NTSB Factual Report

On May 22, 2002, about 0900 eastern daylight time, a Dornier 328-300, N421FJ, operated by Atlantic Coast Airlines as Delta Connection flight 6348, experienced uncommanded yawing while on descent to land at Albany International Airport, Albany (ALB), New York. The airplane was not damaged. There were no injuries to the two certificated airline transport pilots, one flight attendant, and two passengers. Visual meteorological conditions prevailed for the scheduled domestic passenger flight, which originated from the Cincinnati/Northern Kentucky International Airport (CVG), Covington, Kentucky. The flight was on an instrument flight rules (IFR) flight plan, conducted under 14 CFR Part 121.

According to interviews with the flight crew, the first officer was the flying pilot. During the climb to cruise altitude, with the auto-pilot/yaw damper engaged, passing through about 15,000 feet, the flight crew observed an illuminated amber light which indicated an out of rudder trim condition. The first officer disconnected the auto-pilot/yaw damper, and retrimmed the airplane. The flight crew discussed the situation with their maintenance control and elected to continue the flight. The airplane was climbed to FL 290 (29,000 feet) manually, and then the auto-pilot/yaw damper was engaged with no anomalies observed.

After about 1 hour at altitude, the flight crew initiated a descent into Albany. Passing through about 18,000 feet, the airplane began uncommanded yawing, and the first officer disconnected the auto-pilot/yaw damper. However, the oscillations continued unchanged, and the flight crew declared an emergency.

The first officer reported that there was movement of the rudder pedals with the oscillations. She also reported that she could overpower the oscillations with rudder pressure and opposite aileron. During the descent, as the airplane slowed, the oscillations continued; however, the magnitude of the oscillations decreased. As the airplane neared the airport, the yaw damper was re-engaged to see if there was any change in the oscillations. No change was observed, and it was immediately disconnected again.

The first officer further reported that the control tower reported calm winds. However, the onboard computer from the flight management system showed a light wind straight down the runway. The first officer reported that it felt like she was landing the airplane in a crosswind. She initially touched down on one main landing gear, in a wing down attitude.

The airplane was taxied to the gate where the passengers and crew exited the airplane through the main cabin door.

Post incident system checks did not reveal problems or malfunctions. The number one integrated avionics computer (IAC) was removed and retained for further testing. A maintenance test flight was performed, and the airplane returned to service with no problems noted.

The number one IAC was examined under the supervision of an engineer from the Safety Board's Aviation Engineering Division. According to her report:

"Examination of the FDR data from this incident showed that some of the rudder anomalies experienced by the airplane occurred without the autopilot engaged, but the IAC has no inputs into the rudder system when autopilot is disengaged. Also, there appears to be some rudder trim movement just prior to the initial oscillations; in fact, there are some significant changes in rudder trim position that coincide with the initial rudder position movements. There is no IAC input ever into the rudder trim system; rudder trim is controlled by the rudder trim switch in the flight compartment. (Refer to the plot for Autopilot, 22000-29000 feet.)"

"I found no direct link between the Integrated Avionics Computer that was examined and the rudder movement reported by the pilots. The fact that the initial oscillations occurred just after a change in the rudder trim position suggests a possible connection between the two events. However, further examination of system components was not possible by the time the IAC was eliminated from consideration. As of this date (January 13, 2003), the aircraft has been flying for over seven months with no reported recurrence of the rudder system anomaly."

NTSB Probable Cause

Rudder oscillations of an undetermined origin.