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from a remote source and generally need no
adjustment. Heading indicators that do not have this
automatic north-seeking capability are often called
“free” gyros, and require that you periodically adjust
them. You should align the heading indicator with the
magnetic compass before flight and check it at 15-
minute intervals during flight. When you do an in-flight
alignment, be certain you are in straight-and-level,
unaccelerated flight, with the magnetic compass showing
a steady indication.
TURN INDICATORS
Turn indicators show the direction and the rate of turn.
A standard rate turn is 3° per second, and at this rate
you will complete a 360° turn in two minutes. A halfstandard
rate turn is 1.5° per second. Two types of
indicators are used to display this information. The
turn-and-slip indicator uses a needle to indicate direction
and turn rate. When the needle is aligned with the
white markings, called the turn index, you are in a
standard rate turn. A half-standard rate turn is indicated
when the needle is halfway between the indexes.
The turn-and-slip indicator does not indicate roll rate.
The turn coordinator is similar to the turn-and-slip
indicator, but the gyro is canted, which allows it to
sense roll rate in addition to rate of turn. The turn coordinator
uses a miniature aircraft to indicate direction,
as well as the turn and roll rate. [Figure 12-7]
Another part of both the turn coordinator and the turnand-
slip indicator is the inclinometer. The position of
the ball defines whether the turn is coordinated or not.
The helicopter is either slipping or skidding anytime
the ball is not centered, and usually requires an adjustment
of the antitorque pedals or angle of bank to correct
it. [Figure 12-8]
INSTRUMENT CHECK—During your preflight, check
to see that the inclinometer is full of fluid and has no
air bubbles. The ball should also be resting at its lowest
point. Since almost all gyroscopic instruments installed
in a helicopter are electrically driven, check to see that
the power indicators are displaying off indications.
Turn the master switch on and listen to the gyros spool
up. There should be no abnormal sounds, such as a
grinding sound, and the power out indicator flags
should not be displayed. After engine start and before
liftoff, set the direction indicator to the magnetic compass.
During hover turns, check the heading indicator
for proper operation and ensure that it has not precessed
significantly. The turn indicator should also
indicate a turn in the correct direction. During takeoff,
check the attitude indicator for proper indication and
recheck it during the first turn.
MAGNETIC COMPASS
In some helicopters, the magnetic compass is the only
direction seeking instrument. Although the compass
appears to move, it is actually mounted in such a way
that the helicopter turns about the compass card as the
card maintains its alignment with magnetic north.
COMPASS ERRORS
The magnetic compass can only give you reliable
directional information if you understand its limitations
and inherent errors. These include magnetic variation,
compass deviation, and magnetic dip.
MAGNETIC VARIATION
When you fly under visual flight rules, you ordinarily
navigate by referring to charts, which are oriented
Figure 12-7. The gyros in both the turn-and-slip indicator and
the turn coordinator are mounted so that they rotate in a vertical
plane. The gimbal in the turn coordinator is set at an angle,
or canted, which means precession allows the gyro to sense
both rate of roll and rate of turn. The gimbal in the turn-and-slip
indicator is horizontal. In this case, precession allows the gyro
to sense only rate of turn. When the needle or miniature aircraft
is aligned with the turn index, you are in a standard-rate turn.
Gyro
Rotation
Gimbal
Rotation
TURN-AND-SLIP
INDICATOR
Gimbal
Gimbal
Rotation
Gyro
Rotation
Canted Gyro
TURN
COORDINATOR
Horizontal
Gyro
Inclinometer
Figure 12-8. In a coordinated turn (instrument 1), the ball is
centered. In a skid (instrument 2), the rate of turn is too great
for the angle of bank, and the ball moves to the outside of the
turn. Conversely, in a slip (instrument 3), the rate of turn is
too small for the angle of bank, and the ball moves to the
inside of the turn.
12-5
to true north. Because the aircraft compass is oriented
to magnetic north, you must make allowances for the
difference between these poles in order to navigate
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