(a)
When the ALTN EXTENSION switch is pushed, two alternate extension control relays on the MISC ELEC EQUIP panel, P36, are energized. The power to energize these relays is supplied by the 28 VDC BAT. BUS through the LANDING GEAR ALTN EXT CONT circuit breaker. The circuit breaker is installed on the main power distribution panel, P6.
(b)
The ALTN EXT CONT LATCH relay supplies a latching circuit. This circuit continues to supply electrical power to keep the relays energized after the ALTN EXTENSION switch has turned off. The ALTN EXT CONT relay sends electrical power to the power pack. Electrical power is supplied to the power pack by the 28 VDC HOT BAT. BUS through the LANDING GEAR ALTN EXT MOTOR circuit breaker, also on the P6 panel.
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SYSTEM "UP" POWER PACK) SHUTTLE PRESSURE
PRESSURE
VALVE LINE FROM GEAR EXTENSION
RETURN COMPONENTS
SHUTTLE VALVE SHOWN IN ALTERNATE EXTENSION OPERATING MODE (BYPASSES LANDING GEAR UP PRESSURE TO RETURN IF GEAR HANDLE PLACED IN UP POSITION)
Alternate Extension System Shuttle
Valve Operation Schematic
Figure 12
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(c)
The power pack removes fluid from the left hydraulic system reservoir. It supplies the hydraulic fluid to the door lock release actuators for the nose and main landing gear and the hydraulic pressure switch. These actuators extend and mechanically release the internal locks in the door actuators. The safety valves are also turned into the OFF (safety) position. With the door actuators not locked, the landing gear doors now start to free fall open. Hydraulic fluid is also sent to the alternate extension shuttle valve. This prevents fluid flow through the normal system "up" lines. Retraction pressure is then removed if the landing gear selector valve is in the "up" position when alternate extension is used.
(d)
When the door lock release actuators are fully extended, internal valves send the hydraulic fluid through the actuator to the alternate uplock release actuators for the nose and main landing gear . These actuators extend and mechanically release the locks for the landing gears. The landing gears now start to free fall to the down-and-locked position.
(e)
After the alternate uplock release actuators extend, the hydraulic fluid is caught in the system. The pressure then starts to increase. When the pressure gets to the range of 1700 to 2200 psi, the hydraulic pressure switch removes electrical power to the control relays. This stops operation of the power pack.
(f)
This completes the alternate extension cycle. The time for operation is approximately 10 seconds.
(2)
Door Ground Operations
(a)
Open the Doors
1) The alternate extension system, above, is used to open the landing gear doors for ground maintenance. But, the system is not controlled from the flight compartment. The system is operated by movement of the ALL DOORS OPEN ARM switch to ARM and the ALL DOORS OPEN switch to ON at the same time. The ALL DOORS OPEN switch does not operate when the landing gear doors are open.
2) Also, the red DOOR UNSAFE lights warn persons of conditions that are not safe in the landing gear wheel wells. Electrical power is supplied to the lights by the 28 VDC BAT. BUS. The LANDING GEAR ALTN EXT CONT circuit breaker on the P6 panel controls the power supply.
3) The DOOR CLOSED and DOOR LOCKED switches supply inputs for the door not safe condition. When this happens, the switches supply the electrical power to the lights. When the locks on the doors are released and the doors start to open, the DOOR CLOSED switches make the red lights come on. When the door safety valves have turned and locked into the OFF (safety) position, the DOOR LOCKED switches then make the red lights go off.
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(b) Close the Doors
1) The landing gear doors are closed with the normal extension and retraction system (i.e. with the left hydraulic system pressurized and the control lever for the landing gear moved to DN). But, after the doors are opened by the door ground release (or alternate extension), the safety valves are locked to stop the supply of hydraulic pressure to the door actuators. To release the safety valve lock, the MAIN GEAR and NOSE GEAR DOOR CLOSE switches are operated.
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本文鏈接地址：757 AMM 飛機維護手冊 起落架 CHAPTER 32 - LANDING GEAR 2(93)
