3) Comparator channel in coupler provides correct voltage and ac power is correct. 4) Mach signal is good as provided by the air data computer.
(b)
If any one of the above requirements fail, the requirements for mach logic good are no longer satisfied. This opens the solid state switch and breaks the ground to thee e power relay. As a result, the brake clamps the actuator from further movement and the excitation voltage is removed from the actuator motor.
(c)
Power is present at the warning light relay when flaps are up, and circuit breakers are closed. The mach trim warning light remains off when the following requirements provided warning logic good to complete the logic circuit to ground. 1) Power, that is applied to release the brake, is also applied to a logic circuit as an
input to the warning light logic.
2) The inputs that satisfy mach logic good for the power relay are the same requirements to satisfy warning logic good. Failure of any of these inputs will cause the mach trim warning light to illuminate.
3) When flaps are lowered, the warning light is extinguished.
(3) Mach Trim Cockpit Test
(a) The cockpit test circuit is activated by depressing and holding the pushbutton switch on the forward overhead panel which is labeled MACH TRIM TEST. (See figure 1.) After 3 to 4 seconds, the MACH TRIM FAIL light illuminates which indicates the actuator has retracted. (The actuator is normally extended until the mach tuck region is entered.) The MACH TRIM FAIL light remains illuminated 7 to 8 seconds after releasing the test switch, until the actuator is repositioned to its original reference position. Extinguishing of the MACH TRIM FAIL light indicates satisfactory completion of the mach trim cockpit test. The test circuit operates as follows: 1) By depressing the MACH TRIM TEST switch, 28 volts dc is applied to a test
relay which provides a test voltage to the command channel (figure 2) and to the logic circuit (figure 3).
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2) The test voltage applied to the command channel causes the command channel to provide a signal which is different from the one provided by the comparator channel. The test signal is applied to the actuator in the same manner as described in paragraph (l). As the actuator changes length, an error signal is induced in the position feedback synchro. The comparator uses the error signal to compare and detect differences from the air data computer input. Differences cause the comparator to apply comparator not good to the warning logic circuit. This circuit removes the voltage from the warning light relay which allows the MACH TRIM FAIL light to illuminate.
3) When the MACH TRIM TEST switch is released, voltage is held by the engage logic circuit to allow the actuator to return to its original position. After 7 or 8 seconds, the warning logic and engage logic receive logic good voltage. This voltage causes the MACH TRIM FAIL light to extinguish and causes removal of power from the actuator.
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MACH TRIM SYSTEM - TROUBLE SHOOTING
1. General
A. The mach trim system can be checked for proper operation by using the mach trim test switch on overhead panel P5 or the self-test switch on the front of the mach trim coupler. (Refer to Mach Trim System - Adjustment/Test.) Failure of the mach trim system, as indicated by the mach trim failure light on overhead panel P5 and the meter and light on the coupler, usually indicates the coupler is faulty. The mach trim failure light will also be illuminated if the coupler self-test switch is left in any position other than OFF. Replacement of the coupler will correct the majority of any faults which may occur. The mach trim system operates with the autopilot system either engaged or not engaged. If no elevator movement results when the mach trim system is operated and the pitch channel is engaged, the neutral shift sensor is faulty (assuming the pitch channel operates normally). Connectors may also be a source of trouble. If replacement of the coupler does not correct the trouble, connectors and wiring should be checked for damage.
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.
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