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Chapter 17 - Pilot Potpourri: Neat Aeronautical Information
17-7

Fig. 5

Fig. 4

When you move your head or accelerate the airplane (change
speed or direction), fluid within these canals move as shown in
Figure 4, position B. Since the fluid within the canals has a small
amount of inertia (a tendency to resist change of motion), it tends
to remain stationary for a short period of time before moving.
Thus, the canals can be said to rotate around the fluid within them.
Eventually the fluid catches up to the movement of the canal and
the feeling of turning stops (position C). Even though you’re still
turning, you don’t feel it. When you stop turning, however, the
fluid continues its motion for a short period of time because of its
inertia (position D). This signals your mind that you’ve entered a
turn in the opposite direction.
The semicircular canal system was designed as a ground based
system where gravity always pulled the body in one direction—
straight downward. In flight, however, gravity is not the only force
pulling on the body. Centrifugal force as well as gravity tugs on the
seat of the pilot’s pants as shown in Figure 5.
This explains why your eyes may tell you one thing when looking
at the instruments while your brain and your body tell you some-
thing entirely different. For instance, when the body is in a pro-
longed turn, the fluid in the canals eventually comes up to speed
with the canal walls (hair no longer stimulated). If the head is then
tipped or twisted, the fluid once again moves relative to the canal
walls. Now a new sensation of rotation occurs (based on which way
the head was turned) even though the airplane didn’t change its
attitude. Thus, abrupt head movements under instrument or
instrument-like conditions can cause you to perceive maneuvers
that aren’t really happening. This vertigo-type illusion is called the
coriolis illusion. If you try to correct or turn your airplane in com-
pensation for the sensation, you could be in for a real surprise.
Then there’s the somatogravic illusion that results from a rapid
A wise man says,
acceleration (such as you might experience during takeoff) that stim-
“A pilot shouldn’t stop ulates your otolith organs the same way as tilting your head back-
thinking before the wards. This creates the illusion of being in a nose-up attitude, lead-
airplane stops flying . ing you to believe that you must lower the airplane’s nose. A rapid
deceleration followed by a quick reduction of the throttle might pro-
duce the opposite effect, leading you to believe that the airplane has
pitched down. Yikes! Now you know why it’s best to believe your
instruments. When you start work on your instrument rating