As a final note, a flight simulator does not duplicate the aircraft movements in flight to give you the sensation of movement. You can’t tell by looking from the outside what the pilot is experiencing inside. The simulator is fooling the vestibular and otolith system by cleverly creating motions that generate sen- sory perceptions of just about every activity of an airplane. Add vision, and the effect is very real.
Having defined the different situations leading to disorientation, it’s now real- istic to return to traditional terms of spatial disorientation and vertigo in pilot conversation. Just remember that these terms are now generic.
VISUAL ILLUSIONS
Visual illusions affect what the pilot perceives solely through vision, which in turn determines how he or she will respond. Illusions of this kind are a greater source of misguidance in a flying activity such as judging landing height and distances), as opposed to the “illusions” of physiological disorientation described above, where the body automatically responds to physiological cues from the vestibular system and peripheral vision. In other words, visual cues alone can be deceptive without physiological inputs. They are obviously related in that they both can lead to incapacitation especially when the events happen concurrently.
To appreciate the impact that the combination of the semicircular canals has on orientation, try this exercise. Hold your hand straight ahead of your eyes with two fingers spread apart. Now move your hand sideways, back and forth at an increasing rate. Note that it doesn’t take much motion to see the fingers as a blur. Now keep the hand still and move your head sideways, back and forth at an increasing rate. Note how the fingers stay in focus no matter how fast you move your head.
The reason? Semicircular canals act as a gyro, allowing the brain to lock the eyes on a moving object no matter how much or where the head and body moves.
This ability of the eye to fixate on an object is a powerful reflex to motion and difficult to overcome when subjected to some of the following illusions.
VISUAL CUES FOR SPEED, DISTANCE, AND DEPTH OF FIELD
• Comparative sizes of known objects at different distances.
• Relative velocity of images moving across the retina. Nearby appears faster.
• Interposition of known objects. Objects in front of another perceived as closer.
• More distant objects are seen as bluish and blurry.
• Fog, haze, smoke, and smog obscuring horizon, reducing orientation cues.
GOTCHA!
146 Orientation
If the eye can focus and lock onto a fixed object outside, ideally the horizon, the pilot usually can overcome most of the disorienting situations. Vision is a reliable source of information; however, in the absence of outside visual cues, such as in instrument conditions, the chance for disorientation greatly increases.
Autokinetic illusions Autokinesis is the perception of false movement when a static source of light is looked at by the pilot for a period of time (minutes) in the dark (Fig. 8-12). This moving reference point (an illusion) could lead the pilot to visually follow it. It is felt that the cause is the brain’s and eyes’
attempts to find some other point of reference in an otherwise featureless visual field. Prevention is a combination of realizing the eyes must focus on other objects at varying distances, not fixating on one target, and basic scanning.
Oculogyral illusions This effect was previously described under disorienta- tion. From the visual point of view, this illusion is more apparent at night, when vision is already compromised. The pilot perceives the apparent move- ment of a distant object in the visual field as a result of stimulation of the semicircular canals; therefore, in the early stages of a turn to the left, for example, the target appears to move rapidly to the left. After the turn is established and then stopped, the target object appears to move in the oppo- site direction, to the right. The pilot might not correct her aircraft accordingly unless she is scanning her instruments and confirming that she is tracking a moving illusion.
Oculogravic illusions Also called somatogravic illusions, these are perceptions of tilt induced by stimulation of the otolith organs. From the visual perspective, the illusion to the pilot is either a climb or a descent when he has no visual
Figure 8-12
Figure 8-12 To a pilot suffering from autokinesis illusion, a stationary light that is stared at for several seconds in the dark will appear to move.
Visual illusions 147 reference to the horizon. With a climb, the eyes try to compensate with a down- ward tracking, as is common when flying through an updraft. This illusion is also called an elevator illusion. The opposite illusion, familiar in helicopter flight in an autorotation, is the intuitive response of the pilot to change direction and/
or altitude, which can decrease airspeed below a desired level.
Visual-cue illusions In addition to how the body responds (disorientation) to cues from a variety of sources, there are also illusions that are strictly visual that mislead the pilot into erroneous actions. This is not a physiological event as associated with the semicircular canals. This is the brain’s misinterpreting an image as a result of misleading visual cues as transmitted to and received by the eye. The greatest chance of these illusions affecting flight is in the landing phase and, to a lesser degree, in cruise where outside visual cues can be misleading.
Several illusions are explained throughout this text, but here only those gen- erated by what is seen, not affected by our vestibular system, for example, are discussed. The objective is to concentrate on just the illusions as they pertain to actual flight.
Fog and haze Depth perception, the ability of the brain to determine relative dis- tance from visual cues, is compromised by any atmospheric conditions that interfere with light transmission. In addition to obscuring the ground and nearby outside obstacles for avoidance and orientation, fog and haze refract light rays differently than clear air, leading to the eyes perceiving the target object as being out of focus or poorly related to known characteristics of that target.
Contrast, which is important in defining size and shape, is reduced. It becomes difficult to judge distances, especially height above ground and distance from the end of the runway. Light from ground lights, including REIL, VASI, and runway and taxiway lights, are somewhat diffused, losing the visual detail necessary for a precision approach and landing. Penetration into fog can cre- ate the illusion of pitching up.
Water refraction Rain on the windshield refracts incoming light rays, which misleads the pilot concerning the position of the horizon. The perception is that the horizon is below where it actually is, especially on an approach, resulting in a lower approach.
Landing visual illusions Runway width that is narrower than usual or expected tends to make the pilot think she is higher than she actually is and to therefore fly a lower approach (Fig. 8-13). A runway width that is wider than expected has the opposite effect, with the approach being too high.
An upsloping runway or surrounding terrain can result in the illusion that the aircraft is higher (the glide path appears high) than it actually is, result- ing in a lower approach (Fig. 8-14). Downsloping runways have the opposite result; the glide path appears low and the approach might be high.
The absence of ground features such as buildings and trees with which the pilot can relate distance, speed, and position can cause the pilot to think he is higher
148 Orientation
than he is. Flying an approach over water, for example, without frequent referral to instruments will result in a lower approach height (Figs. 8-15 and 8-16).
Recognizing the importance and power of peripheral and central vision inputs to the brain, it should be respected that visual cues can be disorienting in addition to misleading the pilot in this position. It should be obvious that the scan and cross-check of instruments become a crucial part of any flight activity. These illusions are best understood by recognizing their presence in flight, not assuming that being informed of their possible presence will over- come the illusion. Only practice and currency will overcome these illusions.