Attitude Horizon Operational Limitations; Attitude Horizon Troubleshooting; Turn And Bank/Pictoral Rate Instruments - Piper PA-44-180 SEMINOLE Maintenance Manual

OPERATIONAL LIMITATIONS.
Unlike the directional gyro, the attitude horizon has no attitude limits. If, however, pitch reaches 90°, the
"polar effect" is reached and the horizon bar display will rotate 180° to an inverted position and will again rotate
180° when the aircraft is right side up. The modern-day attitude horizon will not be damaged by such an extreme
attitude and will correct itself in a short time. There are no roll limitations to the present attitude horizon. Another
important but not widely understood operating limitation of air driven instruments is erection of the horizon bar
from a full stop and from a residual running condition. This can lead to wasted service time and invalid operating
complaints. When the gyro rotor is at rest and power is applied, the erection mechanism exerts maximum
authority and rapid, noticeable erection results. However, if power is removed from the spinning rotor (such as
when an engine is shut down while briefly discharging passengers) the gyro rotor continues to rotate at high
speed but the erection mechanism is not functional. When power is again applied to the air driven attitude horizon
the erection mechanism again begins to function. However, due to gyro rigidity because of high rotor speed,
erection of the instrument takes considerably longer than normal. In flight, the air-driven attitude horizon exhibits
small errors at roll out after a coordinated turn, skids and small pitch changes after acceleration and deceleration.
The electric attitude horizon exhibits small errors in pitch and roll out from a coordinated turn and also small
pitch changes after acceleration or deceleration. In both cases, the erecting mechanisms quickly return the gyro to
its proper position. The electric attitude horizon is considered generally more efficient in operation and less
subject to error than the air driven attitude horizon.
TROUBLESHOOTING.
Unless an obvious malfunction, such as inability to erect, spinning, or great horizon bar displacement, none of
which can be corrected by manually caging the instrument, requires repair or replacement of the instrument,
service is restricted to the instrument installation and power source. Typical installation examples of attitude
horizon malfunctions are due to such problems as: restricted air flow due to air line kinks or leaks, contaminated
air filters, deteriorating electrical grounds, sagging instrument panel shock mounts, systems regulators, faulty
vacuum/ pressure gauges.
Only after the system has proven to be good should the instrument be "pulled" for replacement or repair.
TURN AND BANK/PICTORIAL RATE INSTRUMENTS.
Unlike the familiar "free" gyro rotor found in the directional and attitude gyros both the turn and bank and the
pictorial rate indicator have captive gyro rotors, the axis of which are attached to the instrument housings. Since
the spinning gyro rotors are literally forced to follow airframe movement, the gyro resists changing position by
exerting precession forces created by the spinning gyro. The greater the "rate of change" the greater the
precession forces, thus, the turn and bank and the pictorial rate indicator ONLY MEASURE MOVEMENT -
NOT POSITION OR DISPLACEMENT. The gyro rotor forces of the turn and bank are presented on the
instrument face by a vertical turn needle and on the pictorial artificial horizon. Although the visual displays are
different the gyro rotor rate detection designs are the same. The gyro motor is mounted at a 6° angle to detect
both yaw and roll motion, but the 60° tilt favors the yaw axis. Due to the great sensitivity of the rate gyro, the turn
needle/ pictorial horizon displays are mechanically dumped to slow or average minute.
PIPER AIRCRAFT
PA - 4 4 - 1 8 0 / 1 8 0 T
AIRPLANE MAINTENANCE MANUAL
— Note —
Air pressure must be 5.5 ± .5 psig.
2J5
3 4 - 3 4 - 0 0
Page 34-11
Revised: May 15, 1989