Reservoir Filter Gear-type pump Low-pressure control valve High-pressure control valve Thermal relief valve Thermal relief valve Pressure switch Gear-up check valve Emergency extend Freefall valve Nose-gear actuator Left main-gear actuator Right main-gear actuator Restrictor Restrictor Down Down Down Gear-up check valve piston Shuttle valve Pressure Return Figure 13-35.  A popular light aircraft gear retraction system that uses a hydraulic power pack in the gear down condition. To raise the gear, the flight deck gear handle is moved to the  gear-up position. This sends current to the electric motor,  which drives the hydraulic gear pump in the opposite direction  causing fluid to be pumped to the gear-up side of the actuators.  In this direction, pump inlet fluid flows through the filter.  Fluid from the pump flows thought the gear-up check valve to  the gear-up sides of the actuating cylinders. As the cylinders  begin to move, the pistons release the mechanical down locks  that hold the gear rigid for ground operations. Fluid from the  gear-down side of the actuators returns to the reservoir through  the shuttle valve. When the three gears are fully retracted,  pressure builds in the system, and a pressure switch is opened  that cuts power to the electric pump motor. The gear are held  in the retracted position with hydraulic pressure. If pressure  declines, the pressure switch closes to run the pump and raise  the pressure until the pressure switch opens again. 13-19