BY ORDER OF THE SECRETARY OF THE AIR FORCE AIR FORCE ...

AFMAN 11-217V1 3 JANUARY 2005 367
somewhat greater than the perceived bank angle, and feel comfortable in it. The
G-excess effect and the illusion of underbank do not necessarily disappear as a
result of the compensatory overbank. The same phenomenon can occur
repeatedly as long as the G-load is maintained. Thus, even though the initial
perceptual error may be small, the accumulation of erroneous compensatory
control input can result in a rapidly developing severe overbank and the
accompanying earthward velocity vector. Remember, the prime time for the G-
excess illusion to happen is during any turning and looking maneuver.
17.3.1.5. Nystagmus. During and immediately after maneuvers resulting from
particularly violent angular accelerations, such as spins and rapid aileron rolls, the
vestibular system can fail to stabilize vision. The eyes can exhibit an uncontrollable
oscillatory movement called nystagmus. This eye movement generally results in an
inability to focus on either flight instruments or outside visual references. Rolling
maneuvers are especially likely to result in visual blurring because of nystagmus.
Normally, nystagmus ceases several seconds after termination of angular
acceleration; but under conditions of vestibular dominance and high task loading,
nystagmus and blurring of vision can persist much longer, even long enough to
prevent recovery.
17.3.2. Visual Illusions (Figure 17.8). A wide variety of visual misperceptions are
known to occur during flight, and the most common illusions are described here. When
flying with NVGs, pilots should be aware that they are susceptible to the same visual
illusions but with additional variations. The image intensification process of the goggles
can intensify the illusion as well as the ambient light.
Figure 17.8. Visual Illusions.