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indicator operate on precession. Precession is the
characteristic of a gyroscope that causes an applied force to
produce a movement, not at the point of application, but at
a point 90° from the point of application in the direction of
rotation. [Figure 3-32]
Turn-and-Slip Indicator
The first gyroscopic aircraft instrument was the turn indicator
in the needle and ball, or turn-and-bank indicator, which
has more recently been called a turn-and-slip indicator.
[Figure 3-33]
The inclinometer in the instrument is a black glass ball sealed
inside a curved glass tube that is partially filled with a liquid
for damping. This ball measures the relative strength of the
force of gravity and the force of inertia caused by a turn.
When the aircraft is flying straight-and-level, there is no
inertia acting on the ball, and it remains in the center of the
tube between two wires. In a turn made with a bank angle
that is too steep, the force of gravity is greater than the inertia
and the ball rolls down to the inside of the turn. If the turn is
made with too shallow a bank angle, the inertia is greater than
gravity and the ball rolls upward to the outside of the turn.
The inclinometer does not indicate the amount of bank, nor
does it indicate slip; it only indicates the relationship between
the angle of bank and the rate of yaw.
3-21
Figure 3-34. The rate gyro in both turn-and-slip indicator and turn
coordinator.
Figure 3-35. A turn coordinator senses rotation about both roll
and yaw axes.
The turn indicator is a small gyro spun either by air or by
an electric motor. The gyro is mounted in a single gimbal
with its spin axis parallel to the lateral axis of the aircraft
and the axis of the gimbal parallel with the longitudinal axis.
[Figure 3-34]
When the aircraft yaws, or rotates about its vertical axis, it
produces a force in the horizontal plane that, due to precession,
causes the gyro and its gimbal to rotate about the gimbal’s
axis. It is restrained in this rotation plane by a calibration
spring; it rolls over just enough to cause the pointer to deflect
until it aligns with one of the doghouse-shaped marks on the
dial, when the aircraft is making a standard rate turn.
The dial of these instruments is marked “2 MIN TURN.” Some
turn-and-slip indicators used in faster aircraft are marked “4
MIN TURN.” In either instrument, a standard rate turn is
being made whenever the needle aligns with a doghouse. A
standard rate turn is 3° per second. In a 2 minute instrument,
if the needle is one needle width either side of the center
alignment mark, the turn is 3° per second and the turn takes
2 minutes to execute a 360° turn. In a 4 minute instrument,
the same turn takes two widths deflection of the needle to
achieve 3° per second.
Turn Coordinator
The major limitation of the older turn-and-slip indicator is that
it senses rotation only about the vertical axis of the aircraft. It
tells nothing of the rotation around the longitudinal axis, which
in normal flight occurs before the aircraft begins to turn.
A turn coordinator operates on precession, the same as the
turn indicator, but its gimbals frame is angled upward about
30° from the longitudinal axis of the aircraft. [Figure 3-34]
This allows it to sense both roll and yaw. Therefore during
a turn, the indicator first shows the rate of banking and once
stabilized, the turn rate. Some turn coordinator gyros are dualpowered
and can be driven by either air or electricity.
Rather than using a needle as an indicator, the gimbal moves
a dial that is the rear view of a symbolic aircraft. The bezel
of the instrument is marked to show wings-level flight and
bank angles for a standard rate turn. [Figure 3-35]
The inclinometer, similar to the one in a turn-and-slip
indicator, is called a coordination ball, which shows the
relationship between the bank angle and the rate of yaw. The
turn is coordinated when the ball is in the center, between the
marks. The aircraft is skidding when the ball rolls toward the
outside of the turn and is slipping when it moves toward the
inside of the turn. A turn coordinator does not sense pitch.
This is indicated on some instruments by placing the words
“NO PITCH INFORMATION” on the dial.
3-22
Figure 3-36. The Kearfott Attitude Heading Reference System (AHRS) on the left incorporates a Monolithic Ring Laser Gyro (MRLG)
(center), which is housed in an Inertial Sensor Assembly (ISA) on the right.
Flight Support Systems
Attitude and Heading Reference System (AHRS)
As aircraft displays have transitioned to new technology,
　
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