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B.  Two ports of the valve are connected to the door open and door close lines of the lift and latch actuators. The other two ports are connected to the pressure and return lines. In position No. 1, the door closed line of the lift actuator is connected to the return line and the door open line at the lift actuator is connected to the pressure line. In position No. 2, the door closed line is connected to the pressure line and the door open line is connected to the return line. The  POS 1 and POS 2 positions are marked adjacent to the control valve manual override lever.
CAUTION:  ELECTRICAL POWER MUST NOT BE SUPPLIED TO THE CONTROL VALVE WHEN THE MANUAL OVERRIDE LEVER IS USED OR DAMAGE TO THE MOTOR MAY OCCUR.
8.  Shutoff Valve
A.  A solenoid -operated shutoff valve is installed on the left wheel well aft bulkhead. The shutoff valve is of the piston type with an internal solenoid connected to the piston. The valve has three ports labeled (1), (2) and (3). When the solenoid is de-energized, the piston connects the output port (2) to the return port (3) and the pressure port (1) is blocked; when energized, the solenoid moves the piston which connects the pressure port (1) to the output port (2) and the return port (3) is blocked. The solenoid uses 28-volt dc power.
9.  Relief Valve
A.  The relief valve, installed at the left wheel well aft bulkhead, is in a bypass line across the output and return lines of the shutoff valve. The relief valve is set for approximately 3500 psi and prevents overpressurization of the cargo door system. It also allows free flow from return to pressure to prevent cavitation within the system during the door closing cycle. 
506 
Aug 15/72  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  52-32-0 Page 15 

10. Flow Limiters
A.  Two flow limiters installed in the hydraulic lines control the speed of the main cargo door during opening and closing. Flow limiting, in one direction only, is done by a variable orifice within the limiter. Variations in flow rate cause a piston to vary the diameter of the orifice. One limiter is installed in the output port of the shutoff valve, the other limiter is installed in the return line between the control valve and shutoff valve.
11. Operation
CAUTION:  DO NOT OPERATE THE MAIN CARGO DOOR WHILE JACKING FORCES ARE BEING APPLIED TO FORWARD BODY JACKING POINT. OPERATION OF THE DOOR CAN RESULT IN DAMAGE TO BOTH DOOR AND FUSELAGE (REF CHAPTER 7, JACKING).
THE MAIN CARGO DOOR MUST NOT BE IN THE CANOPY POSITION WHEN WIND VELOCITY EXCEEDS 60 KNOTS, OR IN THE FULL OPEN POSITION WHEN WIND VELOCITY EXCEEDS 40 KNOTS. THE DOOR ACTUATION SYSTEM IS DESIGNED FOR OPERATION THROUGHOUT FULL DOOR TRAVEL IN WINDS UP TO 40 KNOTS. IF IT BECOMES NECESSARY TO CLOSE THE DOOR IN WINDS EXCEEDING 40 KNOTS, AND DIFFICULTY IS EXPERIENCED DUE TO WIND DIRECTION, THE AIRPLANE SHOULD BE MOVED SO THE DOOR OPENING IS ON THE DOWNWIND SIDE OF AIRPLANE.
A.  The main cargo door may be opened using the control panel, or the manually operated hand pump, or in event of a malfunction in the system, by removing the access panels on the outside of the door for manual actuation of the lockpins and latches. Manual operation with the hand pump is used when hydraulic power and electrical power systems of the airplane are not operating. Normal operation of the door is from the main cargo door control panel, with the airplane on the ground, the parking brake set, hydraulic system B operating, and electrical power on the airplane. The main cargo door circuit is wired through the landing gear safety relay circuit to prevent the cargo door from being opened or closed when the right main gear oleo is extended (Ref Chapter 32, Landing Gear). Oxygen, gasper air, hydraulic and electrical lines are completed to the door with flexible lines at the door hinge; therefore, no disconnects are required before opening the door.
527 
52-32-0 Page 16  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Jun 20/93 

570  Main Cargo Door Electronic Schematic 
Feb 15/78  Figure 6  52-32-0 
Page 17 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

B.  Normal Operation
(1)  
The door is unlocked by pulling on the lock handle on the outside of the door. (The handle can be reached when standing in the forward entryway.) Pulling on the handle causes the eight lockpins to be removed from the rotational path of the eight latch hooks and operates the main cargo door locked switch, thereby completing 28 volts dc to the main cargo door control panel control switches. The main cargo door unlocked light on the control panel and the main cargo door warning light on P5 then illuminate. When the handle is unlocked, the lights at latch viewing windows No. 2 and 7 will extinguish.
　
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