GENERAL.
The hydraulic system components covered in this section consist of the combination hydraulic pump and
reservoir, gear back-up extender actuator assembly, actuating cylinders and hydraulic lines. The brake system,
although hydraulically operated, is not included in this section as it has its own hydraulic system independent
of the gear retraction system. The brake system along with the landing gear and components is covered in
Chapter 32.
This section provides instructions for remedying difficulties which may arise in the operation of the
hydraulic system. The instructions are organized so that the mechanic can refer to: Description of the System,
for a basic understanding of the system; Troubleshooting, for a methodical approach in locating diff i c u l t y ;
Corrective Maintenance, for the removal, repair and installation of components; and Adjustments and Checks,
for the operation of the repaired system.
—CAUTION—
Prior to starting any investigation of the hydraulic system, place
the airplane on jacks. (Refer to Jacking, Chapter 7.)
DESCRIPTION.
Hydraulic fluid is supplied to the landing gear actuating cylinders by an electrically powered, reversible
hydraulic pump located aft of the baggage compartment at the right side of station 156.00. A reservoir is an
integral part of the pump. The pump is controlled by a selector handle on the instrument panel to the left of the
control quadrant. As the handle is selected to either the up or down position, the pump directs fluid through a
single line to either a manifold or a tee fitting (Refer to Figures 29-1 and 29-2 to ascertain which system is
installed) and is then directed on to the actuating cylinders. As fluid pressure increases at one side of a cylinder
piston, fluid at the other side is directed back through the lines to the pump. Depending on the direction of
rotation of the pump, the connecting lines serve either as pressure or return passages to retract or extend the
gear.
A pressure switch is incorporated in the hydraulic system which opens the electrical circuit to the pump
solenoid when the gear fully retracts and pressure in the system increases to a predetermined value. The switch
will continue to hold the circuit open until pressure in the system drops to another predetermined value, at
which time the pump will again operate to buildup pressure as long as the gear selector handle is in up
position. The down position of the handle does not effect the pressure switch.
The hydraulic pump is a gear type unit driven by a 14-volt reversible motor. Maximum pump pressure will
vary depending on which model pump is installed (Refer to Figures 29-1 and 29-2 for pump pressures). To
prevent excessive pressure in the hydraulic system due to fluid expansion, there is a thermal relief valve
incorporated in the pump that will open and allow fluid to flow to the pump reservoir. Other valves in the
pump system channel fluid to the proper outlets during gear retraction or extension. In the base of the pump is
a shuttle valve that allows fluid displaced by the cylinder piston rods to return to the reservoir without back
pressure. During the extension cycle the shuttle valve is unseated by the 400 - 800 psi delivered pressure.
29-00-01
Page 29-01
Revised: November 6, 1979
2B19
PIPER AIRCRAFT
PA-28RT-201 / 201T
MAINTENANCE MANUAL
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