(2)  
The back-drive valve is a manual valve which reverses the RAT pump connections to the center hydraulic system. In the back-drive position, center system pressure is routed to the supply port of the RAT pump. This causes the pump to act as a motor driving the turbine in the normal direction.

 (3)  
The module filters both pressure and case drain fluid from the RAT pump. The filters are a non-cleanable type.

 (4)  
A pressure switch senses pressure output of the RAT pump and turns on the RAT pressure light in the flight compartment.

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
757
MAINTENANCE MANUAL

 (5)  
Check valves prevent the RAT system from being pressurized by the center system while the RAT is not in use. An orifice in the back-drive valve permits a small flow of fluid to warm the strut and pump. The fluid returns to the center system through the pump case drain line.

 (6)  
The volumetric fuse cartridge valve reduces the pressure load on the RAT pump until the turbine reaches governed speed.

 H. Tachometer
 (1)  
The tachometer provides a visual indication of the operating status of the RAT during ground test. The tachometer consists of an indicator and a speed sensor. The tachometer indicator is inside the fairing panel at the lower edge of the right wheel well aft bulkhead.

 (2)  
The tachometer indicator has two indicator lights and a lamp test switch. A green light indicates turbine speed is at the normal governed speed. A red light indicates turbine overspeed. If no light is on, the turbine speed is below the normal governed speed. The lamp test switch is used to check that the indicator lights are not burned out. When the switch is pressed, with the RAT operating, both indicator lights should illuminate.

 (3)  
Power for the tachometer indicator and the source of turbine speed sensing is provided by a speed sensor located in the lower end of the RAT strut between the hub and pump. The speed sensor is a permanent magnet generator that produces an electrical signal which is used to power the indicator lights.

 I. RAT Compartment Door
 (1)  The compartment door is hinged on the outboard side and opens as the RAT is deployed. The door and opening in the fairing have seals to isolate the compartment when the RAT is stowed.
 J. Door Actuation Link
 (1)  The actuation link joins the RAT strut to the compartment door. The link has a bearing at each end. The link forces the door open or closed as the RAT deploys or retracts.
 K. RAT Compartment Access Panel
 (1)  An access panel is located in the outboard wall of the RAT compartment, inside the aft right wing/body fairing. Removal of wing/body fairing panel (196CR) and the RAT compartment access panel (198NZ) allows access to the RAT, if it cannot be deployed normally.
 3. Operation
_________
 A. Functional Description (Fig. 2)
 (1)  In-Flight Deployment
 (a)  
In the air mode, the air/ground system No.1 relay provides 28 volt dc power from the battery bus to the RAT arm Q airspeed switch. The RAT arm Q airspeed switch is actuated by the pitot/static system when airplane speed reaches 80 KIAS to arm the RAT automatic deployment circuit.

 (b)  
If power is lost in both engines, the left and right engine speed cards energize the RAT auto control relay. The auto control relay energizes the auto solenoid to unlock the deployment actuator.
　
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