Incident Overview
A pilot reported that during a visual approach to runway 18, the airplane entered uncommanded rolls. A go-around was performed. According to the pilot, on the second approach the airplane once again experienced an uncommanded roll. The airplane was subsequently landed.
Flight Data Recorder Analysis
Flight data recorder (FDR) data indicated roll oscillations with a maximum roll angle of 15.5 degrees. During these oscillations, the side stick was moved back and forth to its full travel limits.
Pilot-Induced Oscillations (PIO)
The observed airplane movements and the pilot's control inputs were consistent with pilot-induced oscillations (PIO). The airplane was being landed in configuration 3 (Conf 3), which corresponds to 20 degrees of flaps. The handling characteristics in this configuration are described as "more crisp."
Manufacturer Advisory
In 1993, Airbus published a temporary revision to the airplane's Flight Crew Operating Manual (FCOM). The revision stated that the airplane should not be landed in adverse weather conditions, such as gusty winds and turbulence, in Conf 3 due to the possibility of PIO.
Operator Manual Status
Northwest Airlines had not incorporated this advisory information into their operating manuals at the time of the incident.