Piper Cherokee Six Service Manual page 216

PIPER CHEROKEE SIX SERVICE MANUAL
The hydraulic pump is a gear type unit driven by a 12 to 14 volt reversible motor
designed to operate in a pressure range as given in Table VA-I.A relief valve is incorporated
in the pump, which when open allows fluid to flow into the reservoir. Also incorporated in
the system on earlier models is a primary thermal relief valve which prevents excessive
buildup of pressure in the hydraulic system due to expansion. Other valves in the pump,
channel fluid to the proper outlet during retraction or extension of gear. A shuttle valve
located in the base of the pump allows fluid displaced by the cylinder pistons to return to
the reservoir without back pressure. (For specific pressures refer to Table VA-I.)
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.
SEE CAUTION NOTE ON GRID IJ3
Revised: 7/29/77
HYDRAULIC SYSTEM
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