Regional Jet 700 ER/LR (CRJ700)
Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Communication Breakdown; Confusion
Aircraft Equipment Problem Critical; Flight Deck / Cabin / Aircraft Event Other / Unknown
Flight Crew Overcame Equipment Problem
Aircraft; Human Factors
The cause of the Master Warning was a Rudder Configuration issue.  We are absolutely certain that the trims were green; takeoff configuration ok; and all checklist were run. Why the rudder trim was moved out of center on the takeoff roll is unknown; and the captain re-centered it; as well as checking its operation afterward.  As it did not appear to be damaged; broke; or un-operational; we believed it had been accidently bumped or stuck on takeoff.  After checking its position; another Takeoff configuration OK was given and we proceeded on. After running all the checklists and confirming 'Takeoff Configuration OK'; we were cleared for takeoff on 34R.  The First Officer advanced the thrust levers and called 'set thrust'.  Shortly after; approx. 20kts; a Master Warning 'Rudder Configuration' sounded and the captain aborted the takeoff.  The First Officer immediately notified the Tower of the abort and we exited the runway at the first taxiway.  The tower asked if we needed assistance and we declined.  After re-centering the rudder trim; we taxied back to the runway and departed with an uneventful flight. After any Master Warning; regardless of what or why it happened; should call for maintenance to review any past history of similar occurrences; to troubleshoot the issue if needed.
First Officer reports about the cause of a Master Warning sounding due to a Rudder Configuration issue on a CRJ-700 aircraft. Second takeoff attempt aborted when similar Master Warning sounded and 'Rudder Config' appeared. Pilot resets the Rudder Trim a third time and departs. Why the Rudder Trim moved out of center position uncommanded on the takeoff roll; is unknown.