The adjustment knob is used to adjust the wings up or down to
align with the horizon bar. This allows adjustment to the height of the pilot.
Preferably, the adjustment should be made when level on the ground.
When the wings are aligned with the horizon bar, the aircraft
is in level flight. If the wings are above the horizon bar, the aircraft is in a
climb. Wings below the horizon bar indicates a decent. The upper blue part of
the ball represents the sky. The miniature airplane wings (fixed to the case)
represent the wings of the aircraft. In the past, the instrument has been
referred to as "an artificial horizon". When in a left turn, the blue portion of
the ball will have rolled to the right, as though you were looking at the
horizon over the nose of the aircraft. In a right turn, the blue portion will
have rolled to the left.
When the attitude indicator is in operation, gyroscopic
rigidity maintains the horizon bar parallel to the natural horizon. When the
pitch or bank attitude of the aircraft changes, the miniature aircraft, being
fixed to the case, moves with it. These movements of the instrument case with
respect to the gyro are shown on the face of the instrument as pitch and bank
attitude changes of the miniature aircraft with respect to the horizon bar.
Air is sucked through the filter, then through passages in
the rear pivot and inner gimbal ring, then into the housing, where it is
directed against the rotor vanes through two openings on opposite sides of the
rotor. The air then passes through four equally spaced ports in the lower part
of the rotor housing and is sucked out into the vacuum pump or venturi tube.
The chamber containing the ports is the erecting device that
returns the spin axis to its vertical alignment whenever a precessing force,
such as friction, displaces the rotor from its horizontal plane. The four
exhaust ports are each half-covered by a pendulous vane, which allows discharge
of equal volumes of air through each port when the rotor is properly erected.
Any tilting of the rotor disturbs the total balance of the pendulous vanes,
tending to close one vane of an opposite pair while the opposite vane opens a
corresponding amount. The increase in air volume through the opening port exerts
a precessing force on the rotor housing to erect the gyro, and the pendulous
vanes return to a balanced condition.
The limits of the instrument refer to the maximum rotation of
the gimbals beyond which the gyro will tumble. The older type vacuum-driven
attitude indicators have bank limits of approximately 100° to 110°, and pitch
limits of 60° to 70°. If, for example, the pitch limits are 60° with the gyro
normally erected, the rotor will tumble when the aircraft climb or dive angle
exceeds 60°. As the rotor gimbal hits the stops, the rotor precesses abruptly,
causing excessive friction and wear on the gimbals. The rotor will normally
precess back to the horizontal plane at a rate of approximately 8° per minute.
The limits of more recently developed vacuum-driven attitude indicators exceed
those given above.
Many gyros include a manual caging device, used to erect the
rotor to its normal operating position prior to flight or after tumbling, and a
flag to indicate that the gyro must be uncaged before use. Turning the caging
knob prevents rotation of the gimbals and locks the rotor spin axis in its
vertical position. Because the rotor is spinning as long as vacuum power is
supplied, normal manoeuvring with the gyro caged wears the bearings
unnecessarily. Therefore, the instrument should be left uncaged in flight unless
the limits are to be exceeded.
In the caged position, the gyro is locked with the miniature
aircraft showing level flight, regardless of aircraft attitude. When uncaged in
flight, in any attitude other than level flight, the gyro will tend to remain in
an unlevel plane of rotation with the erecting mechanism attempting to restore
the rotor to a horizontal plane. Therefore, should it be necessary to uncage the
gyro in flight, the actual aircraft attitude must be identical to the caged
attitude (that is, straight and level), otherwise, the instrument will show
false indications when first uncaged.
Errors in the indications presented on the attitude indicator
will result from any factor that prevents the vacuum system from operating
within the design suction limits, or from any force that disturbs the free
rotation of the gyro at design speed. Some errors are attributable to
manufacturing and maintenance. These include poorly balanced components, clogged
filters, improperly adjusted valves, and pump malfunction. Such errors can be
minimized by proper installation and inspection.
Other errors, inherent in the construction of the instrument, are caused by
friction and worn parts. These errors, resulting in erratic precession and
failure of the instrument to maintain accurate indications, increase with the
life of the instrument.
Another group of errors, associated with the design and operating principles of
the attitude indicator, are induced during normal operation of the instrument. A
skidding turn moves the pendulous vanes from their vertical position, precessing
the gyro toward the inside of the turn. After return of the aircraft to
straight-and-level, coordinated flight, the miniature aircraft shows a turn in
the direction opposite the skid. During a normal turn, movement of the vanes by
centrifugal force causes precession of the gyro toward the inside of the turn.
Errors in both pitch and bank indications occur during normal
coordinated turns. These errors are caused by the movement of the pendulous
vanes by centrifugal force, resulting in the precession of the gyro toward the
inside of the turn. The error is greatest in a 180° steep turn. If, for example,
a 180° steep turn is made to the right and the aircraft is rolled out to
straight-and-level flight by visual references, the miniature aircraft will show
a slight climb and turn to the left. This precession error, normally 3° to 5°,
is quickly corrected by the erecting mechanism. At the end of a 360° turn, the
precession induced during the first 180° is cancelled out by precession in the
opposite direction during the second 180° of turn. The slight precession errors
induced during the roll-out are corrected immediately by pendulous vane action.
Acceleration and deceleration also induce precession errors,
depending upon the amount and extent of the force applied. During acceleration
the horizon bar moves down, indicating a climb. Control applied to correct this
indication will result in a pitch attitude lower than the instrument shows. The
opposite error results from deceleration. Other errors, such as "transport
precession" and "apparent precession," relate to rotation of the earth and are
of importance to pilots and navigators concerned with high speed and long-range
The application of the foregoing errors as they affect
instrument interpretation will be treated later in Chapter V, "Attitude
Instrument Flying - Airplanes."
Electric Attitude Indicators.
In the past, suction-driven gyros have been favoured over the
electric for light aircraft because of the comparative simplicity and lower
cost. However, the increasing importance of the attitude indicator has
stimulated development of improved electric-driven gyros suited to light plane
installation. Improvements relating to basic gyro design factors, easier
readability, erection characteristics, reduction of induced errors, and
instrument limitations are reflected in several available types. Depending upon
the particular design improvements, the details among different instruments will
vary as to the instrument display and cockpit controls. All of them present, to
a varying degree, the essential pitch and bank information for attitude
Electric gyros may be remotely located, with the gyro
assembly mounted at some convenient location other than behind the instrument
panel, and with the indicator assembly on the instrument panel driven through a
servo motor. Another type is a simpler unit incorporating the gyroscope motor in
the instrument case integral with the indicator assembly. The H-6B attitude
indicator and J-8 gyro-horizon are representative of this type.