THS Actuator Schematic
 Figure 004
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 Schematic of the Control Valve Block - Normal Operation
 Figure 005

 Schematic of the Control Valve Block - Jamming Detection Operation (Blocked

 Control Valve)
 Figure 006
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 B. Mechanical Control Operation
 The input shaft which is connected to the cable control moves the mechanical servoloop mechanism through an override mechanism. The override mechanism installed in the pitch trim actuator makes sure that the mechanical control cancels the electrical control.
 (1)
 Operation with the PTA in the Off Mode If a manual command signal is applied with the handwheel to drive the THS actuator because the PTA is inoperative, the override mechanism stays off. The input signal is transmitted directly to the control valves of the valve block.

 (2)
 Operation of the Override Mechanism If an electrical pitch trim command controls the PTA servomotor, the manual command signal applied through the mechanical input causes the override mechanism to break out. This mechanically disconnects the PTA output from the command loop of the THS actuator. Both control valves are then driven through the manual command signal of the THS actuator. The overriding procedure is possible when the THS actuator is controlled in a fixed position (the handwheel stands still). The overriding procedure is also possible when the THS actuator moves on a demand of a variable electrical command signal (the handwheel turns).

 C. Ground Operation from the Electric Pumps
 During operation with low available flow conditions, the pressure
 maintaining valve keeps the pressure-off brakes released.

 D. Operation in Failure Conditions
 (1) Operation with Loss of the Hydraulic Power
 The THS actuator is usually supplied with power from two independent
 hydraulic systems. When one hydraulic supply to the THS actuator does
 not operate, the related POB is activated.
 The POB stops and holds the hydraulic motor shaft. If this occurs,
 the THS actuator is operated by the other hydraulic system at half
 speed, but the necessary maximum performance is still achieved.

 If a full loss of hydraulic power occurs, the POB and the no-back
 brake operate. They keep the ball screw-jack in the last specified
 position against aerodynamic pressure.

 (2) Operation with a Blocked Control Valve
 When a control valve is blocked, the jamming protection device operates to stop hydraulic power. The control valve opening in the defective circuit lets the hydraulic motor continue to operate. This causes the ball screw to continue to operate and move the feed-back gear after the serviceable control valve reaches its neutral position. This makes the primary detent in the defective control loop break out. The primary detent tries to operate the serviceable control valve to open. As soon as the serviceable control valve is operated, it activates the comparator (COMP) which is connected to both control valves. The piston of the COMP operates both shut-off valves. The shut-off valves isolate the hydraulic power supply from the THS actuator on both circuits. This makes the motor stop which is usually supplied by the defective valve. Thus the POB is activated and stops the ball screw.
 In the event that a control valve blocks in a position that is not in the limits of the protection device:
 -
the two hydraulic motors can operate in the opposite direction at a low speed,

 -
the THS actuator does not move,

 -
the jamming detection is sensitive and makes sure that a permanent flow rate of fluid is supplied to prevent overheating.

 (3) Operation with Disconnected Ball Screw
 If the ball screw is disconnected, the no-back brake holds it in its last signalled position.
 7. Test____
 A. Ground Test
 Different checks can be done when the Yellow and Green hydraulic systems are depressurized. The subsequent tests/checks are:
 -
The operational/test of the valve-jamming protection system.

 -
The functional test of the pressure off brake.

 -
The functional test of the no-back brake assembly.

 -To check the integrity of the ball screw shaft and the tie bar.
 -
The visual inspection of the power gears in position with a borescope.
　
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