Airlines must inspect some Boeing 737 rudder power control units for a possible problem that could cause the rudder to swing on its own.
The Federal Aviation Administration directive, effective immediately, comes after United Airlines said it used an "incorrect bolt" in hundreds of the units for the past 26 years.The unit transfers signals from the pilot's rudder pedals to the hydraulic system that moves the rudder, a vertical tail part that helps steer the aircraft.
United spokeswoman Connie Huff said Tuesday the bolt has caused no rudder problems and was used with FAA approval since 1970. The discovery last fall of some cracks in a control unit led to the directive.
"Such cracking, if not detected, could cause the bearing to seize and, consequently, lead to an uncommanded rudder movement," the airworthiness directive said.
The bolts have been replaced in more than 90 percent of United 737s, Huff said.
Some investigators theorized that rudder malfunctions triggered the 1994 crash of a US Airways (formerly USAir) aircraft at Pittsburgh and the 1991 crash of a United flight at Colorado Springs, Colo., in which the planes suddenly rolled and dived while preparing to land.
Both United and Boeing said the problem was unrelated to the crashes.