The NTSB is investigating whether a rudder problem caused a United Boeing 737 to swing suddenly left and right and bank sharply while descending for landing in Chicago late last month. The pilots reported the airplane suddenly swung to the left and right when it started to descend at about 9,000 ft msl. When the autopilot was disconnected, the airplane banked sharply. Rudder problems are suspected in two fatal U.S. crashes of 737s in 1991 and 1994. In November the FAA proposed an AD to require installation of a redesigned rudder control system on some 2,000 Boeing 737-100s,  -200s, -300s and- 400s within a five-year period. January 14 is the deadline to submit comments.