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This is the situation: A prior to take off check of the DFC90 indicated normal mode annunciator lights in HDG and ALt modes. The HDG bug on PFD was solid but when the bug was moved left or right the yoke would not track. The VS and ALT functions were normal in that the yoke and elevator trim were moved to a climb position. Aileron and Elevator Trim was set in the appropriate settings and the flight controls were free and correct. VMC conditions prevailed and a normal takeoff and climb was accomplished. When the DFC90 was engaged during climb the flight control response was the same as indicated in the ground check; VS and ALT functions were active but the roll function was not. The Fight Director was functional and showing the proper attitude to fly either in HDG or GPSS modes but the aircraft had to be manually controlled to maintain heading. The autopilot and trim circuit breakers were cycled but no change was observed. There were no warning annunciations illuminated and the system acted as if everything was functional. Troubleshooting determined that the aileron trim was inop but that manual control was easily accomplished by overiding the spring pressure. The DFC90 could be engaged and utilized for altitude control, climbs and descents, and the Flight Director could be used for track guidance while manually controlling the ailerons. Maintenance revealed that the relays were functional but the aileron trim motor was inop and the shaft could not be moved by hand.
Question/Comment: It appears as though there is no warning if the trim motor is not functional. The explanation in the Pilot Guide "Reaching Servo Limits Can Cause
Uncommanded Rolls" seems to be different situation than the one encountered. In any event, in this situation, had the weather been IMC and a ground check not conducted; a situational awareness situation would have occurred at a critical time after takeoff. This emphasizes the need for pre-takeoff checks and troubleshooting.
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