Instrument Flight
Background
Rotational Acceleration
The other function of the inner ear is to sense rotational acceleration. This is accomplished by the
semicircular canals which sense head movement in any of the three dimensions. Normally the
semicircular canals work quite well, but their weakness is that the whole system depends on the slight
displacement of fluid within the canals. In the first place, the sensitivity of the canals is limited; a slow
entry into a turn may not get over the threshold of stimulation, and may not give the sensation of entry into
the turn at all. Secondly, when there are sensations, they may be misleading.
False Sensations of Motion
It is easy to see how illusions may arise if you compare the displacement of the fluid in the semicircular
canals to the movement of water in a glass. If the glass is turned rapidly, the water will tend to remain in
motion. The same type of thing happens in the semicircular canals, only on a smaller scale. The
displacement of the fluid in the canals corresponds to the movements of the head only if the rotation is
relatively slow and lasts for a short time. In a long turn or a sudden stop, the liquid behaves almost
independently of the movement of the head; the inner ear transmits false messages to the brain. Consider
how this can produce illusion in flight.
Suppose, during instrument flight, you commence a turn to the right. If your turn is slow, the fluid in the
canals catches up with the motion of the body. If the fluid ceases to move, you will sense that the turning
has stopped. Acting on this information alone, and still wanting to go to the right, you will turn right again,
and get into a much tighter turn which may start a dangerous spiral. If, on the other hand, a relatively
sharp turn is stopped, the fluid in the canals, like the water in a glass, will continue to be displaced, even
after rotation has ceased. This will give the impression of turning in the opposite direction. Again,
depending and acting on the equilibrium senses alone may precipitate entry into a dangerous situation.
WARNING
Extreme care should be taken to limit rapid head movements during descents and turns,
particularly at low altitudes. Cockpit duties should be subordinated to maintaining aircraft
control.
Another illusion is called “the leans.” The aircraft is banked quickly in rough air and a correct sensation of
the attitude results. Then, a slow recovery is performed which does not cross the threshold of angular
motion perception; the senses retain the feeling that the aircraft is still in a bank. The impression may be
so strong that you may lean to one side in an attempt to assume what you suppose to be the vertical.
This sensation is one of the strongest and most frequently experienced in instrument flight. It gives false
impressions of both bank and pitch, particularly after entering a cloud in a turn.
POSTURAL SENSE
The postural sense derives its sensations from the expansion and contraction of muscles and tendons,
touch and pressure, and the shifting of abdominal muscles. Without a visual aid, this sense often
interprets centrifugal force as a false climb or descent. The postural sense is also incapable of sensing
airspeeds without acceleration or deceleration. Therefore, the postural senses, like those of the inner ear,
are unreliable without a visual aid. Without visual reference to the horizon or to flight instruments, you
could interpret a steep turn as a steep climb, or a shallow descending turn as level flight. You must learn
to subordinate these sensations when they conflict with visual reference to the flight instruments.
SIGHT
When blindfolded, you will find that the loss of visual reference to surrounding objects makes it difficult to
stand and nearly impossible to walk a straight line. The inner ear and postural senses are relatively
reliable when standing still. However, their reliability is different on a moving platform, such as an aircraft
in flight. Without the aid of sight, these senses are unable to distinguish gravity, centrifugal force, or small
forces of acceleration and deceleration from one another.
(4-01) Original
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