CEN25LA043

The pilot was practicing aerobatic maneuvers in the experimental biplane. After performing several maneuvers, he initiated a snap roll by applying full throttle, full aft elevator, and full left rudder. The pilot stated that immediately after rudder application, he felt the rudder pedal “lose tension and go to the firewall.” The airplane stopped the snap roll in an inverted spiral descent. The pilot recovered with aileron and elevator, and found that while he still had right rudder control, he was unable to center the rudder due to the unresponsive left rudder pedal. The pilot returned to airport for landing and found that he was able to apply left rudder by pulling on the left rudder cable with his hand. The pilot subsequently landed the airplane, and during the landing roll, the airplane veered to the right. Pulling on the left rudder cable did not provide adequate rudder control authority, and the airplane ground-looped, nosed over, and came to rest inverted, resulting in substantial damage to the vertical stabilizer, lower left wing, and upper wings. Postaccident examination revealed that the left rudder cable had separated at the attachment linkage to the left rudder pedal. Additionally, the right rudder cable thimble at the linkage attachment point was separated and missing a section the same thickness as the linkage. The rudder cables were likely installed with similar configurations, using a thimble to line the inside of the attachment loop and a ferrule to secure the loop in place. The thimble lining on the inside of the right cable loop had failed, with a section of the thimble missing from the apex of the loop. This missing section was large enough for the linkage to fit in the gap, indicating that the linkage was pulling directly on the cable when the cable was in tension. Examination of the rudder cables revealed signatures consistent with severe wear and fatigue. This was exacerbated by contact with the 90° corners of the linkage attachment through hole. The airplane build plans stated that the rudder cable linkage attachment through hole must be chamfered to 45° on both sides on the outside corners. Examination of the linkage attachments revealed that neither the right nor left rudder cable linkage attachment holes had been chamfered in accordance with the build plans. The linkage for both cables and the right cable thimble displayed rubbing damage due to contact with the unchamfered hole. The left cable, before its failure, was likely in a similar condition to the right, and the thimble also likely failed on the left cable. This resulted in fatigue cracking of the cable strands due to repeated rubbing and loading on the unchamfered through hole and the subsequent failure of the left rudder cable. Review of the airframe maintenance logbook revealed no entries for rudder cable replacement. It is likely that the cables were original to the airplane and had been in service for 38 years and 1,751 hours of operation at the time of theaccident. The mechanic who conducted the airplane’s most recent annual condition inspection stated that he did not typically inspect the inside radius of a cable’s attachment loops during his flight control inspection, as long as the cable was otherwise in good condition.

On November 17, 2024, about 1415 central standard time, an experimental Pitts S-1E airplane, N52B, was substantially damaged when it was involved in an accident near Seguin, Texas. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that the purpose of the flight was to practice aerobatic maneuvers. After departing the airport and climbing to altitude, he performed eight maneuvers uneventfully. The ninth maneuver included a 1 ½ rotation snap roll to inverted flight. To initiate the snap roll, he applied full throttle, full aft elevator, and full left rudder. The pilot stated that immediately after rudder application, he felt the rudder pedal “lose tension and go to the firewall.” The airplane transitioned to an inverted “spiral type” descent, and the pilot recovered with aileron and elevator back to upright flight. He found that while he still had right rudder control, he was unable to center the rudder due to the unresponsive left rudder pedal. The pilot returned to the departure airport and found that he was unable to maintain runway alignment during the approach for landing. He performed a go-around and subsequently found that he was able to pull on the left rudder cable with his hand to keep the rudder “streamlined.” The pilot performed a second approach to landing, and during the landing roll, the airplane began to veer to the right. The pilot was unable to correct by pulling on the left rudder cable, and the airplane ground-looped, nosed over, and came to rest inverted, resulting in substantial damage to the vertical stabilizer, lower left wing, and upper wings. Postaccident examination by the pilot revealed that the left rudder cable had separated at the attachment linkage to the left rudder pedal. Additionally, the right rudder cable thimble at the linkage attachment point was separated and missing a section the same thickness as the linkage. The cable was submitted to the National Transportation Safety Board Materials Laboratory for additional examination, which revealed signatures consistent with severe wear and fatigue. The left cable had fractured near the apex of the end loop. The thimble lining for the left rudder cable end loop was not recovered. The left rudder cable strands near the fracture were largely twisted together, with little splaying or fraying at the broken ends. Surface wear consistent with contact or rubbing was noted on strands near the inside of the loop. The fractured ends of the strands were generally separated into two categories. The first set included strands that showed an angle (relative to the axis of the strand) or a knife-edge fracture surface. These surfaces were more numerous on the outside of the loop. The other category of strands were broken with a flat fracture surface with no necking, consistent with progressive cracking under applied stress. Examination of the surfaces with optical microscopy revealedthumbnail-shaped regions consistent with fatigue. These flat fracture surfaces were more numerous toward the inside of the loop, especially near the apex of the loop where the linkage would have been in contact while the assembly was in tension. Some of these cables were deformed and misshapen, consistent with contact damage. These other fracture surfaces also appeared flat with no indication of necking, consistent with fatigue failure. The right rudder linkage, cable, and ferrule were intact, with no significant damage noted. The end loop (where the linkage was attached) contained a teardrop-shaped thimble lining the inside of the loop. A section of the thimble near the top of the loop was missing (see figure). The fracture surfaces on the thimble at this missing section were smeared, consistent with repeated impact or rubbing. The cable showed accumulated brown rust on and between strands in this section, especially on the inside of the loop. When the cable and linkage were pulled in tension, the linkage fit into this missing section of thimble. The attachment hole through the linkage had 90° edges with no chamfer or fillet. Figure. Right rudder cable damage The linkage from the left rudder cable had a similar configuration to the right linkage, with 90° corners on the attachment through hole. These edges showed rubbing damage around the attachment hole. The damage resembled a series of grooves adjacent to one another, localized to the end of the hole that would have been in contact with the cable loop while the cable was under tension. These grooves were visible on both the front and back edges of the hole. The width of the grooves was measured and found to have an average width of about 0.006 inch. The width of the strands that made up the cable were 0.008 inch in diameter. The Pitts S1-E build plans stated that the rudder linkage attachment through hole for the rudder cable must be chamfered to 45° on both sides on the outside corners. Examination revealed that both linkage holes had not been chamfered in accordance with the plans. Review of the airframe maintenance logbook found no entries for cable replacement since the cables were originally installed in 1986 during the airframe assembly. The mechanic who conducted the airplane’s most recent annual condition inspection stated that he did not typically inspect the inside radius of a cable’s attachment loops during his flight control inspection, as long as the cable was otherwise in good condition. He stated that inspecting the inside portion of the attachment loop would require partial removal of the cable to release tension in the system.