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operated safety switches are installed.
A landing gear safety switch, sometimes referred to as
a squat switch, is usually mounted in a bracket on one
of the main gear shock struts. [Figure 11-8] When the
strut is compressed by the weight of the airplane, the
switch opens the electrical circuit to the motor or
mechanism that powers retraction. In this way, if the
landing gear switch in the cockpit is placed in the
RETRACT position when weight is on the gear, the
gear will remain extended, and the warning horn may
sound as an alert to the unsafe condition. Once the
weight is off the gear, however, such as on takeoff, the
safety switch will release and the gear will retract.
Many airplanes are equipped with additional safety
devices to prevent collapse of the gear when the
airplane is on the ground. These devices are called
ground locks. One common type is a pin installed in
aligned holes drilled in two or more units of the
landing gear support structure. Another type is a
spring-loaded clip designed to fit around and hold two
or more units of the support structure together. All
types of ground locks usually have red streamers
permanently attached to them to readily indicate
whether or not they are installed.
EMERGENCY GEAR EXTENSION SYSTEMS
The emergency extension system lowers the landing
gear if the main power system fails. Some airplanes
Figure 11-6. Typical landing gear switches and position
indicators.
Figure 11-7. Typical landing gear switches and position
indicators.
Ch 11.qxd 5/7/04 8:50 AM Page 11-10
11-11
have an emergency release handle in the cockpit,
which is connected through a mechanical linkage to
the gear uplocks. When the handle is operated, it
releases the uplocks and allows the gears to free fall, or
extend under their own weight. [Figure 11-9]
Safety Switch
Landing Gear
Selector Valve
Lock Release
Solenoid
Lock-Pin
28V DC
Bus Bar
Figure 11-8. Landing gear safety switch.
Hand Pump Compressed Gas
Hand Crank
Figure 11-9.Typical emergency gear extension systems.
Ch 11.qxd 5/7/04 8:50 AM Page 11-11
11-12
should then turn on the battery master switch and
ensure that the landing gear position indicators show
that the gear is down and locked.
External inspection of the landing gear should
consist of checking individual system components.
[Figure 11-10] The landing gear, wheel well, and
adjacent areas should be clean and free of mud and
debris. Dirty switches and valves may cause false
safe light indications or interrupt the extension cycle
before the landing gear is completely down and
locked. The wheel wells should be clear of any
obstructions, as foreign objects may damage the gear
or interfere with its operation. Bent gear doors may
Figure 11-10. Retractable landing gear inspection
checkpoints.
On other airplanes, release of the uplock is
accomplished using compressed gas, which is directed
to uplock release cylinders.
In some airplanes, design configurations make
emergency extension of the landing gear by gravity
and air loads alone impossible or impractical. In these
airplanes, provisions are included for forceful gear
extension in an emergency. Some installations are
designed so that either hydraulic fluid or compressed
gas provides the necessary pressure, while others use a
manual system such as a hand crank for emergency
gear extension. [Figure 11-9] Hydraulic pressure for
emergency operation of the landing gear may be
provided by an auxiliary hand pump, an accumulator,
or an electrically powered hydraulic pump depending
on the design of the airplane.
OPERATIONAL PROCEDURES
PREFLIGHT
Because of their complexity, retractable landing gears
demand a close inspection prior to every flight. The
inspection should begin inside the cockpit. The pilot
should first make certain that the landing gear selector
switch is in the GEAR DOWN position. The pilot
Ch 11.qxd 5/7/04 8:50 AM Page 11-12
11-13
be an indication of possible problems with normal
gear operation.
Shock struts should be properly inflated and the
pistons clean. Main gear and nose gear uplock and
downlock mechanisms should be checked for general
condition. Power sources and retracting mechanisms
should be checked for general condition, obvious
defects, and security of attachment. Hydraulic lines
should be checked for signs of chafing, and leakage
at attach points. Warning system micro switches
(squat switches) should be checked for cleanliness
and security of attachment. Actuating cylinders,
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