Incident Description
The instrument flight rules scheduled international flight was in cruise at FL350 with the autopilot engaged when it abruptly rolled into a 30 to 40 degree left bank. Indications showed that the lower rudder had 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 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 hydraulic pressure was relieved.
Investigation Findings
An inspection of the forged aluminum manifold of the lower rudder power control module (PCM) revealed the end portion housing the yaw damper actuator had fractured off from the main portion. The lower rudder PCM and the flight data recorder (FDR) were sent to the NTSB laboratory. FDR data showed an initial lower rudder deflection of 17.5 degrees left, with a subsequent increase to 32 degrees (full) left. The incident airplane has two independently supported and operated rudders providing yaw control; typically the upper and lower rudders operate in unison. The hydraulic actuators for each rudder are controlled by independent PCMs, which are virtually identical.
The fractured PCM was disassembled and inspected. The yaw damper piston was protruding from the fracture, precluding functional testing of the module. Individual components of the PCM were tested, and no anomalies were found. Dimensional checks showed no discrepancies, and metallurgical testing confirmed the manifold material was consistent with 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 PCMs (including those with higher and lower use cycles than the incident airplane's module) were inspected; no similar fractures were found.
Manufacturer and FAA Response
As a result of this incident, the airplane's manufacturer issued an Alert Service Bulletin recommending operators perform ultrasonic inspection on pertinent high-time lower and upper rudder PCMs. The Federal Aviation Administration issued a Notice of Proposed Rule Making for an airworthiness directive that would make the ultrasonic inspection mandatory for all affected airplanes.