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(4)  Before trouble shooting the yaw damper system, ensure hydraulic and electrical power is applied to the airplane and the rudder operates normally without the yaw damper system engaged. The flight control system B switch must be on to engage the yaw damper system. Ensure the yaw damper system, master caution, master dimming bus indicator lights, dim and test, autopilot engage interlock and warning light circuit breakers are closed before attempting to engage the yaw damper system. To prevent misleading indications, test operation of the autopilot disengage lights by pressing lenses to ensure bulbs illuminate. It should be noted that the yaw damper engage light is normally illuminated whenever the yaw damper system is not engaged. The central air data computer does not have to be energized for self-test of the yaw damper system since no interlock voltages are provided to the yaw damper systems. However, the air data computer must be installed since the yaw damper q-potentiometer is used in the system. Voltage through the q-potentiometer is provided by the yaw damper system. Readings taken on the yaw damper coupler self-test meter are calibrated with no airspeed inputs to the central air data computer. Removal of black boxes and system components requires normal safety and maintenance precautions such as removal of power prior to removal of components and capping of all exposed electrical connectors.
CAUTION:  DO NOT REMOVE YAW DAMPER COUPLER FOR AT LEAST 3 MINUTES AFTER REMOVING POWER FROM THE SYSTEM. RATE GYRO MAY BE DAMAGED IF COUPLER IS MOVED BEFORE GYRO IS ALLOWED TO SPIN DOWN.
C.  Autopilot System
(1)  
Autopilot system trouble shooting requires a good knowledge of the pitch and roll channel operation. The pitch and roll channels operate independently in the MAN, HDG OFF or VOR/LOC modes. However, both pitch and roll channels must be engaged to operate in the AUTO APP mode. Failures which occur when operating in the MAN, HDG OFF or VOR/LOC mode can usually be traced to the individual channels since the pitch channel does not depend upon roll channel signals except for versine. Failures which occur when operating in the AUTO APP mode may be more difficult to isolate. A good failure report stating which mode and submode the autopilot system was in at the time of failure is a necessity. Determination of what submode the pitch and roll channels were in at the time of failure and

recognizing the signals and engagement requirements for operation in the submode will help isolate the failure to the pitch or roll channels quickly or help determine whether it was really a problem at all. Failures which occur due to faulty outputs from the attitude reference or gyros, navigation receiver, OMEGA system, air data computer or radio altimeter will usually show up as flag indications on the flight director indicator, course deviation indicator or on the black box due to self-monitoring circuits and associated flag voltages within the components (Ref Chapter 34, Navigation). Trouble shooting procedures given assume the signals from associated systems are valid and analyze only the autopilot system failures (Ref Pitch and Roll Channel Operation Trouble Shooting Charts).

(2)  
The pitch and roll channels have self-test switches on the front panels of their boxes. Each self-test switch is capable of testing the majority of the signal paths within the box. If a signal path malfunction exists within the box, it will usually become apparent by completion of the self-test procedures (Ref Autopilot and Yaw Damper System -Adjustment/Test). The self-test procedures for the pitch and roll channels do not test any system components outside of the black box. Failures which occur outside of the black boxes are usually apparent when operating in the manual mode. It should be noted that the autopilot warning lights illuminate steadily if a self-test switch on the roll or pitch channel (or yaw damper coupler) is left in any position other than OFF.

(3)  
Test points on the front of the boxes may be useful when trouble shooting the system. Expected voltages for some of the test points are given in the adjustment/test procedures as part of system test. The following list indicates each test point and where it is connected on the circuit. Refer to vendor overhaul manual for specific signal for the applicable test point:

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CAUTION:  TEST POINT CIRCUITS ARE NOT PROTECTED AGAINST INADVERTENT SHORTS.
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Pitch Channel
Test Points Description
TP-1 DC ground TP-2 Pitch 28-volt dc circuit breaker TP-3 Stab trim down signal TP-4 Stab trim up signal TP-5 Gain programmer output TP-6 Pitch integrator clamp TP-7 Engage interlock (flaps and stab trim servo) TP-8 Rate gyro filter output TP-9 Vertical path filter output TP-10 Elevator position TP-11 Rate gyro filter input TP-12 Pitch control transformer output TP-13 CWS Q-pot out TP-14 Output to equalizer TP-15 Neutral shift position TP-16 Derived rate input *[1] TP-17 Derived rate Compensated output *[1] TP-18 Derived rate output *[1] TP-19 thru TP-21 Spare
　
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