Accident Details
On February 18, 2021, at approximately 1730 mountain standard time, an Aviat Aircraft A-1C-200, registered as N43LL, was involved in an accident in Carefree, Arizona. The aircraft sustained substantial damage. The commercial pilot and a passenger were not injured. The flight was conducted under Title 14 Code of Federal Regulations Part 91 as a personal flight.
Sequence of Events
The pilot reported that after an uneventful 2.9-hour cross-country flight, he configured the airplane for an approach to runway 24 at the destination airport. During the descent, he encountered a “rudder abnormality.” When he disengaged the autopilot by depressing the red disconnect button on the control stick, the nose of the airplane immediately yawed to the right. The pilot applied left rudder, and the airplane responded normally. He then engaged the yaw damper, and rudder control remained normal.
On final approach, at an altitude of about 800 ft above ground level, the pilot disengaged the yaw damper and maneuvered into a cross-control left slip configuration. He applied left rudder to align the nose with the runway, but the left rudder pedal was ineffective even when pressed to the floor. The pilot elected to land in a nose-right crab position. The airplane touched down on the gravel runway surface, veered to the right quickly, and crossed an asphalt runway surface. The left main landing gear collapsed, and the left wing contacted the ground. The airplane came to rest between the runway and a taxiway with the nose oriented 180° opposite the intended landing direction. The pilot stated that the airplane had made a “classic ground loop.”
Postaccident Findings
A postaccident photograph showed substantial damage to the left aileron. Review of the airframe logbook revealed that the autopilot system had been installed and returned to service on December 31, 2020, with a Hobbs time of 156.1 hours. At the time of the accident, the airplane had accumulated 15.7 hours since the installation.
Postaccident examination of the airplane revealed that the yaw servo bridle cable exhibited no tension and had dislodged from the right pulley assembly attached to the servo tray. The bridle cable was routed under the pulley’s laminated phenolic sheave and was trapped between the yaw servo pulley and the attachment tray. Additionally, a guard pin designed to secure the bridle cable to the phenolic sheave was not present, and the attachment tray’s upper and lower surfaces bore no evidence that the guard pin had been installed. The guard pin is designed to prevent the bridle cable from traveling out of the pulley’s groove past the sheaves.
Further examination of the autopilot hardware identified the following installation deficiencies:
- No cotter keys installed on yaw bridle cable to rudder blocks.
- Torque on bridle cable clamp nuts unable to be measured with a torque wrench due to low torque; estimated between 5 and 10 inch-pounds.
- Gap on both bridle cable clamps measured as 0.026 inches.
- Cable tension on yaw bridle cable for yaw servo was 0 pounds.
- No torque seal observed on bridle cable clamp at cable intersection.
- Cable tensions on yaw and roll servos were 0 pounds.
- Cable tension on pitch servo was 7 pounds.
- Torque on yaw and pitch servo guard pins measured to be 0 inch-pounds.
- Cotter pin missing from castellated nut securing roll pulleys with Part Nos. 115-03343-00 and 115-03344-00.
- Cotter key missing from yaw servo pulley.