(4) Other factors affecting the ability to perceive angular motion:
The threshold of perception of angular acceleration is raised considerably
by vibrations, noise, inattention and anxiety. It is, therefore, highly
probable that angular accelerations of much greater magnitude than specified
by Mulder's law can go unperceived under actual flight conditions. It is
also probable that the threshold for a given individual fluctuates according
to the individual's need for vestibular information. If, for example, a
pilot is jarred into a state of anxiety about his attitude by unusual
turbulence, he will probably reflexively lower his vestibular threshold. In
doing this, he is increasing the likelihood that he will generate false
impressions about his actual attitude and increase the change of the leans.
b. Graveyard Spin. The semicircular canals monitor angular
acceleration to give angular velocity information. They do not perceive
angular velocity per se. When a pilot gets into a spin he undergoes an
initial angular acceleration and perceives the angular motion of the spin
for a short time following the initiation of the angular acceleration.
After this time (15 to 20 seconds), the fluid in the semicircular canals
catches up with the motion giving a perception of discontinuation of motion
even though motion still exists. In other words, the semicircular canals
have equilibrated with the rotating motion and no motion is perceived. If
the pilot then makes the proper control maneuver to stop the spin, he will
undergo an angular deceleration which will be monitored by his semicircular
canals. The central nervous system will interpret this sensation as
representing a spin in the opposite direction, even though his instruments
are telling him that he is not spinning. If deprived of external visual
reference, he will be tempted to make a control correction that spins him in
the direction of the original angular motion.
c. Graveyard Spiral. The graveyard spiral is similar to the graveyard
spin in that the semicircular canals equilibrate to a constant angular
velocity and persisting rotary motion goes unperceived. In the graveyard
spiral, however, the angular velocity is in the form of a coordinated,
banked turn rather than a spin. If a pilot remains in a constant rate,
coordinated turn long enough for his semicircular canal to equilibrate, he
will lose the sensation of turning. If a loss of altitude results from the
decrease in lift due to the bank, the novice pilot may try to correct the
condition by pulling back on the stick or adding power. These maneuvers
only tend to tighten the spiral. Unless he first corrects the bank attitude
he will never recover with power or pitch. Once the spiral has started the
pilot will suffer an illusion of turning in the opposite direction if he
corrects the aircraft to straight and level. Under these conditions he
would not be likely to take the appropriate correction action and would
probably continue tightening the spiral until he either regained good
outside reference or hit the ground.
d. Coriolis Effect. During the mid-1950's, both the Air Force and the
Navy experienced a rash of fatal accidents involving single-seat
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