Hand pumps are used in hydraulic systems to supply fluid under pressure to subsystems, such as the landing gear, flaps, canopy, and bomb-bay doors, and to charge brake accumulators. Systems using hand pumps are classified as emergency systems. Most of these systems may be used effectively during preventive maintenance.

Double-action type of hand pumps are used in hydraulic systems. Double action means that a flow of fluid is created on each stroke of the pump handle instead of every other stroke, as in the single-action type. There are several versions of the double-action hand pump, but all use the reciprocating piston principle, and operation is similar to the one shown in figure 7-13.

This pump consists of a cylinder, a piston containing a built-in check valve (A), a piston rod, an operating handle, and a check valve (B) at the inlet port. When the piston is moved to the left in the illustration, check valve (A) closes and check valve (B) opens.

Fluid from the reservoir then flows into the cylinder through inlet port (C). When the piston is moved to the right, check valve (B) closes. The pressure created in the fluid then opens check valve (A), and fluid is admitted behind the piston. Because of the space occupied by the piston rod, there is room for only part of the fluid; therefore, the remainder is forced out port (D) into the pressure line. If the piston is again moved to the left, check valve (A) again closes. The fluid behind the piston is then forced through outlet port (D). At the same time, fluid from the reservoir flows into the cylinder through check

Figure 7-13.Double-action hydraulic hand pump.

valve (B). Thus, a pressure stroke is produced with each stroke of the pump handle.

Hand pumps are examined frequently for leakage, general condition, and efficiency in operation. To check the operation of a hand pump, the following procedure is recommended:

1. Connect a direct-reading hydraulic pressure gauge into the emergency hand pump pressure line.

2. Insert and lock the hand pump handle in the pump actuating socket.

3. Select an appropriate subsystem to operate, and place its selector valve in an operating position.

4. Actuate the hand pump handle until the unit being operated has completed its movement. Check the pressure gauge for a drop in system pressure.

NOTE: Air in emergency systems will cause the pump handle to spring rapidly to the other end of the stroke.

5. If a pressure drop is indicated, check the system for leakage before removing the pump for repair or replacement.

6. Observe the hand pump handle for piston creep, which indicates that the pump should be removed for repair or replacement.

Removal, replacement, and operational check of hand pumps should correspond to the procedures recommended in the specific MIM.