Piper PA-34-200T Seneca II Service Manual Download

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PIPER SENECA II SERVICE MANUAL

01/01/09

VI - HYDRAULIC SYSTEM

1I2C

SECTION VI

HYDRAULIC SYSTEM

CAUTION: PRIOR TO STARTING ANY INVESTIGATION OF THE HYDRAULIC SYSTEM,

PLACE THE AIRPLANE ON JACKS. (REFER TO JACKING, SECTION II.)

6-1. INTRODUCTION.

The PA-34-200T hydraulic system components covered in this section consist of the combination
hydraulic pump and reservoir, hydraulic pressure switch, free-fall valve 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
landing gear and components is covered in Section VII.

This section provides instructions for remedying difficulties which may arise in the operation of the
hydraulic system. It includes: a system description; troubleshooting recommendations; and instructions
for the removal, repair, and installation of components, as well as adjustments and tests.

6-2. DESCRIPTION.

Hydraulic fluid to the landing gear actuating cylinders is supplied by an electrically powered reversible
pump located in the right forward area of the fuselage nose section. 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 placed in either the up or down position, the pump directs fluid through the
particular pressure line to each individual actuating cylinder. As fluid pressure increases at one side of a
cylinder piston, fluid at the other side is directed back through the other line to the pump. Both lines serve
either as pressure or return passages depending on the rotation of the pump to retract or extend the gear.

A pressure switch is mounted on the pressure line in the right aft side of the nose cone. This switch
opens the electrical circuit to the pump solenoid when the gear fully retracts and pressure in the system
increases to 1800 + 100PSI. The switch will continue to hold the circuit open until pressure in the
system drops to approximately 1700 psi when, at that time, the pump will again operate to build up
pressure as long as the gear selector is in the up position. The down position of the selector handle does
not affect the pressure switch.

Two hydraulic pumps may be encountered:

Original Equipment (Prestolite).

The hydraulic pump is a gear type unit driven by a 12 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 primary thermal relief valve is incorporated on early - O models and is
located directly above the nose gear actuating cylinder. This relief valve will maintain pressure in
the system up to 2350 ± 50 psi. An additional relief valve is incorporated in the pump which will
open at 4000 ± 300 psi and allow fluid to flow into the reservoir. On later - 2 models the external
thermal relief is not used in the system. The relief valve in the pump will open at 2250 ± 50 psi. to
prevent 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 the gear. A shuttle valve
located in the base of the pump allows fluid displaced by the cylinder pistons to return to the
reservoir without backpressure. This shuttle valve has a delivery pressure of 400-800 psi during the
extension cycle.

Piper PA-34-200T Seneca II, Service Manual

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