The position of the stabilizer is shown on the
integrated position indicator located on the left side of
the pilots instrument panel. When the stabilizer is in
the clean configuration, the STAB window of the
indicator shows the word CLEAN. When the
stabilizer is in the dirty configuration, the window
shows a picture of a stabilizer.
The stabilizer actuator (fig. 9-7) is a tandem-type
actuator powered by both flight and combined system
It contains a power valve shuttle, two
tandem-mounted power pistons, a servo ram, an
electrohydraulic servo valve, a lockout actuator, and
parallel and series mode solenoid valves. The
actuator can operate in any of three modes-manual,
series, or parallel.
Refer to figure 9-7 to help you
understand the three modes of operation, as described
in the following paragraphs.
MANUAL MODE.In this mode, the pilot input
alone controls the power valve.
transmitted through linkage to the mechanical input
lever. The auxiliary lever is linked in neutral by the
servo ram centering springs, causing the mechanical
input lever to rotate about its pivot point, moving the
power shuttle valve. As the valve shuttle is displaced
from neutral, a valve error is established, and pressure
is ported to the actuating pistons. The pressure moves
the pistons and the attached stabilizer in proportion to
A mechanical feedback is transmitted through the
differentiating lever, the load-relief bungee, and the
mechanical input lever back to the power valve
shuttle, causing it to return to the neutral position.
For a constant velocity pilot input, a small
constant valve error is established, and the stabilizer
moves at a constant speed. When the pilot input
stops, the power shuttle valve is returned to neutral,
and the stabilizer stops until a new input is introduced.
SERIES MODE.In this mode, input signals
from the automatic flight control system (AFCS) may
be used independently or combined with manual input
to control stabilizer movement. The series mode
solenoid valve is energized, porting flight system
hydraulic pressure to the electrohydraulic servo
valve. Input signals from the AFCS amplifier are
applied to the coils of a torque motor in the servo
valve, regulating flow from the valve to the servo
The servo ram is connected to the auxiliary lever.
Movement of the lever moves the mechanical input
lever floating-pivot point. This movement causes
mechanical input lever rotation about the manual
input point and moves the power shuttle valve,
causing a valve error.
A linear transduce, mounted on the servo ram
center line, provides electrical feedback signals to the
AFCS. Mechanical feedback is provided by the
differentiating lever, as in the manual mode. When
operating in the series mode, control surface
displacement is not reflected at the control stick.
PARALLEL MODE.In this mode, stabilizer
movement is controlled by input signals from the
AFCS alone. Both series and parallel mode solenoid
valves are energized. Flight system pressure is ported
to the electrohydraulic servo valve and the
mechanical input lockout piston.
stabilizes the lockout piston and holds the mechanical
The transducer mounted on the servo ram
provides an electrical signal feedback to the AFCS.
There is no mechanical feedback, since the
mechanical input is locked. Additional electrical
signal feedback is provided by a transducer, which is
mechanically linked to the stabilizer actuating arm.
In the parallel mode, the control stick follows the
motion of the stabilizer. Should the pilot desire to
override the AFCS, he/she can overpower the lockout
actuator with a stick force of 24 pounds.
Stop bolts are attached to the control stick pedal
to limit fore-and-aft stick movement.
current damper dampens out any rapid fore-and-aft
All joints between the pushrods and bell cranks or
idlers contain self-aligning bearings to compensate
for any misalignment during operation and airframe
deflections in flight that might cause binding.
Artificial feel is provided by the artificial-feel
bungee. The bungee consists of two springs, which
have different spring constants. The stick force
caused by the bungee is proportional to stick
displacement. At near neutral, the bungee provides a
high stick force that decreases a short distance from
neutral and gradually increases with the amount of
The electric trim actuator is mechanically
linked to the artificial-feel bungee, and varies the
neutral position of the bungee to provide
longitudinal trim of the aircraft. The actuator
consists of one high-speed and one low-speed
motor, a gearbox, a brake, a ball detent clutch, and a