CEN24LA162

The rudder post’s failure due to corrosion fatigue. Contributing to the accident were the misalignment of the rudder hinge components, which produced mechanical binding and elevated localized stress, and a compromised the protective coating, which allowed corrosion to accelerate fatigue cracking.

The pilot and flight instructor were conducting an instructional flight during which the pilot performed upset recoveries and basic aerobatic maneuvers. During the flight and subsequent taxi, the pilot reported intermittent rudder resistance that he described as a “catch” and an audible “pop.” A post-flight inspection revealed that the lower rudder hinge had separated from the rudder post. Postaccident examination revealed that the rudder post had fractured along the weld toes that attached the middle knuckle of the lower rudder hinge to the post. Although the service manual required inspection of the rudder for damage and cracks, it is likely that the fatigue cracking was obscured by the fabric covering and was not readily apparent during routine visual inspection. Further inspection of the fracture surface revealed features consistent with fatigue cracking that initiated primarily from the exterior surface of the rudder post at the weld edge. Pits and branching microcracks near the crack origins were consistent with corrosion fatigue crack initiation and progression. Wear observed on the hinge knuckles and the vertical stabilizer hinge hardware, along with the measured offset of the upper hinge knuckle from the rudder centerline, was consistent with a misaligned hinge system that produced rubbing and binding during rudder movement. The misalignment was associated with the rudder’s previous damage history and subsequent repair activity. This binding would have increased localized stress at the lower hinge weld toes and contributed to the growth of the fatigue cracks. The rudder post had been finished with a protective coating; however, the presence of corrosion-related features at the crack origins indicated that the coating had been compromised at some time during the rudder’s life and was insufficient to prevent corrosion due to environmental exposure, which contributed to the acceleration of the fatigue cracking. As the fatigue cracks progressed, the remaining tube wall section was reduced until it could no longer sustain operational load applied during rudder movement. The pilot’s reported rudder resistance and audible “pop” were consistent with progressive binding and final separation of the lower rudder hinge from the rudder post. Although minor welding anomalies were identified, they were not located at the fatigue crack origins and did not contribute to the fatigue cracking. Hardness testing showed that the rudder post material met the required tensile strength specifications.

On April 12, 2024, about 0810 central daylight time, an American Champion 7ECA airplane, N793L, was substantially damaged when it was involved in an accident near Longview, Texas. The pilot and flight instructor were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. The purpose of the flight was to practice upset recovery and basic aerobatic maneuvers. The pilot reported that, during a hammerhead turn, he noticed a sensation of something “catching.” While returning to the airport about 1,200 ft above ground level, the pilot was cleared to enter the base leg of the traffic pattern. The pilot then performed a 40° bank turn that he described as “acro-style.” As the airplane rolled back to wings-level flight, the pilot reported a “catch” in the right rudder as he applied right rudder pedal pressure. The pilot and flight instructor tested the rudder for additional interference, but no further anomalies were noted during the remainder of the flight or landing. While taxiing to the ramp after landing, the pilot initiated a left turn. When he applied right rudder to stop the turn, he noted strong resistance. He increased pressure on the rudder pedal, which was followed by an audible “pop” and a sudden decrease in resistance. Subsequent rudder inputs in either direction produced a slight increase in resistance followed by an audible “pop.” A post-flight inspection revealed that the lower rudder hinge had separated from the rudder post. The American Champion 7ECA is a two-seat, high-wing airplane with tandem seating. The rudder post was constructed of 4130 steel tube with a nominal diameter of 1 inch and a wall thickness of 0.035 inches. Postaccident examination of the tail section revealed that the middle knuckle of the lower rudder hinge had separated from the rudder post (see figure 1). The airplane sustained substantial damage to the rudder post. No other damage to the flight control system was observed. Figure 1. Location of damage and view of damaged rudder The rudder assembly was submitted to the NTSB Materials Laboratory for examination. The rudder post was fractured at the toes of the welds that attached the middle hinge knuckle to the post. The fracture followed the weld toes along the left, right, and lower sides of the knuckle. At the upper end of the fracture, the path extended about 0.25 inches above the welds before intersecting. The rudder post wall remained attached to the knuckle piece and was deformed forward relative to the lower welded portion of the wall. Examination of the fracture surfaces using a scanning electron microscope identified features consistent with fatigue. Crack arrest lines and ratchet marks indicated multiple origins at both the outside diameter weld toe and the interior surface of the post. Fine fracture features were partially obscured by oxidation and recontact damage. The exterior surface near the crack origins exhibited small pits and branching microcracks. Testing of the metal’s internal structure revealed a lath-type appearance in the heat-affected zone, which was consistent with the steel having undergone heat treatment after welding. Hardness testing showed that the steel met the required tensile strength specifications. Minor welding anomalies were noted, including weld spatter and a portion of a welding filler rod that was stuck in a weld bead near the upper end of the middle knuckle. These anomalies were not located at the fatigue crack origin. The rudder post was covered by a black protective coating, a white primer, and a blue topcoat. These layers were present over the rudder post, including the areas where the cracks had formed. The lower face of the middle knuckle at the lower hinge exhibited wear and a radial deformation lip. Similarly, the lower bushing from the vertical stabilizer side showed a wear groove consistent with contact with the middle knuckle. Blue paint was observed within these wear areas, indicating that the wear occurred before the most recent painting of the component. A 3D coordinate measurement machine (CMM) scan of the upper hinge assembly revealed a misalignment of the middle knuckle axis relative to the rudder post centerline. The knuckle was rotated clockwise and offset between 0.029 inches and 0.039 inches to the right of the center line. The airplane was manufactured in 2014 and delivered to the same operator in 2014. A review of maintenance records indicated that the airplane had accrued 4,048.4 hours total time at the time of the accident. The most recent 100 hour and annual inspection was performed on February 9, 2024, at 3,957.2 hours total time. According to the maintenance records, the rudder had undergone multiple major maintenance events, including a worn upper rudder hinge bolt requiring replacement in May 2015 at 469.3 hours; damage to the lower portion of the rudder requiring major repair and replacement of lower rudder bearings, bushing, and attaching hardware in May 2018 at 1,450.1 hours; damage to the aft fuselage vertical tail post requiring major repair and replacement of the rudder bearings, bushings, and attaching hardware in June 2019 at 1,925.8 hours; binding of the rudder in September 2019 at 2,062 hours; damage to the bottom of the rudder requiring repair of the rudder in April 2021 at 2,772.8 hours; installation of the repaired rudder with new lower bearings, bushing and attaching hardware in May 2021 at 2,784.0 hours; and a misalignment of the rudder bearings, requiring replacement of rudder bushings, bearings, and attaching hardware in January 2022 at 3,079.4 hours. The American Champion Aircraft Service Manual states that, during preflight inspection, the pilot should ensure there is no external damage, cracks, or operational interference to the control surfaces or fuselage. It further states that on a 100-hour inspection interval to: check rudder hinges, stops and attachments for damage and operation; and to check rudder hinge bolts and bushing for excessive wear. According to FAA Advisory Circular (AC) 43-4B, “Corrosion Control for Aircraft,” East Texas is classified as an area of moderate corrosive exposure for aircraft structures. This document discusses the inspection, identification, and treatment of corrosion and states in part: Corrosion inspection frequency, corrosion identification, and especially corrosion treatment continues to be the responsibility of the operator. These inspections should be accomplished per this AC, the manufacturer’s recommendations, or the operator’s own maintenance program. The procedures in this AC are an acceptable means, but not the only acceptable means, of corrosion treatment. The information in this AC is applicable to aircraft for which the manufacturer has not published corrosion control information. Where the airframe or engine manufacturer has published a recommended corrosion inspection schedule and treatment program, the applicable program must take precedence over the recommendation of this AC. Compliance with FAA AC 43-4B Corrosion Control for Aircraft is not required by the FAA for 14 CFR Part 91 operations. As a result of the NTSB Materials Laboratory findings, American Champion Aircraft Corp. published Service Letter 459. This service letter mandates a visual inspection with 10-times magnification or dye penetrant inspection of the upper and lower bushings, welds, and attach hardware on the vertical stabilizer and rudder at every 100-hour or annual inspection on aircraft that have more than 2,000 hours of service.