Event Description
The takeoff and initial departure were normal. At about 3,000 feet mean sea level, the captain engaged the autopilot. While passing 8,900 feet msl, the airplane experienced a sudden upset, resulting in a rapid roll to the left of 35 degrees.
Flight Crew Actions
The captain countered with 50 to 60 degrees of right aileron input. The autopilot, which had been engaged, disengaged automatically during this pilot control input. The first officer inquired if the captain wanted aileron trim. The captain requested only a small amount, reasoning that he did not want to mask a major problem. He did not feel any vibration, binding, or unusual control feel other than the effort and aileron input required to hold wings level. He observed on the Configuration synoptic page that the right aileron and right elevator were deflected upward, consistent with his control input to lower the right wing against a left rolling tendency. For the remainder of the flight to landing, approximately 45 degrees of yoke deflection was required to maintain a level wing attitude.
Post-Flight Analysis
Analysis of the digital flight data recorder indicated that the aileron trim value was 90 percent of full aileron trim authority prior to pushback, engine start, taxi, and takeoff. Data from the previous inbound flight, which had the same flight crew, showed that the trim changed from 10 to 90 percent authority about three minutes after that flight landed. The trim again shifted following the landing and taxi back of the event flight from 85 percent to 10 percent. The captain stated that he felt the airplane was not tracking properly after takeoff. The airline's debrief revealed that the flight crew did not see or recognize the excessive aileron deflection on the cockpit screens before or after the event worsened and resulted in autopilot disengagement.
During the event, the autopilot was trying to roll out of a turn and was commanding aileron in a direction opposite to the trim position. This caused the aileron torque limiter to reduce aileron authority. With reduced authority, the aileron position could not keep up with the autopilot command, triggering an autopilot disconnect. The airline noted that the Rudder and Aileron check for zero is performed during the Cockpit Preparation Checklist, just prior to the Before Starting Engines Checklist. DFDR data about two-thirds of the way through the flight indicated that the trim did respond when the first officer manipulated it. The data also showed that the trim repositioned to zero at the end of the flight.
Thorough post-accident tests of the system found that the aileron trim control switches and the aileron trim control actuator unit functioned per design specifications, with no mechanical or circuit discrepancies. Testing demonstrated no evidence of any uncommanded motion. Tests on the Aileron and Rudder Trim Control Module also revealed no faults.