What happened
Following takeoff, the aircraft experienced a flight control malfunction characterized by cyclic yawing and rolling motions from left to right. The captain attempted to rectify the issue by cycling the yaw damper off and on to ensure it was engaged; however, the yaw and roll continued. As airspeed increased, the amplitude of the yaw appeared to increase.
Due to the instability, the crew declared an emergency and requested an immediate return to the airport, with emergency equipment placed on standby. The aircraft landed in a normal fashion after a total flight time of six minutes. There were no injuries to the crew or passengers, and the aircraft sustained no damage.
The investigation
An inspection of the Stability Augmentation Computer (Part 2587826-3, S/N 0270890) was conducted at the TWA Maintenance Facility. Investigators determined that the yaw rate gyro within the Stability Augmentation Computer had been installed in a reverse position, oriented 180 degrees opposite from the correct orientation.