About 15 years ago in a DC-9 we took off and at about 500' AGL the airplane began to slowly yaw from side to side. The Captain took control of the plane (he had experienced it once before) and decided quickly that we would level off on downwind and return, with the emergency equipment standing by. Went fast through climb, in-range and approach checklists. Weather was perfect, mid-morning (at TYS). We both forgot that there was a procedure in the COM for "uncontrolled rudder", or whatever it was called. But he did pull back the Rudder Power Lever, which removed right hydraulic system pressure. The only slight hang-up is that the final approach speed is at a minimum of 135 (or Vref+5, if higher), until landing assured. He also switched off the yaw damper. The yawing was smooth and not constant. On final, there were no more incommanded rudder movements. At the gate, using a "cherry picker" stand, we never figured out if it was the yaw damper or a combination of anomaliesA few planes had a water leak into the gear wells, from wet carpet under the overwing exits. The ailerons would almost freeze in flight-those planes descended to lower, warmer altitudes. Guess they kept under control using autopilots and flight spoilers.
A buddy in a Connie Kalitta Learjet declared an emergency to Boston (BOS) Departue control. They had uncommanded fuel transfer into wing tip tanks! They decided to immediately land at a high speed, around 180 knots
, John told me. Much slower could have meant a loss of roll control. I'll bet they got on the brakes really fast and hard. Learjet technical reps never figured out what went wrong.