System B System A Alternate brakes Ground spoilers Autopilot A Flight spoilers Nose wheel steering Landing gear Alternate nose wheel steering Normal brakes Trailing edge flaps Autopilot B Flight spoilers No. 2 thrust reverser Yaw damper No. 1 thrust reverser Rudder Ailerons Leading edge flaps & slats Auto slats Elevator + elevator feel Standby system Landing gear transfer unit M M M EDP EDP ACMP ACMP ACMP Figure 12-64.  Boeing 737 hydraulic system (simplified). Landing Gear Transfer Unit Indications Boeing 777 Hydraulic System Standby Hydraulic System 12-42 The Boeing 777 is equipped with three hydraulic systems.  The left, center, and right systems deliver hydraulic fluid at a  rated pressure of 3,000 psi (207 bar) to operate flight controls,  flap systems, actuators, landing gear, and brakes. Primary  hydraulic power for the left and right systems is provided  by two EDPs and supplemented by two on-demand ACMPs.  Primary hydraulic power for the center system is provided  by two electric motor pumps (ACMP) and supplemented  by two on-demand air turbine-driven pumps (ADP). The  center system provides hydraulic power for the engine  A master caution light illuminates if an overheat or low  pressure is detected in the hydraulic system. An overheat  light on the flight deck illuminates if an overheat is detected  in either system A or B and a low-pressure light illuminates  if a low pressure is detected in system A and B. The standby hydraulic system is provided as a backup if  system A and/or B pressure is lost. The standby system can  be activated manually or automatically and uses a single  electric ACMP to power: The purpose of the landing gear transfer unit is to supply the  volume of hydraulic fluid needed to raise the landing gear at  the normal rate when system A EDP is lost. The system B  EDP supplies the volume of hydraulic fluid needed to operate  the landing gear transfer unit when all of the following  conditions are met: •  Leading edge flaps and slats (extend only) •  Standby yaw damper •  Aircraft airborne. •  No. 1 engine rpm drops below a limit value. •  Landing gear lever is up. •  Either or both main landing gear not up and locked. •  Thrust reversers • Rudder