N7082A

HISTORY OF FLIGHT

At approximately 1450 eastern daylight time on May 11, 1999, American Airlines (American) flight 916, an Airbus A300-605R (N7082A), experienced high rudder pedal forces and uncommanded rudder motion during final approach to runway 9R at Miami International Airport, Miami, Florida. The motion started soon after the autopilot was turned off in preparation for landing. (The autopilot remained off throughout the remainder of the flight.) Flight data recorder (FDR) data show small rudder oscillations within the range of about 1.0 degree left to about 4.0 degrees right when the airplane was at an altitude of approximately 1,450 feet and an airspeed of 152 knots. The flight crew switched off the yaw dampers, but the uncommanded rudder motion worsened, and the crew switched the yaw dampers back on. During the airplane's descent and approach (at an airspeed of 137 knots), the amplitude of the rudder oscillations reached as high as 5 degrees left and 12 degrees right. The FDR data indicate that the airplane descended to about 360 feet before a go-around was initiated and the airplane began to climb. Large rudder oscillations continued during the climb, and maximum lateral accelerations of +0.342 to -0.240 g were recorded. Although the uncommanded rudder motion continued during the go-around, the crew was able to land on runway 27L successfully. No passengers or crewmembers were injured, and the airplane was not damaged. The flight was a regularly scheduled flight from Bogota, Colombia, to Miami being conducted under 14 Code of Federal Regulations Part 121.

During postincident tests conducted at the American maintenance hangar in Miami, investigators reproduced rudder behavior similar to that experienced on flight 916 by pressurizing the green hydraulic servo control system and energizing at least one of the flight control computers (FCC). (The airplane has three independent hydraulic systems, designated as green, yellow, and blue.) The autopilot remained off. Rudder operation returned to normal when either testing condition was not satisfied.

After the rudder anomaly had been reproduced several times, the FCC fault data for the incident flight were reviewed by American technical specialists, who reported that the data indicated faults with the autopilot yaw actuator and the No. 1 FCC. (FCC fault data are recorded when an uncommanded autopilot disengagement occurs.)

After the rudder testing was complete, the autopilot yaw actuator was removed from the airplane at Safety Board request and replaced with a new yaw actuator to determine whether the rudder anomaly remained and to preserve any failures within the original actuator. The incident actuator, including the hydraulic lines, torque limiter, connector, and wiring, was inspected before and after it was removed. No anomalies were noted. No shorts were found during continuity checks between the FCC connectors and the autopilot yaw actuator (pin-to-pin and pin-to-ground). With the new yaw actuator installed in the airplane, investigators repeated the test series. No rudder stiffness or uncommanded movements were noted during any testing.

A300 AUTOPILOT ACTUATOR DESCRIPTION

The A300-600 contains three autopilot actuators: one each for the pitch, roll, and yaw axes. The actuators are identical although the output lever is unique to each position. When the autopilot is active, the actuators transform signals from the FCC into commands for movement of the respective flight controls.

Each actuator consists of two independent, redundant control channels-the green and yellow systems-each with its own electric and hydraulic operation. Operation of each actuator is controlled by two pairs of solenoids (one pair for each channel). Each solenoid pair consists of a main valve solenoid that controls hydraulic pressure to the actuator and an engagement valve solenoid that controls engagement of the autopilot clutch for autopilot commands. Except for tags on the wiring to the end caps that distinguish the two main valves (both marked "P") from the two engagement valves (both marked "E"), the four solenoids (part number 38215-262) are identical in appearance. The main valve solenoid is always energized in the ON position when the respective FCC is powered, and the FCCs are continuously powered in flight. Once the appropriate autopilot mode is engaged, the engagement valve solenoids for the roll and pitch autopilot actuators are energized and actuator movement is controlled by FCC commands. For the autopilot yaw actuator, the leading edge slats must also be extended to at least 15 degrees before the engagement valve solenoid is energized.

For each autopilot actuator channel, the FCC monitors the position of the autopilot clutch to ensure that it agrees with the autopilot selection made by the crew. If the FCC detects a disagreement between the autopilot clutch and the autopilot selection, the FCC will command the respective main valve solenoid to close, thus depressurizing that channel of the autopilot actuator.

AUTOPILOT YAW ACTUATOR EXAMINATION

The autopilot yaw actuator was produced by Lucas Aerospace, which is now known as TRW Aeronautical Systems. Although the actuator was the original unit installed in the airplane at delivery, American had replaced all four solenoids with upgraded units during a C check on January 19, 1998. Safety Board investigators transported the incident airplane's autopilot yaw actuator, serial number (S/N) 2011, to the Lucas facility in St. Ouen, France, for examination. The examinations revealed two main findings: (1) the wiring to the two main valve solenoids had been cross-connected, and (2) a foreign-particle contaminant was found in the green system engagement valve solenoid.

Cross-connected Wiring of the Two Main Valve Solenoids

Examination of the actuator at the Lucas facility revealed that its two main valve solenoids were cross-connected so that the electrical connection for the yellow system was connected to the green system solenoid, and vice versa. In this configuration, the FCC for the green system would actually control the main valve for the yellow system, and the FCC for the yellow system would control the main valve for the green system.

After the cross-connected wiring on N7082A was discovered, Airbus issued an All Operators Telex (AOT) on May 27, 1999, to all operators of Airbus A300, A310, and A300-600 airplanes. The AOT specified that a one-time inspection be conducted within 10 days to confirm proper connection of the main valve solenoids. Both the French Directeur General de L'Aviation Civile and the Federal Aviation Administration subsequently issued airworthiness directives on June 30 and August 24, 1999, respectively, to require the AOT actions.

As a result of the AOT checks, another American A300-600 (N3075A) autopilot yaw actuator (S/N 1630) was discovered with cross-connected autopilot solenoid wiring. American indicated that it had installed new solenoids on the three actuators (pitch, roll, and yaw) on this airplane and the rest of its A300 fleet between July 1997 and July 1999. At the time the new solenoids were installed, procedures did not specifically include a check to ensure that the solenoid wiring was installed properly. Both Airbus and Lucas have since developed new procedures to ensure that the autopilot actuator solenoids are properly wired.

Solenoid Contamination

Investigators found that a cylindrical particle in the green autopilot engagement valve solenoid, S/N 4287, on the incident airplane's yaw actuator had prevented the solenoid from disengaging. The particle was analyzed at the Lucas facilities under supervision by the French Bureau Enquêtes-Accidents. Analysis revealed that the particle was composed of a copper-zinc alloy covered by a layer of gold and nickel. Spectroscopic examination and analysis of the solenoid components indicated that the particle did not originate from solenoid S/N 4287. The particle measured 0.65 millimeter (diameter) by 0.34 millimeter (length).

Further examination of the incident airplane's autopilot yaw actuator revealed contaminants in two of the other three solenoids (S/Ns 4358 and 4315). The contaminants consisted of particles in the small cross-hole of the solenoid pushbutton. Lucas conducted several tests using the biggest particle but was unable to make it jam the solenoid.

Investigators checked other A300-600 autopilot actuator solenoids for contamination, including the autopilot yaw actuator (S/N 1630) from N3075A, which was the other A300 that had been discovered to have the cross-connected solenoid. That actuator was sent to the Lucas facility in St. Ouen for examination, where investigators found that one solenoid (S/N 4513) contained three contaminants and another solenoid (S/N 4671) contained one. The first two contaminants in solenoid 4513 were thin curls of aluminum bronze (the same material as the solenoid's push button); one measured 0.8 by 0.4 millimeter, and the other measured 0.4 by 0.4 millimeter. The third contaminant found in solenoid S/N 4513 was a particle of organic material containing traces of titanium and silicon, consistent with paint pigment. This particle measured 0.15 by 0.12 millimeter. Solenoid S/N 4671, the other solenoid found with a contaminant, contained a particle of acrylic resin; its size was not noted.

To further explore the issue of solenoid contamination, Lucas examined the eight solenoids from two similar autopilot actuators (S/N 1828 and S/N 2432) and found contaminants in two of the solenoids from autopilot actuator 1828. In one solenoid, the single contaminant was identified as cellulose (consistent with wood fibers). In the other, the single contaminant was identified as lacquer. Lucas indicated that these contaminants were minor and would not have prevented operation of the solenoids. No contamination was found in the so...