Hydraulic System Description - Piper PA-44-180 SEMINOLE Maintenance Manual

GENERAL
The PA-44-180/180T may be equipped with either a Prestolite (S/N's 44-7995002 thru 44-8195026 and all
PA-28 -180T's) or Oildyne hydraulic pump (S/N's 4495001 and up) Prestolite pumps may be replaced with
Oildyne pumps.
The hydraulic components covered in this chapter consist of the combination hydraulic pump and reservoir,
free fall valve, actuating cylinders and hydraulic lines. The brake system, although hydraulically operated, is
not included in this chapter as it has its own hydraulic system independent of the gear retraction system. Refer
to Chapter 32 for information on the brake system.
PRIOR TO STA RTING ANY INVESTIGATION OF THE
HYDRAULIC SYSTEM, PLACE THE AIRPLANE ON
JACKS (REFER TO JACKING, CHAPTER 7)
DESCRIPTION.
Hydraulic fluid to the landing gear actuating cylinders is supplied by an electrically powered reversible
pump located in the aft fuselage on the battery shelf between stations 156 and 191. A reservoir is an integral
part of the pump. The pump is controlled by a selector switch on the instrument panel to the left of the control
quadrant. As the switch is placed in either the up or down position, the pump directs fluid through the particular
pressure line to each individual actuating cylinder. Both lines serve either as pressure or return passages
depending on the rotation of the pump to retract or extend the gear (Refer to Figures 1 and 2).
A pressure switch is installed on a cross fitting connected to the pump mount assembly. During 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. The
switch will continue to hold the circuit open until pressure in the system drops to 200 to 400 psi. At that time
the pump will again operate to build up pressure as long as the gear selector handle is in the up position. The
down position of the selector does not affect the pressure switch. (For specific pressure refer to Chart 2901.)
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 Chart 2901. 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 opens at 2250 ± 250 psi, allowing fluid to flow into the reservoir. The Oildyne pump's
relief valve opens at 3000 ± 200 to 300 psi. 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 back pressure. (For specific pressures refer to Chart
2902.)
Also in the system is a bypass or free-fall valve, operated by the emergency gear knob, that releases
hydraulic pressure to permit the gear to free fall, with spring assistance on the nose gear, should a malfunction
in the pump system occur. The knob must be pulled out for emergency extension. To prevent the gear from
extending too fast, there is a special restriction elbow on the nose gear retraction line. The knob must be
pushed in when the hydraulic system operational checks are being conducted. The emergency gear knob is
located on the instrument panel to the left of the control quadrants.
PIPER AIRCRAFT
PA - 4 4 - 1 8 0 / 1 8 0 T
AIRPLANE MAINTENANCE MANUAL
— CAUTION —
2B20
29-01-00
Page 29-01
Revised: June 20, 1995