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provided. The complexity of the retractable landing
gear system requires that specific operating procedures
be adhered to and that certain operating limitations not
be exceeded.
LANDING GEAR SYSTEMS
An electrical landing gear retraction system utilizes an
electrically driven motor for gear operation. The
system is basically an electrically driven jack for
raising and lowering the gear. When a switch in the
cockpit is moved to the UP position, the electric motor
operates. Through a system of shafts, gears, adapters,
an actuator screw, and a torque tube, a force is
transmitted to the drag strut linkages. Thus, the gear
retracts and locks. Struts are also activated that open
and close the gear doors. If the switch is moved to the
DOWN position, the motor reverses and the gear
moves down and locks. Once activated the gear motor
will continue to operate until an up or down limit
switch on the motor’s gearbox is tripped.
A hydraulic landing gear retraction system utilizes
pressurized hydraulic fluid to actuate linkages to raise
and lower the gear. When a switch in the cockpit is
moved to the UP position, hydraulic fluid is directed
into the gear up line. The fluid flows through
sequenced valves and downlocks to the gear
actuating cylinders. A similar process occurs during
gear extension. The pump which pressurizes the fluid
in the system can be either engine driven or
electrically powered. If an electrically powered pump
is used to pressurize the fluid, the system is referred
to as an electrohydraulic system. The system also
incorporates a hydraulic reservoir to contain excess
fluid, and to provide a means of determining system
fluid level.
Regardless of its power source, the hydraulic pump is
designed to operate within a specific range. When a
sensor detects excessive pressure, a relief valve within
the pump opens, and hydraulic pressure is routed back
to the reservoir. Another type of relief valve prevents
excessive pressure that may result from thermal expansion.
Hydraulic pressure is also regulated by limit
Ch 11.qxd 5/7/04 8:50 AM Page 11-9
11-10
switches. Each gear has two limit switches—one
dedicated to extension and one dedicated to retraction.
These switches de-energize the hydraulic pump after
the landing gear has completed its gear cycle. In the
event of limit switch failure, a backup pressure relief
valve activates to relieve excess system pressure.
CONTROLS AND POSITION INDICATORS
Landing gear position is controlled by a switch in the
cockpit. In most airplanes, the gear switch is shaped
like a wheel in order to facilitate positive identification
and to differentiate it from other cockpit controls.
[Figure 11-6]
Landing gear position indicators vary with different
make and model airplanes. The most common types of
landing gear position indicators utilize a group of
lights. One type consists of a group of three green
lights, which illuminate when the landing gear is down
and locked. [Figure 11-6] Another type consists of one
green light to indicate when the landing gear is down
and an amber light to indicate when the gear is up. Still
other systems incorporate a red or amber light to
indicate when the gear is in transit or unsafe for
landing. [Figure 11-7] The lights are usually of the
“press to test” type, and the bulbs are interchangeable.
[Figure 11-6]
Other types of landing gear position indicators consist
of tab-type indicators with markings “UP” to indicate
the gear is up and locked, a display of red and white
diagonal stripes to show when the gear is unlocked, or
a silhouette of each gear to indicate when it locks in
the DOWN position.
LANDING GEAR SAFETY DEVICES
Most airplanes with a retractable landing gear have a
gear warning horn that will sound when the airplane is
configured for landing and the landing gear is not
down and locked. Normally, the horn is linked to the
throttle or flap position, and/or the airspeed indicator
so that when the airplane is below a certain airspeed,
configuration, or power setting with the gear retracted,
the warning horn will sound.
Accidental retraction of a landing gear may be
prevented by such devices as mechanical downlocks,
safety switches, and ground locks. Mechanical
downlocks are built-in components of a gear retraction
system and are operated automatically by the gear
retraction system. To prevent accidental operation of
the downlocks, and inadvertent landing gear retraction
while the airplane is on the ground, electrically
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