NTSB Identification: DCA99IA058.
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Scheduled 14 CFR operation of AMERICAN AIRLINES
Incident occurred Tuesday, May 11, 1999 in MIAMI, FL
Probable Cause Approval Date: 11/14/2002
Aircraft: Airbus Industrie 300-600, registration: N7082A
Injuries: 129 Uninjured.
NTSB investigators used data provided by various sources and may not have traveled in support of this investigation to prepare this aircraft incident report.
During final approach to the airport and after the crew turned the autopilot off, American Airlines flight 916 began experiencing uncommanded rudder motion and high rudder pedal forces. The flight crew executed a go-around about 360 feet above ground level. Despite continued difficulties with the rudder, the flight crew was able to land the airplane successfully. Examination of the autopilot yaw actuator revealed two main findings: (1) the wiring to the two main valve solenoids had been cross connected, and (2) a foreign-particle contaminant in the green system engagement valve solenoid had prevented disengagement of the yaw actuator. Investigators found that when these two conditions exist, turn coordination system inputs can cause uncommanded rudder movements. Inspections of A300 and A310 airplanes after the incident resulted in the discovery of another American A300 with cross-connected solenoid wiring. American indicated that it had installed new autopilot actuator solenoids on this airplane and the rest of its A300 fleet. At the time the solenoids were installed, American's procedures did not include a check for proper installation of the solenoid wiring. The manufacturers of the airplane and the actuator have since incorporated new procedures to ensure that the solenoids are wired properly. As part of the investigation, numerous additional solenoids were examined for the presence of contaminants. Although contaminants were discovered on several other solenoids, they were smaller than that found on the incident airplane's engagement solenoid and did not affect solenoid functionality. Audits of the solenoid assembly process revealed the need to update production and inspection documentation and the need to include measures to prevent foreign particles from contaminating adjacent workstations. The manufacturer of the solenoids took corrective actions that included improving production facilities, implementing more stringent sampling and inspection procedures, and clarifying assembly instructions.
The National Transportation Safety Board determines the probable cause(s) of this incident to be: Uncommanded rudder movement caused by a contaminant in the engagement valve solenoid that prevented disengagement of the autopilot yaw actuator and cross-connected wiring of the autopilot yaw actuator's main valve solenoids by the operator. Factors contributing to the incident were the lack of proper controls in the solenoid assembly process and the lack of procedures to detect miswired solenoids. Full narrative available
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