(2)  
The two main cargo door control switches must be operated in sequence to raise the door to the full open position and fully close the door. Switch No. 1 is operated first to unlatch and open the door to the canopy position. Switch No. 2 is operated to raise the door from the canopy position to full open. Closing the door requires switch No. 2 be operated first to lower the door to the canopy position. Switch No. 1 can then be operated to close the door.

(3)  
Holding switch No. 1 to UP TO CANOPY completes 28 volts dc through a canopy position B switch to the main cargo door motor-operated control valve (Fig. 7). The motor drives the valve to POS 1 which connects the hydraulic pressure line to the "open" line of the lift and latch actuators and then closes an internal switch which removes 28 volts dc from the "open" motor winding. The internal switch then connects power to the main cargo door shutoff valve. Another switch within the control valve connects a circuit from the canopy position A switch to the "close" motor winding. The main cargo door shutoff valve opens and applies hydraulic pressure to the "open" side of the lift actuator and latch actuator, provided the airplane is on the ground and the parking brake is set. Due to a restrictor in the lift actuator, the latch actuator operates first to unlatch the door, open it far enough to break the seal and to operate the door position switch. When the door is unlatched, the lift actuator opens the door approximately 87 degrees. At this point the canopy position B switch is operated by a cam driven by the lift bellcrank and 28-volt dc power is disconnected from the solenoid-operated shutoff valve to isolate system B pressure from the actuators and hydraulically lock the door in the canopy position.

NOTE:  Due to possible internal or external hydraulic leakage, the door may settle from canopy (a hydraulic lock position) to ram lock. In this case, to raise the door up from ram lock position, it is necessary to actuate control switch No. 1 up to canopy, to raise the door sufficiently to operate position B switch. This makes 28 volts dc available to the shutoff valve through switch No. 2.
520 
52-32-0 Page 18  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Feb 01/76 

501  Main Cargo Door Hydraulic Schematic 
Aug 15/72  Figure 7  52-32-0 
Page 19 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

(4)  
Operating switch No. 2 to FULL OPEN completes 28 volts dc to the canopy position B switch (now repositioned) which completes power to the shutoff valve. The shutoff valve then opens and applies pressure to the system. Since the control valve is still open, hydraulic pressure is again applied to the "open" side of the lift actuator and the door rises to the full open position. As the door passes the 90-degree open position, a second cam on the lift actuator bell crank operates the canopy position A switch which connects switch No. 2 DOWN TO CANOPY control circuits to the control valve. The door will raise to full open provided switch No. 2 is held until the door has reached its maximum height. Door full open position is with door overcenter and the lift actuator bottomed out.

(5)  
If either switch No. 1 or 2 is released before the door has reached the canopy position or the full open position, 28 volts dc through the main cargo door position switch ensures the control valve is in POS 1 and the door is hydraulically locked open. This also occurs any time either switch is released with the door position switch in the "door open" position.

(6)  
Operating switch No. 2 to DOWN TO CANOPY completes 28 volts dc through canopy position A switch to the control valve motor to drive the valve to POS No. 2. The control valve connects the hydraulic pressure line to the "closed" line of the lift actuator and latch actuator. An internal switch within the control valve connects power to the shutoff valve when the control valve reaches position No. 2. The shutoff valve then opens and connects hydraulic pressure to the system. However, due to the spring cartridge assembly holding the latch actuator overcenter, the latch actuator cannot operate until the door reaches the closed position. The lift actuator lowers the door at a rate controlled by the return line flow limiter. At approximately 90 degrees the canopy position A switch operates and disconnects switch No. 2 from the shutoff valve and the position 2 side of the control valve. The control valve remains in position 2 as long as switch No. 2 is held in DOWN position. Hydraulic pressure is removed from the door actuators but door motion continues due to door weight. At approximately 87 degrees the canopy position B switch operates and connects switch No. 1 to the shutoff valve and the POS 1 side of the control valve but no electrical power is available until switch No. 1 is held in FULL CLOSE position. The door continues to the ram-lock position, which is slightly below where the canopy position B switch operates. Release of switch No. 2 connects 28 volts dc through the door position switch to the POS 1 side of the control valve which rotates the control valve to the POS 1 position creating a hydraulic lock. (See figures 4 and 5 for switch locations.)
　
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