Gyro Horizon; Air Driven Attitude Horizon; Electrically Driven Attitude Horizon; Operational Limitations - Piper Navajo Chieftain PA-31-350 Service Manual

NAVAJO CHIEFTAIN SERVICE MANUAL
10-28. REMOVAL OR REPLACEMENT.
(Refer to Paragraph 10-94 of this section).
10-29. GYRO HORIZON.
10-30. GENERAL: Both air and electric Attitude Horizons are used, depending on the option package
installed. Both types are displacement type gyros with "FREE" rotors mounted in Gimbal Assemblies. It
is important to consult the A.F.C.S. Manufacturer's Service Manual if a Flight Director or AutoPilot is
coupled to the Attitude Horizon.
10-30a. AIR DRIVEN ATTITUDE HORIZON: The air driven Attitude Horizon is driven by the pneumatic
system, which is supplied by engine driven dry pneumatic pumps either on pressure or vacuum, or from
cooled and regulated bleed air on Turbine engine installations. It is of prime importance to realize that air
VOL U M E and not air PRESSURE, spins the gyro rotor. The air filter can become contaminated and restrict
air flow. reducing gyro rotor speed, while the pressure regulator will automatically adjust air pressure within
proper limits. (THE GYRO AIR FILTER MUST BE CLEAN OR REPLACED. BEFORE ADJUSTING
GYRO AIR PRESSU RE.) Air flow directed at the gyro buckets, causes the rotor to spin approximately 17.000
to 22.000 R.P.M.. thus, producing the gyroscopic ability to remain rigid in space. The instrument case moves
freely about the spinning gyro rotor in three dimensions by the use of a Gimbal Assembly. The resulting
displacement in both pitch and roll is mechanically displayed on the instrument face, providing an artificial
horizon reference which portrays airframe attitude at any given moment. The display is stable and can show
minute attitude changes of only 1
°
. Unlike the Directional Gyro. the erection mechanism activity can be seen
by a rapidly wobbling and leveling horizon bar. when power is first applied. The instrument can be adjusted
for parralex through a knob on the instrument face, this knob when held to the "IN" position engages forks
which cage the gyro rotor.
10-30b. ELECTRICALLY
DRIVEN ATTITUDE
HORIZON: These gyros contain rotors which are
electrically driven, with the gyro rotor acting as the armature of an induction motor. Brush assemblies are not
used since the friction would result in abnormal precession, the rotorarmature
is inductively excited. The
electric A.H. is subject to the same operational requirements of the air driven A.H.. except for the method of
obtaining rotor rotation and the design of the erection mechanism.
10-30c. OPERATIONAL
LIMITATIONS:
Unlike the D.G. the A.H. has no attitude limits. If. however.
pitch reaches 90
°
the "polar effect" is reached and the Horizon Bar display will rotate 180
°
when the aircraft
is again right side up. The modern-day A.H. 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 A.H. 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 gvro rotor is at rest and power is applied, the erection mechanism exerts maximum authority and
rapid and noticeable erection results. However, if power is removed from the spinning rotor (engine 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 A.H., the erection mechanism
again begins to function, but due to gyro rigidity. because of high rotor speed, erection of the instrument is
considerably longer than normal. In flight the air-driven A.H. exhibits small errors at roll out aftercoordinated
turns, skids, and small pitch changes after acceleration and deceleration. The electric A.H. exhibits small
errors in pitch and roll after 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 A.H. is considered generally more efficient in operation and less subject to errorthan the air driven A.H.
Revised: 10/15/82
INSTRUMENTS
3B9