Regional Jet 700 ER/LR (CRJ700)
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude
Flight Crew Diverted; Flight Crew Landed As Precaution; Flight Crew Returned To Departure Airport
Aircraft
On climb out passing through approximately 7;000 FT; we received an 'EFIS COMP MON' caution message immediately followed by an AP TRIM IS ND (Autopilot Trim is Nose Down) caution message. I instinctively looked at the elevator trim display and observed the trim running nose down. I immediately pushed the stab trim disconnect and instructed the First Officer to do the same. I assumed control of the aircraft and told the First Officer to request a level off at 10;000 FT from ATC. As he was doing that I disconnected the Autopilot and found it to be excessively out of trim to the point that it was very difficult to control. I instructed him to run the Stab Trim Runaway checklist. Although we never heard the trim clacker; it seemed at the time the more prudent checklist to run.I had the First Officer request a turn back towards [departure airport] as a precaution although we had not committed yet to a return. During this turn I lost some altitude due to the excessive nose down trim as the primary cause but was able to gain it back. Upon further reflection; I decided to run the AP TRIM IS ND checklist because I felt that the Autopilot was the more probable cause. I reengaged the trim and trimmed the aircraft for level flight. We did not experience any trim runaway.After briefing the First Officer about the Autopilot possibly driving the trim nose down; a reengaged the Autopilot per the checklist. Immediately the trim started moving nose down so I once again disconnected the trim and the Autopilot. I reengaged the trim and left the Autopilot off. I determined the Autopilot had malfunctioned and since at this time we had full control of the aircraft; I elected to not [get priority handling]. I had the First Officer request a return to [departure airport] and instructed him to notify the flight attendants and to ACARS dispatch. We returned and landed uneventfully.
CRJ-700 Captain reported returning to departure airport after experiencing an autopilot malfunction that drove the stabilizer trim to a nose-down position.