Carefree, AZ, USA
N43LL
AVIAT AIRCRAFT INC A-1C-200
The pilot reported that, during the approach to land, he disconnected the autopilot, and the airplane immediately yawed to the right. The pilot applied left pedal, engaged the yaw damper, and continued the approach. He subsequently disengaged the yaw damper and entered a cross-control slip to descend, and he was unable to make an effective left rudder input. The airplane touched down on the gravel runway surface and veered to the right. The airplane then crossed over to the asphalt runway surface and subsequently ground-looped. The airplane’s autopilot had been installed in the airplane 7 weeks before the accident. Postaccident examination of the airframe revealed that the autopilot hardware installation was substandard. Specifically, the guard pin, which was designed to prevent the bridle cable from traveling out of the pulley’s groove past the sheaves, was not installed to the yaw servo tray and the right pulley assembly, which allowed the yaw servo bridle cable to separate from the pulley’s phenolic sheave. Additionally, the bridle cable, which was undertensioned, became trapped between the yaw servo pulley and the attachment tray and prohibited the pilot from applying left rudder. As a result, the pilot was unable to maintain directional control during the landing roll. The guard pin is designed to prohibit the bridle cable from being able to travel out of the pulley’s groove past the sheaves.
On February 18, 2021, about 1730 mountain standard time, an Aviat Aircraft A-1C-200, N43LL, sustained substantial damage when it was involved in an accident in Carefree, Arizona. The commercial pilot and passenger were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that, after an uneventful 2.9-hour cross-country flight, he configured the airplane for the approach to runway 24 at the destination airport. While descending the airplane, the pilot encountered a “rudder abnormality.” Specifically, the pilot stated that, 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. To counteract the yawing motion, the pilot applied left rudder, and the airplane responded normally. The pilot then engaged the yaw damper, and rudder control remained normal. During final approach, the pilot disengaged the yaw damper at an altitude of about 800 ft above ground level and maneuvered into a cross-control left slip configuration. He subsequently applied left rudder to align the nose of the airplane with the runway, but the left rudder pedal was not effective, even when it was 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 over the asphalt runway surface. The airplane’s left main landing gear collapsed, and its left wing contacted the ground. The airplane came to rest between the runway and the taxiway with the nose of the airplane oriented 180° opposite the intended landing direction. The pilot stated that the airplane had made a “classic ground loop.” A postaccident photograph of the airplane revealed that the left aileron sustained substantial damage. 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 autopilot 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, the postaccident photographs revealed that a guard pin designed to secure the bridle cable to the phenolic sheave was not present and the attachment tray’s upper and lower surface bore exhibited no evidence indicating 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 revealed the following installation deficiencies: o No cotter keys installed on yaw bridle cable to rudder blocks. o Torque on bridle cable clamps nuts unable to be measured with torque wrench because of low torque. Torque estimated to be between 5 and 10 inch pounds. o Gap on both bridle cable clamps measured as 0.026 inches. o Cable tension on bridle cable for yaw servo was 0 pounds. o No torque seal observed on bridle cable clamp at intersection of cable. o Cable tensions on yaw and roll servos was 0 pounds. o Cable tension on pitch servo was 7 pounds. o Torque on yaw and pitch servo guard pins measured to be 0 inch pounds. o Cotter pin missing from castellated nut securing roll pulleys with Part Nos. 115 03343-00 and 115-03344-00. o Cotter key missing from yaw servo pulley.
The substandard installation of the airplane’s autopilot system hardware, which resulted in a malfunction of the rudder control system during flight and a loss of directional control during the landing roll.
Source: NTSB Aviation Accident Database
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