To system pressure manifold To system return manifold Orifice check valve Main-gear sequence valve Main-gear sequence valve Left main-gear actuating cylinder Left gear uplock Right gear uplock Right gear downlock Left gear downlock Gear-door sequence valve Gear-door actuator Nose-gear downlock Nose-gear uplock Nose-gear actuator Gear-door actuator Gear-door sequence valve Right main-gear actuating cylinder Orifice check valve Down Landing gear selector in gear-up position Up Up Close Close Up Up C D A B Figure 13-37.  A simple large aircraft hydraulic gear retraction system.  13-21 can be retracted. At the same time, the actuator cylinder on  each gear also receives pressurized fluid to the gear-up side  of the piston through an unrestricted orifice check valve. This  drives the gear into the wheel well. Two sequence valves  (C and D) also receive fluid pressure. Gear door operation  must be controlled so that it occurs after the gear is stowed.  The sequence valves are closed and delay flow to the door  actuators. When the gear cylinders are fully retracted, they  mechanically contact the sequence valve plungers that open  the valves and allow fluid to flow into the close side of the  door actuator cylinders. This closes the doors. Sequence  valves A and B act as check valves during retraction. They  In the system illustrated in  Figure 13-37 , when the flight deck  gear selector is moved to the gear-up position, it positions  a selector valve to allow pump pressure from the hydraulic  system manifold to access eight different components. The  three downlocks are pressurized and unlocked so the gear  and do not require hydraulic power. There are many gear and  gear door arrangements on various aircraft. Some aircraft  have gear doors that close to fair the wheel well after the  gear is extended. Others have doors mechanically attached  to the outside of the gear so that when it stows inward, the  door stows with the gear and fairs with the fuselage skin. 