Boeing 747-400NORTHWEST AIRLINES INC

The instrument flight rules scheduled international flight was in cruise flight at FL350 with the autopilot engaged, when it abruptly rolled into a 30 to 40 degree left bank. There were indications that the lower rudder moved to the left blowdown limit, and remained there. Emergency procedures failed to correct the problem, and the flight diverted to the nearest airport for an emergency landing. As the airspeed decreased during the approach, the lower rudder deflected further to the left. The flightcrew used asymmetric engine thrust to maintain heading. After landing, the lower rudder remained deflected fully to the left, and could not be repositioned until the hydraulic pressure was relieved. An inspection of the forged aluminum manifold of the lower rudder power control module (PCM) revealed the end portion which houses the yaw damper actuator had fractured off from the main portion of the manifold. The lower rudder PCM and the flight data recorder (FDR) were sent to the NTSB laboratory. The data retrieved from the FDR showed an initial lower rudder deflection of 17.5 degrees left, and a subsequent increase to 32 degrees (full) left. The incident airplane has two independently supported and operated rudders, which provide yaw control. Typically, the upper and lower rudders operate in unison. The hydraulic actuators for the lower and upper rudders are controlled by independent power control modules. The power control modules for both the upper and lower rudders are virtually identical. The fractured power control module was disassembled and inspected. The yaw damper piston was protruding from the fracture, and precluded functional testing of the module. The individual components of the power control module were tested, and no anomalies were found. Dimensional checks showed no discrepancies, and metallurgical testing showed the manifold material was consistent with the manufacturer's specifications. Metallurgical examination revealed a mode of crack initiation and growth consistent with fatigue. A non-destructive inspection process was developed, and a group of similar power control modules were inspected. The group contained power control modules with higher and lower use cycles than the incident airplane's power control module. No similar fractures were found. As a result of this incident, the airplane's manufacturer has issued an Alert Service Bulletin which recommends operators perform an ultrasonic inspection on pertinent high-time lower and upper rudder power control modules. The Federal Aviation Administration has issued a Notice of Proposed Rule Making for a airworthiness directive which would make the ultrasonic inspection mandatory for all affected airplanes.