Stabilizer Trim Motor Stabilizer trim actuation on the  aircraft is provided by two 3,000-  psi, constant-displacement, nine-piston, bent-axis hydraulic  motors. Each motor produces  77.3 in-lb torque at 2,250 psid  with a rated speed of 2,700 rpm  and an intermittent speed of  4,050 rpm. Displacement is  0.216 in$^{2}$/rev; weight is 3.9 lb. Emergency Passenger Door Actuator Plays a critical role in  safety; extends to open  door when actuated by nitrogen  gas pressure. Assembly  reaches full extension in 2.75  to 4.16 seconds with output force  of 2,507 to 2,830 lb.  Trailing edge flap actuation is provided by  one 3,000-psi, constant-displacement, nine- piston, bent-axis hydraulic motor. The motor  produces 21.4 in-lb torque at 2,250 psid with  a rated speed of 3,750 rpm and an inter- mittent speed of 5,660 rpm. Displacement is  0.596 in$^{2}$/rev; weight is 6.5 lb. Engine-Driven Pump Hydraulic power for the left and right systems is  supplied by two 48-gpm variable-displacement,  3,000-psi pressure compensated in-line pumps.  Displacements 3.0 in$^{2}$/rev; weight is 40.1 lb. Ram Air Turbine Pump A 3,025 psi in-line piston pump  provides 20 gpm at 3,920 rpm,  delivering hydraulic power for  the priority flight control surfaces  in the event both engines are  lost or a total electrical power  failure occurs. Displacement  is 1.25 in$^{2}$/rev; weight is 15 lb. Nose Wheel Steering System Consists of a digital electronic controller, hydro- mecharical power unit, mounting collar, tiller,  and rudder pedal positon sensors. The hydro- mecharical power unit (an integrated assembly)  includes all the hydraulic valving, power  amplification, actuation, and damping  components. AC Motor Pump Auxiliary power is provided  by a 3,110-psi, 12-gpm,  8,000-rpm fluid-cooled  motor pump. Some AC  motor pumps feature a  ceramic feed-through  design. This protects the  electrical wiring from being  exposed to the caustic  hydrauic fluid environment. Leading Edge Slat Drive Motor Leading edge slat actuation on the  aircraft provided by one constant- displacement, nine-piston, bent-axis  hydraulic motor. The motor produces  544.3 in-lb torque at 2,250 psid with  a rated speed of 3,170 rpm and an  intermittent speed of 4,755 rpm.  Displacement is 1.52 in$^{2}$/rev; weight  is 13.21 lb. Hydraulic Motor- Driven Generator The hydraulic motor-driven  generator (HMDG) is a servo-  controlled, variable  displacement, inline axis-piston  hydraulic motor integrated  with a three-stage, brushless  generator. The HMDG is  designed to maintain a  steady state generator  output frequency of 400 ±2V (at the point of regulation)  over a rated electrical output  range of 10kVA. Power Transfer Unit The transfer of hydraulic power  (but not fluid) between the left  and right independent hydraulic  system is accomplished with a  nonreversible power transfer  unit (PTU) that provides an  alternate power source for the  leading and trailing edge flaps  and the landing gear, including  nose gear steering, which are  normally driven by the left  hydraulic system. The PTU  consists of a bent-axis hydraulic  motor driving a fixed displacement,  in-line pump. Rated speed is 3,900  rpm. Displacement of the pump is  1.39 in$^{2}$/rev and displacement of  the motor is 1.52 in$^{2}$/rev. The unit weight is 35 lb. Trailing Edge Flap Drive Motor System A reservoir Keel beam Forward bulkhead System B reservoir Standby reservoir FWD Figure 12-62.  Large aircraft hydraulic systems. Figure 12-63.  Hydraulic reservoirs on a Boeing 737. PTU uses system A pressure to drive a hydraulic motor. The  hydraulic motor of the PTU unit is connected through a shaft  with a hydraulic pump that can draw fluid from the system  B reservoir. The PTU can only transfer power and cannot  transfer fluid. The PTU operates automatically when all of  the following conditions are met: •  System B EDP pressure drops below limits. •  Aircraft airborne. •  Flaps are less than 15° but not up. 12-41