conditions, but has instruments to keep him spatially oriented under those
conditions where false perceptions may be experienced.
a. The Leans. The most common form of spatial disorientation is the
leans. According to Mulder's law, any constant angular motion slower than
2.5 degrees per second will not be perceived. Since a constant angular
motion is a function of both the rate of angular acceleration and the time
over which the acceleration is applied, there are several situations where
angular motion may not be perceived.
(1) High rates of angular acceleration over a relatively short
period of time: An example would be the motion of an aircraft rotating on
its roll axis with an acceleration of 8 degrees per second per second for a
quarter of a second. The constant angular motion in this case would be 2
degrees per second (8 X 0.25 = 2), which is below the 2.5 degrees per second
rate required for perception.
(2) Slow rates of angular acceleration over a longer period of time:
For example, the motion of an aircraft rotating on its roll axis with an
acceleration of a quarter of a degree per second per second for a period of
8 seconds. Here the constant angular motion of the aircraft would again be
2 degrees per second (0.25 X 8 = 2) and would go unperceived. If either of
these conditions recur over time, considerable angles of bank could be
achieved without perception. Once this unperceived bank is detected by
reference to attitude instruments the pilot will apply control pressure to
correct the attitude. If the correction is accomplished with a constant
angular motion faster than 2.5 degrees per second the pilot will perceive
the motion in the direction of correction. Since only the corrective motion
has been perceived, when the aircraft Is returned to level flight
(instrument indication), the pilot will sense himself in a bank equal to the
amount of correction. Even though the pilot believes his instruments, he
will be compelled to align his body with the perceived vertical and actually
lean in the direction of the original (subthreshold) roll as depicted in
Figure 18).
(3) Slow angular correction (acceleration in the opposite direction
to rapid angular acceleration): The leans can be generated in the opposite
way as well. If an aircraft is rolled in one direction with a perceivable
rate of motion (suprathreshold), and the pilot corrects the change in
attitude very slowly and smoothly (subthreshold), the leans may result. The
fact that only one of the motions has been perceived will again cause the
pilot (who believes his instruments) to align his body with the perceived
vertical and lean in the direction opposite the original roll.
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