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B.  The air data computer supplies mach signals and a mach good logic voltage to the mach trim coupler. Failure of one of the mach potentiometers in the air data computer will cause loss of the mach good logic to the mach trim coupler, thereby causing the mach trim failure light to illuminate. If a failure originates in the air data computer, a module failure indicator will be operated on the front panel of the computer, thereby indicating a need for replacement of the air data computer. Checking for mach good logic can be done by looking for voltage at TP-8 or by positioning coupler self-test switch to 5. The meter should indicate in the red, if not, the air data computer is faulty. Pitot static inputs for the air data computer are not required for basic trouble shooting. Faults which may occur when pitot pressure is applied during a test of the system may be corrected by replacing the air data computer.
2.  Mach Trim Components
A.  Mach Trim Coupler
(1)  
The mach trim coupler is located on the El equipment rack in the electrical and electronics compartment. The mach trim coupler consists of a rack assembly and five plug-in card assemblies. The card assemblies are two amplifier-generator assemblies, the amplifier-monitor assembly, control amplifier assembly, and the power supply.

(2)  
The rack assembly acts as a base for the mach trim coupler and supplies the necessary support and interconnection for the plug-in card assemblies. The unit houses self-test components and test points. The coupler contains automatic self-monitoring circuitry to electrically decouple the actuator and illuminate the warning light in the event of failure in electronic circuitry or actuator.

(3)  
The coupler receives signals which are computed as an elevator command signal. This signal is compared with the actuator feedback signal and then drives the actuator motor.

B.  Mach Trim Actuator
(1)  The mach trim actuator consists of a servomotor, a position sensor, limit stops, gearbox, brake, and actuator screw. The actuator has a 115 volt, 400 cps, single-phase motor which drives the gearbox;
500 
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the gearbox then drives the actuator screw. The direction of screw movement is determined by the application of the proper voltage polarity to the motor control winding. The mach trim coupler provides 28 volts dc to the actuator brake mechanism to unlock the actuator and allow its movement when the actuator is engaged and operating. When power is applied to the motor and brake, actuator motion is limited to fully extended and fully retracted positions by both electrical and mechanical limit stops.
C.  Mach Trim Controls
(1)  
The mach trim flap switch, located in the aft upper right portion of wheel well area, applies 28 volts dc to the mach trim coupler when the flaps are lowered. This signal prevents illumination of the mach trim warning light. See figure 1 for flap switch location.

(2)  
The mach trim cockpit test switch is a pushbutton switch on overhead panel P5. This switch provides a means of testing the mach trim system from the cockpit.

(3)  
The mach trim failure light is located adjacent to the mach trim test switch on the overhead panel. If the flaps are up, the light will illuminate whenever the cockpit test switch is depressed or whenever there is a malfunction in the mach trim system. The light will also illuminate if the coupler self-test switch is left in any position other than OFF.

3.  Operation
A.  Engagement
(1)  The mach trim system engages automatically if airplane speed is in the mach tuck region, air data computer information is valid, system power is present, and computed actuator position and actual position agree.
B.  Disengagement
(1)  
The system automatically disengages when airspeed is below mach tuck region. When disengaged, the warning light remains out and an electromechanical brake locks the actuator to prevent changes in actuator length. Automatic disengagement with warning light illumination takes place when the following conditions exist.

(a)  
The computed actuator position disagrees with the actual actuator position for more than 1 second.

(b)  
Air data computer information is invalid.

(c)  
Failure of electromechanical brake to actuate below mach tuck region.

(d)  
E;ectrical power failure to mach trim system.

 

(2)  
When disengagement is caused by any of the above causes, the warning light will illuminate. The warning light will extinguish when the flaps are lowered.
　
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