508  Main Gear Hydraulic System Schematic 
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9.  Main Gear Isolation Valves
A.  The main gear isolation valves are provided to enable maintenance to be carried out on the landing gear, and landing gear hydraulic components, without depressurizing the hydraulic system. The valve consists of a dual unit in a modular package (Fig. 9). The valves are normally lockwired in the open position. The isolation valve modular package is mounted in the left wheel well.
10. Operation
A.  Retraction
(1)  When the control lever is moved to UP position, hydraulic pressure is then directed to the modular package and to the uplock actuator. After passing through the modular package, pressure reaches the up port of the main actuator and the uplock port of the downlock actuator. Flow to the main actuator is controlled by flow limiting valves. Flow to the downlock actuator is restricted. Flow from the downlock actuator is unrestricted. Therefore, the downlock actuator quickly unlocks the gear and starts the side strut folding before full pressure builds up in the main actuator (Fig.\3). When the gear retracts, the uplock actuator has locking pressure, but stop links prevent the uplock hook from moving to locked position. On the final movement to ful1 retraction, the uplock roller strikes a projection on the uplock hook; this triggers stop links to flip over and release the hook. The pressurized lock actuator then snaps the hook into final locked position. In the same movement, the stop links engage in locking detents to hold the hook in position.

Main Landing Gear Isolation Valve  516 
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B.  Extension
(1)  When the control lever is moved to DN position, hydraulic pressure is directed to the main gear actuator, the lock port of the downlock actuator, and to the modular package. After passing through the modular package pressure reaches the unlock port of the uplock actuator. Pressure to the uplock actuator is restricted. Pressure from the uplock actuator is unrestricted. Therefore, the actuator moves quickly to unlock the gear. After retraction, as soon as retraction pressure leaves the main actuator, the full weight of the gear hangs on the uplock hook. To prevent the possibility of main actuator extension force further loading the hook and so causing noisy disengagement of the uplock, the transfer cylinder goes into action. On the first surge of pressure in the main actuator down line, the cylinder transfers pressure to the main actuator up line. Loss of transfer cylinder pressure from the up line is delayed by the flow limiting valve in the modular package. For a moment, pressure is equal on both ends of the main actuator, and the piston differential area creates an up force. The uplock actuator extends to unlock the hook during the up force period, and the gear extends. On extension, locking pressure on the downlock actuator pulls the lock strut into the overcenter locked position. Spring bungees apply a force to keep the strut in locked position.
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MAIN LANDING GEAR EXTENSION AND RETRACTION - TROUBLE SHOOTING
1.  General
A.  Trouble shooting the main gear retraction mechanism requires the airplane to be jacked for gear retraction per Chapter 7, Airplane Jacking. External hydraulic power from a test bench capable of a varying flow rate and electrical power must be provided. Three men are required. One stationed in the control cabin to operate the landing gear control lever, one stationed at the hydraulic test bench to vary retraction speed or stop the gear in an intermediate position, and one stationed at the wheel well to observe the gear action. The control cable system must be correctly rigged and hydraulic reservoirs filled to correct level. Close observation of the gear action should reveal the cause of the trouble.
2.  Trouble Shooting Charts

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MAIN GEAR EXTENSION AND RETRACTION - ADJUSTMENT/TEST
1.  General
A.  For all tests an hydraulic test bench may be connected to the ground service module. System A is pressurized by setting the parking brake and opening the interconnect valve (Ref Chapter 29, Hydraulic Power).
　
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