who looks at a building below and, seeing it to be closer than such buildings usually are, seeks a higher approach slope. By the same token, focal vision and size constancy are responsible for poor height and distance judgments pilots sometimes make when flying over terrain having an unfamiliar composition (Fig. 17). A reported example of this is the tendency to misjudge the approach height when landing in the Aleutians, where the evergreen trees are much smaller than those to which most pilots are accustomed. Such height-estimation difficulties are by no means restricted to the approach and landing phases of flight. One fatal mishap occurred during air combat training over the Southwest desert when the pilot of a high-performance fighter aircraft presumably misjudged the aircraft's height over the desert floor because of the small, sparse vegetation and was unable to arrest a deliberate descent to a ground-hugging altitude.
Figure 15.Effect of runway width on the pilot's image of runway (left) and the potential effect on approach flown (right). a. Accustomed width--normal approach. b. A narrow runway makes the pilot feel the aircraft is higher than it actually is, which results in the approach being too low and the flare being too late. c. A wide runway gives the illusion of being closer than it actually is--the pilot tends to approach too high and flares too soon.
Figure 16. Potential effect of the slope of the terrain under the approach on the approach slope flown. a. The terrain slopes down to the runway; the pilot thinks the approach is too shallow and steepens it. b. Upsloping terrain makes the pilot think the aircraft is too high, which is corrected by making the approach too shallow. Aerial PerspectiveAerial perspective also can play a role in deceiving the pilot, and the approach-to-landing regime again provides examples. In daytime, fog or haze can make a runway appear farther away as a result of the loss of visual discrimination. At night, runway and approach lights in fog or rain appear less bright than they do in clear weather and can create the illusion that they are farther away. It has even been reported that a pilot can have an illusion of being banked to the right, for example, if the runway lights are brighter on the right side of the runway than they are on the left. Another hazardous illusion of this type can occur during approach to landing in a shallow fog or haze, especially during a night approach. The vertical visibility under such conditions is much better than the horizontal visibility, so that descent into the fog causes the more distant approach or runway lights to diminish in intensity at the same time that the peripheral visual cues are suddenly occluded by the fog. The result is an illusion that the aircraft has pitched up, with the concomitant danger of a nose-down corrective action by the pilot. Absent Focal CuesA well-known pair of approach-to-landing situations that create illusions because of the absence of adequate focal visual orientation cues are the smooth-water (or glassy-water) and snow-covered approaches. In a seaplane, one's perception of height is degraded substantially when the water below is still. For that reason, a seaplane pilot routinely just sets up a safe descent rate and waits for the seaplane to touch down, rather than attempting to flare to a landing when the water is smooth. A blanket of fresh snow on the ground and runway also deprives the pilot of visual cues with which to estimate height above ground, thus making the approach extremely difficult. Again, approaches are not the only regime in which smooth water and fresh snow cause problems. A number of aircraft have crashed as a result of pilots maneuvering over smooth water or snow-covered ground and misjudging their height above the surface. Absent Ambient CuesTwo runway approach conditions that create considerable difficulty for the pilot, by requiring focal vision alone to accomplish what is normally accomplished with both focal and ambient vision, are the black-hole and whiteout approaches. A black-hole approach is one that is made on a dark night over water or unlighted terrain to a runway beyond which the horizon is indiscernible, the worst case being when only the runway lights are visible (Fig. 18). Without peripheral visual cues to help orient the aircraft relative to the earth, the pilot tends to feel that the aircraft is stable and situated appropriately but that the runway itself moves about or remains malpositioned (is down-sloping, for example). Such illusions make the black-hole approach difficult and dangerous, and often result in a landing far short of the runway. A particularly hazardous type of black-hole approach is one made under conditions wherein the earth is totally dark except for the runway and the lights of a city situated on rising terrain beyond the runway. Under these conditions, the pilot may try to maintain a constant vertical visual angle for the distant city lights, thus causing the aircraft to arc far below the intended approach slope as it gets closer to the runway (Fig. 19).24 An alternative explanation is that the pilot falsely perceives through ambient vision that the rising terrain is flat, which leads to a lower-than-normal approach. An approach made under whiteout conditions can be as difficult as a black-hole approach, and for essentially the same reason--lack of sufficient ambient visual orientation cues. There are actually two types of whiteout, the atmospheric whiteout and the blowing-snow whiteout. In the atmospheric whiteout, a snow-covered ground merges with a white overcast, creating a condition in which ground textural cues are absent and the horizon is indistinguishable. Although visibility may be unrestricted in the atmospheric whiteout, there is essentially nothing to see except the runway or runway markers; an approach made in this condition must therefore be accomplished with a close eye on the altitude and attitude instruments to prevent spatial disorientation and inadvertent ground contact. In the blowing-snow whiteout, visibility is restricted drastically by snowflakes, and often those snowflakes have been driven into the air by the propeller or rotor wash of the affected aircraft. Helicopter landings on snow-covered ground are particularly likely to create blowing-snow whiteouts. Typically, the helicopter pilot tries to maintain visual contact with the ground during the sudden rotor-induced whiteout, gets into an unrecognized drift to one side, and shortly thereafter contacts the ground with sufficient lateral motion to cause the craft to roll over. Pilots flying where whiteouts can occur must be made aware of the hazards of whiteout approaches, as the disorientation induced usually occurs unexpectedly under visual rather than instrument meteorological conditions. Another condition in which a pilot is apt to make a serious misjudgment is in closing on another aircraft at high speed. When the pilot has numerous peripheral visual cues by which to establish both the aircraft's position and velocity relative to the earth and the target's position and velocity relative to the earth, the difficulty of tracking and closing is not much different from what it would be on the ground giving chase to a moving quarry. When relative position and closure rate cues must come from foveal vision alone, however--as is generally the case at altitude, at night, or under other conditions of reduced visibility--the tracking and closing problem is much more difficult.
An overshoot, or worse, a midair collision, can easily result from the perceptual difficulties inherent in such circumstances, especially when the pilot lacks experience in an environment devoid of peripheral visual cues.
Figure 19. A common and particularly dangerous type of black-hole approach, in which the pilot falsely perceives the distant city to be on flat terrain and arcs below the desired approach slope A related phenomenon that pilots need especially to be aware of is the dip illusion. It occurs during formation flying at night, when one aircraft is in trail behind another. To avoid wake turbulence and maintain sight of the lead aircraft, the pilot in trail flies at a small but constant angle below the lead aircraft by placing the image of the lead aircraft in a particular position on the windscreen and keeping it there. Now suppose the pilot is told to "take spacing" (separate) to 5 nautical miles (10 km). For every 1° below lead the trailing pilot flies, he or she is lower than lead by 1.7% (sin 1°) of the distance behind lead. Thus, if the trail pilot is 2° below lead and keeps the image of the lead aircraft at the same spot on the windscreen all the way back to 5 miles, the trail aircraft will descend to about 1100 ft (350 m) below the lead aircraft. To make matters worse, when the aircraft in trail slows down to establish separation, its pitch attitude increases by several degrees, and if the pilot does not compensate for this additional angle and tries to maintain the lead aircraft image in the same relative position, the altitude difference between the two aircraft can be doubled or even tripled. In the absence of ambient visual orientation cues, the pilot cannot detect the large loss of altitude without monitoring the flight instruments, and may inadvertently "dip" far below the intended flight path. Clearly this situation would be extremely hazardous if it were to occur at low altitude or during maneuvers in which altitude separation from other aircraft is critical. One puzzling illusion that occurs when ambient visual orientation cues are minimal is visual autokinesis (Fig. 20). A small, dim light seen against a dark background is an ideal stimulus for producing autokinesis. After 6 to 12 seconds of visually fixating the light, one can observe it to move at up to 20° /sec in a particular direction or in several directions in succession, but there is little apparent displacement of the object fixated. In general, the larger and brighter the object, the less the autokinetic effect. The physiologic mechanism of visual autokinesis is not entirely understood. One suggested explanation for the autokinetic phenomenon is that the eyes tend to drift involuntarily, perhaps because of inadequate or inappropriate vestibular stabilization and that checking the drift requires efferent oculomotor activity having sensory correlates that create the illusion. Whatever the mechanism, the effect of visual autokinesis on pilots is of some importance. Anecdotes abound of pilots who fixate a star or a stationary ground light at night, and seeing it move because of autokinesis, mistake it for another aircraft and try to intercept or join up with it. Another untoward effect of the illusion occurs when a pilot flying at night perceives a relatively stable tracked aircraft to be moving erratically when in fact it is not; the unnecessary and undesirable control inputs the pilot makes to compensate for the illusory movement of the target aircraft represent increased work and wasted motion at best and an operational hazard at worst. To help avoid or reduce the autokinetic illusion, the pilot should try to maintain a well-structured visual environment in which spatial orientation is unambiguous. Because this is rarely possible in night flying, it has been suggested that (1) the pilot's gaze should be shifted frequently to avoid prolonged fixation of a target light; (2) the target should be viewed beside, through, or in some other reference to a relatively stationary structure such as a canopy bow; (3) the pilot should make eye, head, and body movements to try to destroy the illusion; and (4) as always, the pilot should monitor the flight instruments to help prevent or resolve any perceptual conflict. Equipping aircraft with more than one light or with luminescent strips to enhance recognition at night probably has helped reduce problems with autokinesis.
Figure
20. Visual autokinesis. A small, solitary light or small group of lights
seen in the dark can appear to move, when in fact they are stationary. Nearly everyone who drives an automobile has experiencedone very common linear vection illusion: when we are stopped at a stoplight and a large, presumably stationary vehicle in the adjacent lane creeps forward, a compelling illusion that our own car is creeping backward can result (prompting a swift but surprisingly ineffectual stomp on the brakes). Similarly, when one is sitting in a stationary train and the train on the adjacent track begins to move, a strong sensation that one's own train is moving in the opposite direction can be experienced (Fig. 21a). Linear vection is one of the things that make close formation flying so difficult, because the pilot can never be sure whether his or her own aircraft or that of the lead or wingman is responsible for the perceived relative motion. Angular vection occurs when peripheral visual cues convey the information that one is rotating; the perceived rotation can be in pitch, roll, yaw, or other plane. Although angular vection illusions are not common in life, they can be generated readily in a laboratory by enclosing a subject in a rotating striped drum. Usually within 10 seconds after the motion begins, the subject perceives that he or she rather than the drum is rotating. A pilot can experience angular vection if the anticollision light on the aircraft is left on during flight through clouds or the revolving reflection provides a strong ambient visual stimulus, rotation in the yaw plane. Another example of vection illusions is the so-called "Star Wars" named after the popular motion picture famous for its vection-inducing effects. This phenomenon involves linearly and angularly moving of ground lights off the curved inside surface of a fighter aircraft which create in the pilot disconcerting sensations of motion that conflict with the actual motion of the aircraft. Fortunately, vection illusions are not all bad. The most advanced simulators depend on linear and angular vection to create the illusion of angular motion (Fig. 21b). When the visual flight environment is dynamically portrayed by flight simulators with a wide field-of-view and infinity-optics, the illusion of actual flight is so compelling that additional mechanical motion is often not even needed (although mechanically generated motion-onset cues do seem to improve the fidelity of the simulation). False Horizons and Surface PlanesSometimes the horizon perceived through
ambient vision is not horizontal. Quite naturally, this misperception
of the horizontal creates illusions in flight. A sloping cloud deck, for example, is very difficult
to perceive as anything
but horizontal if it extends for any great distance into the peripheral
visual field (Fig. 22). Uniformly sloping terrain, particularly featureless terrain, can also create an illusion
of horizontality with disastrous results,
for the pilot thus deceived. Many aircraft have crashed as a result
of pilot's entering a canyon with an apparently level floor, only to
find that floor actually rose faster than the airplane could climb.
At night, the lights of a city built on sloping terrain can create the
false impression that the extended plane of the city lights
is the horizontal plane of the earth's surface as already noted (Fig.
19). A distant rain shower can obscure the real horizon and create the
impression of a horizon at the proximal edge (base) of the rainfall. If
the shower is seen just beyond the runway during an approach to landing,
the pilot can misjudge the pitch attitude of the aircraft and make inappropriate
pitch corrections on the approach.
Figure
21. Vection illusions. a. Linear vection. In this example, the adjacent
vehicle seen moving aft in the subject's peripheral vision causes the
sensation of moving forward. b. Angular vection. Objects seen revolving
around the subject in the flight simulator result in a perception of self-rotation
in the opposite direction--in this case, a rolling motion to the right.
Pilots are especially susceptible to misperception
of the horizontal while flying at night (Figs. 23a and 23b). Isolated
ground lights can appear to the pilot as stars, creating the perception
of a nose-high or one-wing-low attitude. Flying under such a false impression
can, of course, be fatal. Frequently, no stars are visible because of
overcast conditions. Unlighted areas of terrain can then blend with
the dark overcast to create the illusion that the unlighted terrain
is part of the sky. One extremely hazardous situation is that in which
a takeoff is made over an ocean or other large body of water that cannot
be distinguished visually from the night sky. Many pilots in this situation
have perceived the shoreline receding beneath them to be the horizon,
and some have responded to this false percept with disastrous nose-down
control input. Figure
22. A sloping cloud deck, which the pilot misperceives as a horizontal
surface.
Pilots flying at
high altitudes can sometimes experience difficulties in control of aircraft
attitude, because at high altitudes the horizon is lower with respect
to the plane of level flight than it is at the lower altitudes most pilots
are accustomed to flying. As a reasonable approximation, angle of depression
of the horizon in degrees equals the square root of altitude in kilometers.
A pilot flying at an altitude of 49,000 ft (15 km) sees the horizon almost
4° below the extension of the horizontal plane of aircraft. By visually
orienting to the view from the left cockpit window, a pilot might be inclined
to fly with the left wing 4° down to level it with horizon. If the pilot
does this and then looks out the right window, the wing would be seen
8° above the horizon, with half of that elevation due to the erroneous
control input. The pilot also might experience problems with pitch control
because the depressed horizon can cause a false perception of 4° nose-high
pitch attitude.
Figure 23. Misperception of the horizontal
at night. a. Ground lights appearing to be stars cause the
earth and sky to blend and a false horizon to be perceived. b. Blending
of overcast sky with unlighted terrain or water causes the horizon to
appear lower than is actually the case. One very important aspect of ambient visual
orientational cuing is in the stabilizing effect of the surrounding
instrument panel, glare shield, canopy bow or windshield frame, and especially
the reflections of panel and other cockpit structures off the windshield
or canopy at night. The stable visual provided by these objects tends
to cause the aircraft motion to appear not above the threshold for vestibular
motion though it may be well above the usual threshold for vestibular
motion. While flying at night or in instrument weather, a pilot may thus
have a sense of security because no motion is felt, due to the apparently
ambient visual environment. Of course, this falsely stabilizing effect
occurs when the visual environment contains the usually valid ambient
references (natural horizon, earth's surface, etc.).
Another result of false ambient visual
orientational cuing is the sun illusion. On the ground, we are accustomed
to seeing brighter surroundings above and darker below, regardless
of the position of the sun. The direction of this gradient in light intensity
thus helps us orient with respect to the surface of the earth. In clouds,
however, such a gradient generally does not exist, and when it does, the
lighter direction is generally toward the sun and the darker direction
is away from it. But the sun is not always directly overhead; as a consequence,
a pilot flying in a thin cloud layer may perceive falsely that the sun
is directly overhead. This prompts the pilot to bank in the direction
of the sun, hence the name of this illusion.
Finally, the disorienting effects of the
northern lights and of aerial flares should be mentioned. Aerial
refueling at night in high northern latitudes often is made quite difficult
by the northern lights, which provide false cues of verticality to the
pilot's peripheral vision. Similarly, when aerial flares are dropped,
they may drift with the wind, creating false cues of verticality. Their
motion also may create vection illusions. Another phenomenon associated
with use of aerial flares at night is the "moth" effect. The size of the
area on the ground illuminated by a dropped flare slowly decreases as
the flare descends. Because of the size constancy mechanism of visual
orientation discussed earlier, a pilot circling the illuminated area may
tend to fly in a descending spiral with gradually decreasing radius. Another
important factor is that the aerial flares can be so bright as to reduce
the apparent intensity of the aircraft instrument displays and thereby
minimize their orientational cuing strength.
Somatogyral Illusion, Oculogyral
Illusion, Coriolis
Illusion, Somatogravic
Illusion, Inversion
Illusion, G-Excess
Effect, Oculogravic
Illusions, The Leans
The vestibulocerebellar axis processes
orientation information from the vestibular, visual, and other sensory
systems. In the absence of adequate ambient visual orientation cues,
the inadequacies of the vestibular and other orienting senses can result
in orientational illusions. It is convenient and conventional to discuss
the vestibular illusions in relation to the two functional components
of the labyrinth that generate them--the semicircular ducts and the
otolith organs. In flight under conditions of reduced
visibility, somatogyral illusions can be deadly. The graveyard spin is
the classic example of how somatogyral illusions can disorient a pilot
with fatal results. This situation begins with the pilot intentionally
or unintentionally entering a spin, let's say to the left (Fig. 27). At
first, the pilot perceives the spin correctly because the angular acceleration
associated with entering the spin deviates the cupulae the appropriate
amount in the appropriate direction. The longer the spin persists, however,
the more the sensation of spinning to the left diminishes as the cupulae
return to their resting positions. On trying to stop the spin to the left
by applying the right rudder, the angular deceleration causes the pilot
to perceive a spin to the right, even though the only real result of this
action was termination of the spin to the left. A pilot who is ignorant
of the possibility of such an illusion is then likely to make counterproductive
left-rudder inputs to negate the unwanted erroneous sensation of spinning
to the right. These inputs keep the airplane spinning to the left, which
gives the pilot the desired sensation of not spinning but does not bring
the airplane under control. To extricate himself from this very hazardous
situation, the pilot must read the aircraft flight instruments and apply
control inputs to make the instruments give the desired readings (push
right rudder to center the turn needle, in this example). Unfortunately,
this may not be so easy to do. The angular accelerations created by both
the multiple-turn spin and the pilot's spin-recovery attempts can elicit
strong but inappropriate vestibulo-ocular reflexes, including nystagmus.
In the usual terrestrial environment, these reflexes help stabilize the
retinal image of the visual surround; in this situation, however, they
only destabilize the retinal image because the visual surround (cockpit)
is already fixed with respect to the pilot. Reading the flight instruments
thus becomes difficult or impossible, and the pilot is left with only
false sensations of rotation to rely on for spatial orientation and aircraft
control.26
Although the lore of early aviation
provided the graveyard spin as an illustration of the hazardous nature
of somatogyral illusions, a much more common example occurring all too
often in modern aviation is the graveyard spiral (Fig. 28). In this situation,
the pilot has intentionally or unintentionally entered into a prolonged
turn with a moderate amount of bank. After a number of seconds in the
turn, the pilot loses the sensation of turning because the cupula-endolymph
system cannot respond to constant angular velocity. The percept of being
in a bank as a result of the initial roll into the banked attitude also
decays with time because the net gravitoinertial force vector points toward
the floor of the aircraft during coordinated flight (whether the aircraft
is in a banked turn or flying straight and level), and the otolith organs
and other graviceptors normally signal that down is in the direction of
the net sustained gravitoinertial force. As a result, when trying to stop
the turn by rolling back to a wings-level attitude, the pilot feels not
only a turning in the direction opposite to that of the original turn,
but also a bank in the direction opposite to that of the original bank.
Unwilling to accept this sensation of making the wrong control input,
the hapless pilot rolls back into the original banked turn. Now the pilot's
sensation is compatible with a desired mode of flight, but the flight
instruments indicate a loss of altitude (because the banked turn is wasting
lift) and a continuing turn. So the pilot pulls back on the stick and
perhaps adds power to arrest the unwanted descent and regain the lost
altitude. This action would be successful if the aircraft were flying
wings-level, but with the aircraft in a steeply banked attitude it tightens
the turn, serving only to make matters worse. Unless the pilot eventually
recognizes what is occurring and rolls out of the unperceived banked turn,
the aircraft will continue to descend in an ever-tightening spiral toward
the ground, hence the name graveyard spiral.
Figure 27. The Graveyard Spin. After
several turns of a spin the pilot begins to lose the sensation of spinning.
When trying to stop the spin, the resulting somatogyral illusion of
spinning in the opposite direction makes the pilot reenter the original
spin. (The solid line indicates actual motion; the dotted line indicates
perceived motion. illusion of spinning in the opposite
direction makes the pilot reenter the original spin. (The solid line
indicates actual motion; the dotted line indicates perceived motion. Figure
28. The Graveyard Spiral. The pilot in a banked turn loses
the sensation of being banked and turning. Upon trying to establish
a wings-level attitude and stop the turn, the pilot perceives a bank
and a turning in the opposite direction from the original banked turn.
Unable to tolerate the sensation of making an inappropriate control
input, the pilot banks back into the original turn.
Whereas a somatogyral illusion is
a false sensation, or lack of sensation, of self-rotation in a subject
undergoing unusual angular motion, an oculogyral illusion is a false sensation
of motion of an object viewed by such a subject.27 For example, if a vehicle with a subject inside is rotating about a vertical
axis at a constant velocity and suddenly stops rotating, the subject experiences
not only a somatogyral illusion of rotation in the opposite direction,
but also an oculogyral illusion of an object in front moving in the opposite
direction. Thus, a somewhat oversimplified definition of the oculogyral
illusion is that it is the visual correlate of the somatogyral illusion;
however, its low threshold and lack of total correspondence with presumed
cupular deviation suggest a more complex mechanism. The attempt to maintain
visual fixation during a vestibulo-ocular reflex elicited by angular acceleration
is probably at least partially responsible for the oculogyral illusion.
In an aircraft during flight at night or in weather, an oculogyral illusion
generally confirms a somatogyral illusion: the pilot who falsely perceives
a turning in a particular direction also observes the instrument panel
to move in the same direction.
Coriolis Illusion The vestibular Coriolis effect, also
called the Coriolis cross-coupling effect, vestibular cross-coupling
effect, or simply the Coriolis illusion, is another false percept that
can result from unusual stimulation of the semicircular duct system.
To illustrate this phenomenon, let us consider a subject who has been
rotating in the plane of the horizontal semicircular ducts {roughly
the yaw plane) long enough for the endolymph in those ducts to attain
the same angular velocity as the head: the cupulae in the ampullae of
the horizontal ducts have returned to their resting positions, and the
sensation of rotation has ceased {Fig. 29a). If the subject then nods
forward in the pitch plane, let's say a full 90° for the sake of simplicity,
the horizontal semicircular ducts are removed from the plane of rotation
and the two sets of vertical semicircular ducts are inserted into the
plane of rotation {Fig. 29b). Although the angular momentum of the subject's
rotating head is forcibly transferred at once out of the old plane of
rotation relative to the head, the angular momentum of the endolymph
in the horizontal duct is dissipated more gradually. The torque resulting
from the continuing rotation of the endolymph causes the cupulae in
the horizontal ducts to be deviated, and a sensation of angular motion
occurs in the new plane of the horizontal ducts--now the roll plane
relative to the subject's body. Simultaneously, the endolymph in the
two sets of vertical semicircular ducts must acquire angular momentum
because these ducts have been brought into the plane of constant rotation.
The torque required to impart this change in momentum causes deflection
of the cupulae in the ampullae of these ducts, and a sensation of angular
motion in this plane--the yaw plane relative to the subject's body--results.
The combined effect of the cupular deflection in all three sets of semicircular
ducts is that of a suddenly imposed angular velocity in a plane in which
no actual angular acceleration relative to the subject has occurred.
In the example given, if the original constant-velocity yaw is to the
right and the subject's head pitches forward, the resulting Coriolis
illusion experienced is that of suddenly rolling and yawing to the right.
Figure
29. Mechanism of the Coriolis illusion. A subject
rotating in the yaw plane long enough for the endolymph to stabilize
in the horizontal semicircular duct (a) pitches the head forward (b).
Angular momentum of the endolymph causes the cupula to deviate, and
the subject perceives rotation in the new (i.e., roll) plane of the
horizontal semicircular duct, even though no actual rotation occurred
in that plane. A particular perceptual
phenomenon experienced occasionally by pilots of relatively high-performance
aircraft during instrument flight has been attributed to the Coriolis
illusion because it occurs in conjunction with large movements of the
head under conditions of prolonged constant angular velocity. It consists
of a sensation of rolling and/or pitching that appears suddenly after
the pilot's attention has been diverted from the instruments in front
and his or her head is moved to view some switches or displays elsewhere
in the cockpit. This illusion is especially deadly because it is most
likely to occur during an instrument approach, a phase of flight in which
altitude is being lost rapidly and cockpit chores (e.g., radio frequency
changes) repeatedly require the pilot to break up his instrument cross-check.
The sustained angular velocities associated with instrument flying are
insufficient to create Coriolis illusions of any great magnitude, however;28 and another mechanism (the G-excess effect) has been proposed to explain
the illusory rotations experienced with head movements in flight.29 Even if not responsible for spatial disorientation in flight, the Coriolis
illusion is useful as a tool to demonstrate the fallibility of our nonvisual
orientation senses. Nearly every military pilot living today has experienced
the Coriolis illusion in the Barany chair or some other rotating device
as part of physiological training, and for most of these pilots it was
then they first realized that their own orientation senses really cannot
be trusted--the most important lesson of all for instrument flying.
Somatogravic
Illusion The otolith organs are responsible
for a set of illusions known as somatogravic illusions. The mechanism
of illusions of this type involves the displacement of otolithic membranes
on their maculae by inertial forces so as to signal a false orientation
when the resultant gravitoinertial force is perceived as gravitational
(and therefore vertical). Thus, a somatogravic illusion can be defined
as a false sensation of body tilt that results from perceiving as vertical
the direction of a nonvertical gravitoinertial force. The illusion of
pitching up after taking off into conditions of reduced visibility is
perhaps the best illustration of this mechanism. Consider the pilot of
a high-performance aircraft waiting at the end of the runway to take off.
Here, the only force acting on the otolithic membranes is the force of
gravity, and the positions of those membranes on their maculae signal
accurately that down is toward the floor of the aircraft. Suppose the
aircraft now accelerates on the runway, rotates, takes off, cleans up
gear and flaps, and maintains a forward acceleration of 1 g until reaching
the desired climb speed. The 1 G of inertial force resulting from the
acceleration displaces the otolithic membranes toward the back of the
pilot's head. In fact, the new positions of the otolithic membranes are
nearly the same as they would be if the aircraft and pilot had pitched
up 45°, because the new direction of the resultant gravitoinertial force
vector, if one neglects the angle of attack and climb angle, is 45° aft
relative to the gravitational vertical (Fig. 30). Naturally, the pilot's
percept of pitch attitude based on the information from the otolith organs
is one of having pitched up 45°; and the information from nonvestibular
proprioceptive and cutaneous mechanoreceptive senses supports this false
percept, because the sense organs subserving those modalities also respond
to the direction and intensity of the resultant gravitoinertial force.
Given the very strong sensation of a nose-high pitch attitude, one that
is not challenged effectively by the focal visual orientation cues provided
by the attitude indicator, the pilot is tempted to push the nose of the
aircraft down to cancel the unwanted sensation of flying nose-high. Pilots
succumbing to this temptation characteristically crash in a nose-low attitude
a few miles beyond the end of the runway. Sometimes, however, they are
seen to descend out of the overcast nose-low and try belatedly to pull
up, as though they suddenly regained the correct orientation upon seeing
the ground again. Pilots of carrier-launched aircraft need to be especially
wary of the somatogravic illusion. These pilots experience pulse accelerations
lasting 2 to 4 seconds and generating peak inertial forces of +3 to +5
Gx. Although the major acceleration is over quickly, the resulting illusion
of nose-high pitch can persist for half a minute or more afterward, resulting
in a particularly hazardous situation for the pilot who is unaware of
this phenomenon. 30
Figure 30. A somatogravic illusion
occurring on takeoff. The inertial force resulting from the
forward acceleration combines with the force of gravity to create a
resultant gravitoinertial force directed down and aft. The pilot, perceiving
down to be in the direction of the resultant gravitoinertial force,
feels the aircraft is in an excessively nose-high attitude. Do not be misled by the above example
into believing that only pilots of high-performance aircraft suffer the
somatogravic illusion of pitching up after takeoff. More than a dozen
air transport aircraft are believed to have crashed as a result of the
somatogravic illusion occurring on takeoff.31 A relatively slow aircraft, accelerating from 100 to 130 knots over a
l0-second period just after takeoff, generates +0.16 Gx on the pilot.
Although the resultant gravitoinertial force is only 1.01 G, barely perceptibly
more than the force of gravity, it is directed 9° aft, signifying to the
unwary pilot a 9° nose-up pitch attitude. Because many slower aircraft
climb out at 6° or less, a 9° downward pitch correction would put such
an aircraft into a descent of 3° or more--the same as a normal final-approach
slope. In the absence of a distinct external visual horizon or, even worse,
in the presence of a false visual horizon (e.g., a shoreline) receding
under the aircraft and reinforcing the vestibular illusion, the pilot's
temptation to push the nose down can be overwhelming. This type of mishap
has happened at one particular civil airport so often that a notice has
been placed on navigational charts cautioning pilots flying from this
airport to be aware of the potential for loss of attitude reference.
Although the classic graveyard spiral
was indicated earlier to be a consequence of the pilot's suffering a somatogyral
illusion, it also can be said to result from a somatogravic illusion.
A pilot who is flying "by the seat of the pants" applies the necessary
control inputs to create a resultant G-force vector having the same magnitude
and direction as that which the desired flight path would create. Unfortunately,
any particular G vector is not unique to one particular condition of aircraft
attitude and motion, and the likelihood that the G vector created by a
pilot flying in this mode corresponds for more than a few seconds to the
flight condition desired is remote indeed. Specifically, once an aircraft
has departed a desired wings-level attitude because of an unperceived
roll, and the pilot does not correct the resulting bank, the only way
to create a G vector that matches the G vector of the straight and level
condition is with a descending spiral. In this condition, as is always
the case in a coordinated turn, the centrifugal force resulting from the
turn provides a Gy force that cancels the Gy component of the force of
gravity that exists when the aircraft is banked. In addition, the tangential
linear acceleration associated with the increasing airspeed resulting
from the dive provides a +Gx force that cancels the -Gx component of the
gravity vector that exists when the nose of the aircraft is pointed downward.
Although the vector analysis of the forces involved in the graveyard spiral
is somewhat complicated, a skillful pilot can easily manipulate the stick
and rudder pedals to cancel all vestibular and other nonvisual sensory
indications that the aircraft is turning and diving. In one mishap involving
a dark-night takeoff of a commercial airliner, the recorded flight data
showed that the resultant G force which the pilot created by his control
inputs allowed him to perceive his desired 10° to 12° climb angle and
a net G force between 0.9 and 1.1 G for virtually the whole flight, even
though he actually leveled off and then descended in an accelerating spiral
until the aircraft crashed nearly inverted.
The inversion illusion is a type of somatogravic
illusion in which the resultant gravitoinertial force vector rotates
backward so far as to be pointing away from rather than toward the earth's
surface, thus giving the pilot the false sensation of being upside down.
Figure 31 shows how this can happen. Typically, a steeply climbing high-performance
aircraft levels off more or less abruptly at the desired altitude. This
maneuver subjects the aircraft and pilot to a -Gz centrifugal force
resulting from the arc flown just prior to level-off. Simultaneously,
as the aircraft changes to a more level attitude, airspeed picks up
rapidly, adding a +Gx tangential inertial force to the overall force
environment. Adding the -Gz centrifugal force and the +Gx tangential
force to the 1-G gravitational force results in a net gravitoinertial
force vector that rotates backward and upward relative to the pilot. This
stimulates the pilot's otolith organs in a manner similar to the way a
pitch upward into an inverted position would. Even though the semicircular
ducts should respond to the actual pitch downward, for some reason this
conflict is resolved in favor of the otolith-organ information, perhaps
because the semicircular-duct response is transient while the otolith-organ
response persists, or perhaps because the information from the other mechanoreceptors
reinforces the information from the otolith organs. The pilot who responds
to the inversion illusion by pushing forward on the stick to counter the
perceived pitching up and over backward only prolongs the illusion by
creating more -Gz and +Gx forces, thus aggravating the situation. Turbulent
weather usually contributes to the development of the illusion; certainly,
downdrafts are a source of -Gz forces that can add to the net gravitoinertial
force producing the inversion illusion. Again, do not assume one must
be flying a jet fighter to experience this illusion. Several reports of
the inversion illusion involve crews of large airliners who lost control
of their aircraft because the pilot lowered the nose inappropriately after
experiencing the illusion. Jet upset is the name for the sequence of events
that includes instrument weather, turbulence, the inability of the pilot
to read the instruments, the inversion illusion, a pitch-down control
input, and difficulty recovering the aircraft because of resulting aerodynamic
or mechanical forces.33
Figure
31. The inversion illusion. Centrifugal and tangential inertial
forces during a level-off combine with the force of gravity to produce
a resultant gravitoinertial force that rotates backward and upward with
respect to the pilot, causing a false percept of suddenly being upside
down. Turbulent weather can produce additional inertial forces that
contribute to the illusion. (Adapted from Martin and Jones. 32) Figure 32. Mechanism of G-excess
illusion. In this oversimplified illustration, the subject
in a 1-G environment (upper half of figure) experiences the result of
a 0.5-G pull on the utricular otolithic membranes when the head is tilted
30° off the vertical, and the result of a 1-G pull when the head is
tilted a full 90°. The subject in a 2-G environment (lower half of figure)
experiences the result of a 1-G pull when the head is tilted only 30°.
The illusory excess tilt perceived by the subject is attributed to external
forces (lower right). Note that the actual plane or the utricular macula
slopes 20-30° upward. Figure 33. The G-excess illusion
during a turn in flight. G-induced excessive movement of the
pilot's otolithic membranes causes the pilot to feel an extra amount
of head and body tilt, which is interpreted as an underbank of the aircraft
when looking up to the inside of the turn. Correcting for the illusion,
the pilot overbanks the aircraft and it descends. The elevator illusion was originally
thought of as a visual phenomenon like the oculogravic illusion, except
that the false percept was believed to result from a vestibulo-ocular
reflex generated by a change in magnitude of the +Gz force instead of
by a change in its direction. When an individual is accelerated upward,
as in an elevator, the increase in +Gz force elicits a vestibulo-ocular
reflex of otolith-organ origin (the elevator reflex) that drives the eyes
downward. Attempting to stabilize visually the objects in a fixed position
relative to the observer causes those objects to appear to shift upward
when the G force is increased. The opposite effect occurs when the individual
is accelerated downward; the reduction in the magnitude of the net gravitoinertial
force to less than +1 Gz causes a reflex upward shift of the direction
of gaze, and the immediate surroundings appear to shift downward. (The
latter effect also has been called the oculoagravic illusion because of
its occurrence during transient weightlessness.) Although the described
visual effect undoubtedly contributes to the expression of the elevator
illusion, it is not essential for its generation, since the illusion can
occur even in the absence of vision, as just noted.
The
Leans By far the most common vestibular
illusion in flight is the leans. Virtually every instrument-rated pilot
has had or will get the leans in one form or another at some time during
his or her flying career. The leans consists of a false percept of angular
displacement about the roll axis (i.e., an illusion of bank) and is frequently
associated with a vestibulospinal reflex, appropriate to the false percept,
that results in the pilot's actually leaning in the direction of the falsely
perceived vertical (Fig. 34). The usual explanations of the leans invoke
the known deficiencies of both otolith-organ and semicircular-duct sensory
mechanisms. As indicated previously, the otolith organs are not reliable
sources of information about the exact direction of the true vertical
because they respond to the resultant gravitoinertial force, not to gravity
alone. Furthermore, other sensory inputs can sometimes override otolith-organ
cues and result in a false perception of the vertical, even when the gravitoinertial
force experienced is truly vertical. The semicircular ducts provide false
inputs in flight by responding accurately to some roll stimuli but not
responding at all to others because they are below threshold. For example,
a pilot who is subjected to an angular acceleration in roll so that the
product of the acceleration and its time of application does not reach
some threshold value, say 2°/sec, does not perceive the roll. Suppose
that this pilot, who is trying to fly straight and level, is subjected
to an unrecognized and uncorrected 2°/sec roll for 10 seconds: a 20° bank
results. If the unwanted bank becomes suddenly apparent and is corrected
by rolling the aircraft back upright with a suprathreshold rate, the pilot
experiences only half of the actual roll motion that took place--the half
resulting from the correcting roll. As the aircraft started from a perceived
wings-Ievel position, the pilot upon returning to an actual wings-Ievel
attitude is left with the illusion of having rolled into a 20° bank in
the direction of the correcting roll and experiences the leans. Even though
the pilot may be able to fly the aircraft properly by the deliberate and
difficult process of forcing the attitude indicator to read correctly,
the leans can last for many minutes, seriously degrading flying efficiency
during that time. Figure
34. The leans, the most common of all vestibular illusions in night. Falsely perceived to be in a right bank, but flying the aircraft straight
and level by means of the flight instruments, this pilot is leaning to
the left in an attempt to assume an upright posture compatible with the
illusion of bank.
Interestingly,
pilots frequently get the leans after prolonged turning maneuvers and
not because of alternating subthreshold and suprathreshold angular motion
stimuli. In a holding pattern, for example, the pilot rolls into a 3°/sec
standard-rate turn, holds the turn for 1 minute, rolls out and flies straight
for 1 minute, turns again for 1 minute, and so on until traffic conditions
permit the continuation of the flight toward its destination. During the
turning segments, the pilot initially feels the roll into the turn and
accurately perceives the banked attitude. But as the turn continues, the
percept of being in a banked turn dissipates and is replaced by a feeling
of flying straight with wings level, both because the sensation of turning
is lost when the endolymph comes up to speed in the semicircular ducts
(somatogyral illusion) and because the net G force being directed toward
the floor of the aircraft provides a false cue of verticality (somatogravic
illusion). Upon rolling out of the turn, the pilot's perception is of
a banked turn in the opposite direction. With experience, a pilot learns
to suppress this false sensation quickly by paying strict attention to
the attitude indicator. Sometimes, however, pilots cannot dispel the illusion
of banking--usually when they are particularly busy, unfortunately. The
leans also can be caused by misleading peripheral visual orientation cues,
as mentioned in the section entitled "Visual Illusions." Roll vection
is particularly effective in this regard, at least in the laboratory.
One thing about the leans is apparent: there is no single explanation
for this illusion. The deficiencies of several orientation-sensing systems
in some cases reinforce each other to create the illusion; in other cases,
the inaccurate information from one sensory modality for some reason is
selected over the accurate information from others to create the illusion.
Stories have surfaced of pilots suddenly experiencing the leans for no
apparent reason at all, or even of experiencing it voluntarily by imagining
the earth to be in a different direction from the aircraft. The point
is that one must not think that the leans, or any other illusion for that
matter, occurs as a totally predictable response to a physical stimulus;
there is much more to perception than stimulation of the end-organs.
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