A B Check valve Piston rod Piston O-ring O-ring Outlet port Outlet port Inlet port Inlet port Operating handle Operating handle Figure 12-20.  Rotary hand pump. Figure 12-19.  Double action hand pump. Classification of Pumps Power-Driven Pumps 12-16 Many of the power driven hydraulic pumps of current  aircraft are of variable delivery, compensator-controlled  type. Constant delivery pumps are also in use. Principles  of operation are the same for both types of pumps. Modern  aircraft use a combination of engine-driven power pumps,  electrical-driven power pumps, air-driven power pumps,  power transfer units (PTU), and pumps driven by a RAT.  For example, large aircraft, such as the Airbus A380, have  A rotary hand pump may also be employed. It produces  continuous output while the handle is in motion.  Figure 12-20 shows a rotary hand pump in a hydraulic system. All pumps may be classified as either positive displacement  or nonpositive displacement. Most pumps used in hydraulic  systems are positive displacement. A nonpositive- displacement pump produces a continuous flow. However,  because it does not provide a positive internal seal against  slippage, its output varies considerably as pressure  varies. Centrifugal and impeller pumps are examples of  nonpositive-displacement pumps. If the output port of a  nonpositive-displacement pump was blocked off, the pressure  would rise, and output would decrease to zero. Although  the pumping element would continue moving, flow would  stop because of slippage inside the pump. In a positive- displacement pump, slippage is negligible compared to  the pump’s volumetric output flow. If the output port were  plugged, pressure would increase instantaneously to the point  that the pump pressure relief valve opens. When the piston is moved to the right, the pressure in the  chamber left of the piston is lowered. The inlet port ball check  valve opens, and hydraulic fluid is drawn into the chamber.  At the same time, the rightward movement of the piston  forces the piston ball check valve against its seat. Fluid in the  chamber to the right of the piston is forced out of the outlet  port into the hydraulic system. When the piston is moved to  the left, the inlet port ball check valve seats. Pressure in the  chamber left of the piston rises, forcing the piston ball check  valve off of its seat. Fluid flows from the left chamber through  the piston to the right chamber. The volume in the chamber  right of the piston is smaller than that of the left chamber due  to the displacement created by the piston rod. As the fluid  from the left chamber flows into the smaller right chamber,  the excess volume of fluid is forced out of the outlet port to  the hydraulic system. two hydraulic systems, eight engine-driven pumps, and three  electrical driven pumps. The Boeing 777 has three hydraulic  systems with two engine driven pumps, four electrical driven  pumps, two air driven pumps, and a hydraulic pump motor  driven by the RAT.  [Figure 12-21  and  12-22]