A CRJ900 rudder trim began an uncommanded oscillating motion which triggered the EICAS warning CONFIG RUDDER during takeoff; so the Captain rejected the takeoff and returned to the gate.
Synopsis
A CRJ900 rudder trim began an uncommanded oscillating motion which triggered the EICAS warning CONFIG RUDDER during takeoff; so the Captain rejected the takeoff and returned to the gate.
Narrative
From boarding door closure all aspects of our operation were normal. I called for and the First Officer ran and completed all appropriate checklists. A few moments after the First Officer had set the three trims; he made reference that they may have moved. He re-centered them and once again they were normal. At the time I didn't think much of this; as perhaps he may have bumped the trim switches or perhaps didn't look at them quite right the first time. I am aware of the memo concerning the importance of neutral trim settings. The First Officer was the flying pilot. I transferred the aircraft control to the First Officer. The thrust was set beyond seventy percent. It was around eighty six percent and stable. Several moments later; at 120 knots; the EICAS alerted us of a 'CONFIG RUDDER' warning. Remembering the memo's statement 'by centering the trims for takeoff; you eliminate the possibility of receiving a CONFIG AILERON or RUDDER warning due to sensor accuracy/fluctuation; [so] I elected to reject the takeoff thinking this was the safest course of action. I am aware that it is possible for the aircraft to fly in a runaway trim condition but wasn't sure if this may have been the beginning of more messages to follow for a larger problem. We had 2000 feet of runway remaining when we came to a stop. I set the brake; the First Officer advised ATC of the abort and he ran the rejected takeoff non-normal checklist. I elected to taxi back to the gate asking that CFR follow us as I thought the brake temperatures could increase with possibility of fire. During our taxi back to the gate the First Officer and I watched the trim indications move on their own from centered green to out of center white and back. This continued up until I left the aircraft for the day. Rudder trim would 'swing' as much as eighty percent nose right and aileron would swing as much as thirty percent. At the gate with ninety percent of the passengers off the airplane CFR noticed a small amount of smoke coming off the right main gear. I happened to be on the phone with Maintenance Control and they ask that ARFF not put any water/material on that area. I asked that CFR continue to observe the wheel until the brake temperature readouts were normal (green) and no evidence of smoke. I released ARFF and continued to work with Maintenance Control and Dispatch. Local Maintenance came to the aircraft and in conjunction with my air carrier Maintenance and local Maintenance they asked I leave the airplane as is and powered up at the gate. Although I am not an aircraft technician; I believe there may be a problem with uncommanded trim movements. Our first indication of this may have been passed over as 'bumping the switch' or not having it perfectly centered the first time. Prior to taking the runway I am one hundred percent certain the trims were centered. I believe that some time after we started the takeoff roll the rudder trim began to 'drift' on its own and sent the warning message. During our runway exit the rudder trim drifted back to center and the warning went away. It was during the taxi back and time at the gate that the First Officer and I could watch the trim move without input from us. Knowing that the memo addresses this issue; and knowing that the trims were centered prior to taking the runway it was in my best judgment that rejecting the takeoff prior to our V1 of 137 knots was the safest course of action.
Second reporter narrative
Given that we checked and centered the trims twice prior to the takeoff roll; I believe that there was a fault; or condition that was causing the aileron and rudder trims to drift. Because they were drifting in both directions it lead to the warning being gone by the time we exited the runway. We were both aware of the importance of centering the trims per memo 10-19; which is why we are confident that prior to applying takeoff power the trims were centered for takeoff.
NASA callback
The reporter stated that he believes this event is not the first occurrence of its type because a company memo is emphatic about ensuring all trims are centered. The memo goes on to say that if this anomaly occurs below V1 continue the takeoff and return to land. The reporter stated that this guidance goes against his years of training which taught him never to takeoff with a flight control problem. The reporter indicated that if a trim tab is not zero and N1 is above seventy percent the system will alert with an EICAS message; a Master Warning and a verbal warning. This aircraft's trim is electric so it is possible that a computer was causing the movement or that an indicating problem exists and the two trims were not actually moving.The crew did not go out and physically look at the trim tabs. A Contract Mechanic verified with the crew what the flight control page was indicating. Company mechanics flown in to examine the aircraft did not see the reported fault. The reporter believes this is at least the second uncommanded trim movement event but he does not know if the previous one was this aircraft or not.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.