WASHINGTON, Dec 18 (Reuters) - Pilots had to "wrestle" a United Airlines jetliner to the ground last week after an apparent rudder malfunction during a flight to Chicago, authorities said on Tuesday. Federal investigators said Flight 578 from St. Louis was at 9,000 feet on Dec. 13 when the nose of the Boeing 737-300 swung to the right. After disconnecting the auto pilot and a rudder control device, the crew struggled hard to regain control of the plane. One official said the pilots had to "wrestle" the aircraft to a safe landing at O'Hare airport in Chicago. After landing, the crew said they heard a bang after the rudder swung to the left and then right. None of the 93 passengers aboard were hurt. National Transportation Safety Board investigators are analyzing the plane's flight data recorder for clues. One regulatory official said the problem was reported to authorities as an uncommanded rudder movement. Investigators determined that rudder malfunctions were the likely cause of two fatal 737 crashes in 1991 and 1994. Those disasters prompted Boeing (BA) to replace certain parts in all 737 rudder systems. Last month, the Federal Aviation Administration proposed the industry outfit its fleet of 2,000 U.S.-registered 737s with new rudder control systems over a five-year period at a cost of $364 million. United is a unit of UAL Corp. (UAL) ©2001 Reuters Limited.