locking fingers to open as the piston shaft assembly is
compression spring, and down-lock switch. The
operation of the ball-lock actuator is described in the
retracted into the cylinder.
following paragraphs.
During normal extension of the landing gear (view
When the landing gear is down and locked, the
B of figure 11-4), hydraulic pressure is directed from
ball-lock actuator will be in the position shown in view
the selector valve to the normal extension port of the
A of figure 11-3. Notice the locking ball bearings are
integral shuttle valve. This pressurized fluid forces the
being held in the ball bearing race detents by the inner
piston towards the extended position. As the piston
lip of the ball-lock plunger. Since no hydraulic
comes in contact with the locking fingers, hydraulic
pressure exists while in this position, the
pressure and spring tension are required to force the
spring-loaded, ball-lock plunger is held in its retracted
piston over the fingers while fully extending the piston
position, allowing the down-lock switch to be actuated
shaft assembly. At the same time the piston is being
by the groove portion of the piston shaft.
forced over the locking fingers, it contacts the
cam-shaped lower end of a toggle shaft, which extends
When the landing gear selector valve is positioned
radially into the cylinder area, thereby rotating the
to its retracted (UP) position, pressurized fluid is
shaft. Movement of the toggle shaft is transmitted to
allowed to enter the actuator through its only port. This
the main landing gear down-limit switch, which is
pressurized fluid forces the ball-lock plunger to the
attached to the outer surface of the cylinder. This
right, which simultaneously allows the ball bearings to
indicates the cylinder is in the locked position.
drop free from their detents in the bearing race and
actuate the down-lock switch, as shown in view B of
Control Surface Actuating Cylinder
figure 11-3. As soon as the locking ball bearings are
released, the piston shaft assembly retracts, as shown
Actuators are used in conjunction with
in view C of figure 11-3, and unlocks the landing gear.
power-operated flight control systems. Their function
When the landing gear completes its UP cycle, the
is to assist the pilot in handling the aircraft, in the same
selector valve returns to neutral, trapping hydraulic
way as power steering aids in handling an automobile.
fluid within the actuator until the next cycle begins.
In a power-operated flight control system, all the
FINGER-LOCK ACTUATOR.--The actuating
force necessary for deflecting the control surface is
cylinder shown in figure 11-4 is a double-action,
supplied by hydraulic pressure. A hydraulic actuator
two-port, finger-lock, balanced actuator. This type of
incorporated in the control linkage operates each
actuator is currently installed as a main landing gear
movable surface. Some aircraft manufacturers refer to
component on some aircraft. It incorporates an inner
these units as power control cylinders; however, all
cylinder to equalize the displacement of fluid on either
flight control system actuators and power control
side of the piston.
cylinders perform the same function, and are similar in
As shown in view A of figure 11-4, an integral,
principle of operation.
finger-type, spring-loaded, mechanical lock is also
A typical flight control surface actuator is shown in
incorporated within the actuator to lock the piston shaft
figure 11-5. This is a tandem-type hydraulic unit,
assembly in the extended position. The finger-lock
which means, in this case, that two control valves are
actuator has a down-limit switch mounted on and
incorporated within a common housing. One of the
through the cylinder area, which indicates when the
control valves is connected to the aircraft's primary
landing gear is down and locked; also, an added feature
flight control hydraulic system, while the other is
that is common on landing gear actuators is an integral
connected to a separate hydraulic system.
shuttle valve. The shuttle valve allows connection of
T h i s i s a t y p i c a l a r r a n g e m e n t s i n c e N av y
both the normal extension hydraulic fluid line and the
specifications require two independent hydraulic
emergency pneumatic extension pressure line. The
systems for operation of the primary flight control
operation of the finger-lock actuator is described in the
systems on all high-performance aircraft.
following paragraphs.
Although a synchronizing rod interconnects the
When the pilot positions the selector valve in the
two control valves in the actuator mechanically, they
landing gear retracted position, view A of figure 11-4,
are not interconnected hydraulically. The purpose of
hydraulic pressure is directed to the cylinder's retract
the synchronizing rod is to equalize the flow of fluid
port. Hydraulic pressure entering the cylinder
into the actuator piston chambers.
overcomes piston spring force, which permits the
11-4
