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against the apparent miniature aircraft position. Disregarding
precession error, small deviations from straight coordinated
flight can be readily detected on the scale pointer. The
banking index may be graduated as shown in Figure 5-17,
or it may be graduated in 30° increments.
The instrument depicted in Figure 5-17 has a scale pointer
that moves in the same direction of bank shown by the
miniature aircraft. In this case, the aircraft is in a left 15°
bank. Precession errors in this instrument are common
and predictable, but the obvious advantage of the attitude
indicator is an immediate indication of both pitch attitude
and bank attitude in a single glance. Even with the precession
errors associated with many attitude indicators, the quick
attitude presentation requires less visual effort and time for
positive control than other flight instruments.
Heading Indicator
The bank attitude of an aircraft in coordinated flight is shown
indirectly on the heading indicator, since banking results in
a turn and change in heading. Assuming the same airspeed
in both instances, a rapid movement of the heading indicator
(azimuth card in a directional gyro) indicates a large angle
of bank, whereas slow movement reflects a small angle of
bank. Note the rate of movement of the heading indicator
and compare it to the attitude indicator’s degrees of bank.
The attitude indicator’s precession error makes a precise
check of heading information necessary in order to maintain
straight flight.
When deviations from straight flight are noted on the heading
indicator, correct to the desired heading using a bank angle no
greater than the number of degrees to be turned. In any case,
limit bank corrections to a bank angle no greater than that
required for a standard rate turn. Use of larger bank angles
requires a very high level of proficiency, and normally results
in overcontrolling and erratic bank control.
Turn Coordinator
The miniature aircraft of the turn coordinator gives an indirect
indication of the bank attitude of the airplane. When the
miniature aircraft is level, the airplane is in straight flight.
When the miniature airplane is aligned with one of the
alignment marks and the aircraft is rolling to the left or right
the indication represents the roll rate, with the alignment
marks indicating a roll of 3° per second in the direction of
the miniature aircraft. This can be seen in level flight when
a bank is introduced either to the left or the right. The turn
coordinator’s indicator will indicate the rolling motion
although there is no turn being made. Conversely, a pedal
input to the right or left causes the aircraft to turn momentarily
about its vertical axis (with no rolling motion) with an
indication of turn on the turn coordinator. After the turn
becomes stabilized and the aircraft is no longer rolling, the
turn coordinator displays the rate of turn with the alignment
marks equaling a turn of 3° per second. The turn coordinator
is able to display both roll and turn parameters because its
electrically powered gyroscope is canted at an angle. As a
result, the turn-and-slip indicator provides both roll and turn
indications. Autopilots in general aviation today use this
instrument in determining both roll and turn information.
After the completion of a turn, return to straight flight is
accomplished by coordinated aileron and rudder pressure to
5-8
Figure 5-18. Skid Indication.
Figure 5-19. Slip Indication.
level the miniature aircraft. Include the miniature aircraft in
the cross-check and correct for even the smallest deviations
from the desired position. When this instrument is used to
maintain straight flight, control pressures must be applied
very lightly and smoothly.
The ball of the turn coordinator is actually a separate
instrument, conveniently located under the miniature aircraft
because the two instruments are used together. The ball
instrument indicates the quality of the turn. If the ball is off
center, the airplane is slipping or skidding. That is, if the
coordinator’s miniature airplane is tilted right and the ball is
displaced to the right, the aircraft is in a skid. [Figure 5-18] If
however, the miniature airplane is tilted to the right with the
ball off-center to the left, the aircraft is in a slip. [Figure 5-19]
If the wings are level and the airplane is properly trimmed,
the ball will remain in the center, and the airplane will be
in straight flight. If the ball is not centered, the airplane is
improperly trimmed.
To maintain straight-and-level flight with proper trim, note
　
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