Piper Cherokee LANCE, Service Manual

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PIPER CHEROKEE SIX / LANCE SERVICE MANUAL
VA - HYDRAULIC SYSTEM 07/15/06
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Also in the system is a by-pass or free-fall valve that allows the gear to drop should a malfunction in the
pump system occur. To prevent the gear from extending too fast, there is a special restrictor nipple on the
main gear retraction line. The valve is controlled manually or by a gear back-up extension device that is
operated by a pressure sensing device which lowers the gear regardless of gear selector handle position,
depending upon airspeed and engine power (propeller slipstream). Gear extension occurs even if the
selector is in the up position, at airspeeds below approximately 118 MPH with engine power off. The
device also prevents the gear from retracting at airspeeds below approximately 93 MPH at sea level with
full power, though the selector switch may be in the up position. This speed increases with reduced
power and/or increased altitude. The sensing device operation is controlled by a differential air pressure
across a flexible diaphragm which is mechanically linked to the hydraulic valve and an electrical switch
which actuates the pump motor. A high pressure and static air source for actuating the diaphragm is
provided in a mast mounted on the left side of the fuselage above the wing. Manual override of the
device is provided by an emergency gear lever located between the front seats to the right of the flap
handle.
The emergency gear lever, used for emergency extension of the gear, manually releases hydraulic
pressure to permit the gear to free-fall with spring assistance on the nose gear. The lever must be held in
the downward position for emergency extension. This same lever, when held in the raised position, can
be used to override the system, and gear position is controlled by the selector switch regardless of
airspeed/power combinations. The lever must also be held in the raised position when hydraulic system
operational checks are being conducted. An override latch allows the emergency extension lever to be
retained in the up override position. The latch is disengaged by pulling up on the extension lever. The
lever includes a centering device to return the handle to neutral, when not latched in override. An auto
extension off light is mounted below the gear selector switch, and flashes to indicate whenever the latch
is in use. The auto extension off light is controlled by a switch and flasher mounted under the flap handle
cover.
For a description of the landing gear and electrical switches, refer to Section VIA, Landing Gear and
Brake System.
CAUTION: PRIOR TO STARTING ANY INVESTIGATION OF THE HYDRAULIC SYSTEM,
PLACE THE AIRPLANE ON JACKS. WITH THE AIRPLANE ON JACKS, PULL THE
EMERGENCY EXTENSION LEVER UP AND LATCH IN OVERRIDE POSITION
THUS PREVENTING THE BUILDUP OF UNNECESSARY PRESSURE ON THE
ACTUATING CYLINDERS AND CONNECTING HYDRAULIC LINES WHEN THE
GEAR IS RAISED OR LOWERED MANUALLY. FAILURE TO COMPLY WITH THESE
INSTRUCTIONS COULD RESULT IN THE BUILDUP OF SUFFICIENT PRESSURE
TO UNLOCK THE DOWNLOCK MECHANISM ALLOWING THE GEAR TO
COLLAPSE WHEN THE WING JACKS ARE REMOVED. PRIOR TO REMOVING THE
AIRPLANE FROM JACKS, PUSH THE EMERGENCY EXTENSION LEVER DOWN,
TURN ON THE MASTER SWITCH AND SELECT GEAR DOWN, OBSERVE THAT
ALL THREE GREEN LIGHTS INDICATING THE LANDING GEAR IS DOWN AND
LOCKED ARE ENERGIZED. TURN MASTER SWITCH OFF.