Pressure Return 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 Up Up Up Gear-up check valve piston Shuttle valve Figure 13-36.  A hydraulic power pack gear retraction system in the gear up condition. Large Aircraft Retraction Systems  Large aircraft retraction systems are nearly always  powered by hydraulics. Typically, the hydraulic pump is  driven off of the engine accessory drive. Auxiliary electric  hydraulic pumps are also common. Other devices used in a  hydraulically-operated retraction system include actuating  cylinders, selector valves, uplocks, downlocks, sequence  valves, priority valves, tubing, and other conventional  hydraulic system components. These units are interconnected  so that they permit properly sequenced retraction and  extension of the landing gear and the landing gear doors. The correct operation of any aircraft landing gear retraction  system is extremely important.  Figure 13-37  illustrates an  example of a simple large aircraft hydraulic landing gear  system. The system is on an aircraft that has doors that  open before the gear is extended and close after the gear is  retracted. The nose gear doors operate via mechanical linkage  13-20