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

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AFMAN 11-217V1 3 JANUARY 2005
17.3.2.10. Induced Motion Illusion. Induced motion illusion is the perceived motion
of objects that are not actually moving, when other objects are physically moving
instead. Induced motion is most vivid with indiscernible backgrounds such as “blackhole”
conditions (e.g., the illusion of rising light can be an undetected descent).
17.3.2.11. Moth Illusion. The visual illusion experienced when trying to stay
oriented with a descending light source. The misperception is the result of failing to
detect the actual downward motion because attention is centered on keeping the
intensity of the light constant and failing to detect the downward movement.
17.3.3. Somatosensory Illusions.
17.3.3.1. The Seat-of-the-Pants Sense. Pilots can be deceived if they interpret the
pressure sensations experienced during flight as meaning the same thing they would
in an earth bound situation (i.e., pressure on the seat-of-the-pants indicates down). In
flight, this pressure sensation is misleading because during coordinated flight, the
force resulting from centripetal acceleration and gravity are always toward the floor
of the aircraft. Thus, pilots can never tell through the pressure sensors which
direction is the true vertical.
17.3.3.2. Giant Hand Illusion. The giant hand phenomenon is a subconscious reflex
behavior, generated by vestibular or somatosensory inputs that interfere with the
pilot’s conscious control of the aircraft. This illusion gives the impression that some
external force is pushing on the aircraft or holding it in a certain attitude. When
disorientation is primarily about the roll axis, as with the leans or graveyard spiral, the
pilot may see deviation from the desired attitude on the attitude indicator, apply the
appropriate stick pressure to roll the aircraft to reduce the unwanted bank angle, and
discover that efforts to roll the aircraft appear to be resisted. The aircraft either seems
to not let the pilot roll or, once the airplane has been rolled to the proper attitude, it
seems to roll back to the original attitude as if a giant hand were pushing a wing
down. When the disorientation is about the pitch axis, as it is when a somatogravic
illusion of pitch-up occurs during forward acceleration, the pilot may experience what
feels as excessive nose down trim and the aircraft appears to resist efforts by the pilot
to pull the nose up, as if a giant hand were pushing the nose down. There is little
research relating to our understanding of the Giant Hand Illusion. To date it has not
been satisfactorily reproduced on the ground. It appears to be most commonly
experienced during night air refueling operations.
17.3.3.2.1. Reflex Actions. The giant-hand phenomenon is thought to occur as a
result of pilot reflex actions during disorientation. Remember, our reflexes are
geared to a ground-based environment and, therefore, rely on vestibular and
somatosensory inputs to determine which way is up. During disorientation, the
desired control input is in conflict with the reflex input, giving the illusion of
some external force acting on the aircraft.
17.3.3.2.2. Overcoming the Giant Hand. To overcome the giant hand illusion,
the pilot should momentarily remove his or her hand from the control stick to
interrupt the reflex response. A positive effort must then be made on the controls
to move the attitude indicator to the proper attitude. Some pilots have reported
that they used their fingertips or knees to move the controls to keep the illusion