Piper PA-32R-301T SARATOGA S, Maintenance Manual

Page: 369 / 956

HYDRAULIC POWER (Cont.)
B. Description (cont.)
A pressure switch is installed on a cross fitting connected to the pump mount assembly. During landing
gear retraction the pressure switch is the primary means to shut down the pump. This switch opens
the electrical circuit to the pump solenoid when the gear fully retracts and the pressure in the system
increases to 1800 ± 100 psi. As long as the gear selector handle is in the up position the switch will
continue to hold the circuit open until system pressure drops to 200 to 400 psi. At that time, the pump
will again operate to build up pressure to prevent the gear from free falling. The pressure switch has no
effect on the system when the gear selector is in the down position.
Both the Prestolite and Oildyne hydraulic pumps are gear type units driven by a 14-volt reversible
motor. designed to operate in a pressure range of 2000 to 2500 psi. To prevent excessive buildup of
pressure in the hydraulic system due to expansion, a thermal relief valve is incorporated in both the
Prestolite and Oildyne pumps. The Prestolite pump’s relief valve will open at 2250 ± 250 psi allowing
fluid to flow into the reservoir. The Oildyne pump’s relief valve will open at 3000 ± 500 psi Other
valves in both pumps channel fluid to the proper outlet during retraction or extension of gear. A shuttle
valve located in the base of the both pumps allows fluid displaced by the cylinder pistons to return to
the reservoir without back-pressure. (Refer to Figure 1 and Chart 1 for specific pressures.)
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. This valve is manually controlled by a lever or push-pull type cable
located between the two pilot seats on the elevator trim quadrant.'
– NOTE–
Beginning with airplane s/n’s 3213003 and up, Piper Aircraft
no longer installs the automatic gear extender system. Piper
kit, P/N 765 199, provides information concerning the removal
of the backup landing gear extension system
Models that have not had the automatic backup gear extender removed, have a lever that can manually
extend the gear, or allow the gear to extend automatically by the use of a backup extension device that
operates by a pressure sensing mechanism that lowers the landing gear regardless of the gear selector
handle position. Its operation depends on airspeed and engine power (propeller slipstream). Gear
extension occurs even if the selector is in the up position, at airspeeds below approximately 103 KIAS
with engine power off. The device also prevents the gear from retracting at airspeeds below approxi-
mately 81 KIAS 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 con-
trolled 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.
PIPER AIRCRAFT
PA-32R-301/301T
MAINTENANCE MANUAL
29-00-00
Page 29-4
Reissued: July 1, 1993
2E24