East Hampton, NY, USA
N46PD
SEAMAX AIRCRAFT M-22
The pilot was conducting a local flight when, about 15 minutes into the flight, a witness heard a loud “crack” and saw a wing separate and the airplane spiral down into the water. Postaccident examination of the wreckage revealed that the right wing had separated from the airplane and came to rest in trees about 700 ft from the main wreckage. The airplane incorporated a foldable wing design that was provided as an option by the manufacturer. When on the ground, the wings detached from the upper fuselage and rotated about the wing strut attachment bolts to fold against the fuselage for transportation or storage. Examination of the right wing revealed a shear fracture of the nut threads at the location where the outboard end of the wing strut attached to the lower side of the wing via a bolt. The fractured nut thread piece showed indications of ductile overstress fracture. Following the nut fracture, the strut separated from the attachment bolt, leaving the bolt intact in the fitting attached to the wing spar. The wing rotated upward from air loads following the strut separation, resulting in overload upward bending fractures at the wing root. Witness marks and fracture patterns at the wing root were consistent with the wing being locked in place at the time of the failure. The bolt contact surfaces on the thread flanks showed indications of repeated contact loading. Examination of the corresponding nut on the left wing revealed threads that were partially fractured, rolled, and displaced, with similar rolled thread deformation as observed on the right wing nut. Hardness and dimensional checks of the nut from the left wing strut outboard attachment were consistent with the specified dimensions and material tensile strength requirements specified by the airplane manufacturer. The similarity in damage patterns and evidence of repeated contact loading signatures in both nuts suggest that the nut from the right wing strut outboard attachment likely fractured due to repeated contact loading of a loose joint, resulting in a progressive overstress fracture of the nut threads. The partially fractured threads in the left wing strut attachment nut likely represented an earlier stage of the failure progression in the joint. The manufacturer specified the nut used for attaching the upper end of the wing strut to the wing as an AN364-524 thin profile self-locking nut, which could be all metal or could have a non-metallic insert (the nuts in the accident airplane had a non-metallic insert). The AN364-524 specification has been inactive for design since 1957 and was superseded by MS21083 (for the non-metallic locking option) in 1991. The MS21083 specification noted that these nuts were intended for use in shear applications; however, review of the aircraft design suggests that the bolt at the outboard attachment location would be loaded primarily in tension. Therefore, the use of the nut at the wing strut outboard attachment location would appear to be inconsistent with the design intent and usage limitations for the MS21083 nut. Additionally, the attachment bolts at each end of the struts served as axes of rotation for folding the wings for storage; however, a positive locking device that ruptured or sheared material before joint clamping forces were relieved was not used at either of the wing strut attachment joints. Since the integrity of the joints attaching the wing struts was critical to the safety of flight, the use of the MS21083 nut without a positive locking device appears to be inconsistent with its usage limitations. Following the accident, the airframe manufacturer issued a safety directive, which recommended replacement of the existing bolts and non-metallic locking nuts with a drilled bolts, castellated nuts, and cotter pins.
HISTORY OF FLIGHTOn October 6, 2022, about 1229 eastern daylight time, a Seamax Aircraft Ltd M-22, N46PD, was substantially damaged when it was involved in an accident near East Hampton, New York. The private pilot was fatally injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. According to surveillance video from East Hampton Town Airport (JPX), East Hampton, New York, the pilot arrived at the airplane’s hangar at 1159. At 1206, he pulled the airplane out of the hangar, and subsequently performed a preflight inspection, entered the airplane, shut the canopy, and started the engine at 1209. At 1214, the airplane began to taxi for departure. ADS-B data indicated that the airplane departed runway 28 at 1219. The airplane proceeded south until reaching the Atlantic Ocean, turned east to parallel the shoreline, then made a turn to the north. ADS-B contact was lost as the airplane was over Three Mile Harbor. A witness reported that he heard the engine of an airplane flying over, which drew his attention. Then, he heard a loud “crack,” saw a wing separate from the airplane, and saw the airplane spiral down into the water. The airplane came to rest in 3 to 4 ft of water in Three Mile Harbor. The right wing came to rest in a tree about 770 ft from the main wreckage. AIRCRAFT INFORMATIONThe accident airplane incorporated a foldable wing design that was provided as an option by the manufacturer. When on the ground, the wings could be detached from the upper fuselage and rotated about the wing strut attachment bolts to fold against the fuselage for transportation or storage. The wing struts were structural members that span between the lower right or lower left side of the fuselage at the inboard end of each strut to a point near mid span on the lower side of each wing. AIRPORT INFORMATIONThe accident airplane incorporated a foldable wing design that was provided as an option by the manufacturer. When on the ground, the wings could be detached from the upper fuselage and rotated about the wing strut attachment bolts to fold against the fuselage for transportation or storage. The wing struts were structural members that span between the lower right or lower left side of the fuselage at the inboard end of each strut to a point near mid span on the lower side of each wing. WRECKAGE AND IMPACT INFORMATIONThe engine was impact separated from the fuselage but remained attached to the engine mounts. There were no holes in the crankcase. All 4 cylinders remained attached to the engine. The carburetors were impact separated from the engine but remained attached through fuel lines. Both carburetor butterfly valves could be operated by hand. The spark plugs were removed and examined. All were light grey in color and exhibited minimal wear. The rocker box covers were removed and no anomalies were noted with the rocker arms or valve springs. The propeller remained attached to the engine. One propeller blade remained attached to the propeller hub. It exhibited chordwise scratching and leading edge gouging. Another blade was separated from the propeller hub, was located in the vicinity of the main wreckage, and exhibited chordwise scratching. The third propeller blade was not recovered. Examination of the airframe revealed that the fuselage was fragmented. Flight control continuity was confirmed from the flight control surfaces to the cabin through multiple breaks and overload failures. The right stabilator remained attached to the empennage and exhibited leading edge damage. The inboard section of the left stabilator remained attached to the empennage. The outboard section was impact separated and located in the vicinity of the main wreckage. The rudder remained attached to the vertical stabilizer and exhibited impact damage. The left wing was fragmented, but the left aileron, left flap, and left wingtip were located. The left wing strut was impact separated from the wing and the strut attach point bolt remained secure with the nut. The entire right wing was separated from the main wreckage and located in a tree about 770 ft from the main wreckage. The aileron remained attached; the flap was separated from the wing. The wing strut was separated. The bolt attaching the right strut to the right wing remained attached to the wing and was not fractured; however, the nut was not present. The bolt threads did not exhibit significant damage. The right and left wing strut assemblies were retained and sent to the NTSB Materials Laboratory for further examination. Examination of the nut thread piece that was removed from the right wing strut outboard attachment bolt revealed that the orientation of the threads relative to contact with the bolt threads was evident by contact patterns on the opposing thread flanks and by deformation in the thread fracture surfaces around the outside circumference. The thread piece consisted of between 2 to 3 threads. The threads closest to the clamping face were deformed such that the surface at the inside diameter rotated toward the clamping face. The thread closest to the clamping face had secondary fractures where pieces of the rotated inside diameter surface were fractured away. The contact faces exhibited a battered appearance with rounded pits and deformed metal shavings, features consistent with repeated contact loading. The fracture surfaces around the perimeter were examined using a scanning electron microscope. The surfaces showed many areas of smearing that obliterated fracture features and other areas where elongated dimple features were observed, consistent with ductile overstress fracture in a plane of shear stress. The nut from the left strut outboard attachment location had 2 to 3 threads between the clamping face and the red nonmetallic locking insert. The two threads closest to the clamping face were deformed and twisted with the inside diameter face rotated toward the clamping face. The threads were also partially fractured and displaced axially toward the clamping face. The bolt thread contact flanks had a battered appearance with rounded pits and metal shavings consistent with repeated contact loading. Following the accident, the Federal Aviation Administration (FAA) issued Special Airworthiness Information Bulletin (SAIB) 2022-24 on December 23, 2022, in which it recommended that all pertinent Seamax service bulletins be complied with and that repetitive inspections for fastener security of the strut attachment hardware be completed before each flight. On February 23, 2023, Seamax Aircraft issued Service Alert (SA) SA_001_23, which required a one-time inspection of the attachment bolts and nuts at the outboard ends of the left and right struts. In the service alert, the hardware included in the outboard strut attachment was specified as an AN5-10C bolt and an AN364-524 nut. On September 19, 2023, Seamax Aircraft issued SD_003_23, which recommended replacement of the AN5-10C bolts and AN364-524 nuts on the attachment points of the upper terminal of the wing strut. The specified replacement hardware included an AN5-7 drilled bolt, an AN310-5 castle nut, and an MS24665-285 cotter pin. Subsequently, the FAA updated SAIB 2022-24 to SAIB 2022-24R1 on November 14, 2023, recommending continued inspection of the wing strut hinge points before each flight, as well as replacement of the nuts and bolts in conformance with SD_003_23. According to Air Force-Navy (AN) Aeronautical Standard AN364 revision 4, dated July 22, 1969, the standard was inactive for design after March 13, 1957. The AN364 standard was canceled on October 21, 1991, and for self-locking low height nuts with nonmetallic inserts, was superseded by Military Specification Sheet (MS) MS21083. According to note 3 in MS21083, revision H, dated September 28, 1993, the MS21083 nuts were intended for shear applications only. Additionally, use of the nuts was subject to the limitations of MS33588. According to MS33588, revision E, dated December 28, 1984, self-locking nuts shall not be used “…where loss of the bolt would affect safety of flight unless the threaded parts are held by a positive locking device that requires shearing or rupture of materials before torsional loads would relieve the initial stresses of the assembly.” Additionally, self-locking nuts shall not be used “on any externally threaded part that serves as an axis of rotation…,” unless the loads acting on the joint do not act to relieve initial stresses or unless the nut is secured with a positive locking device as previously described. The axial height of the nut as measured using calipers was 0.250 inch, which was the maximum height listed for an AN364-524 nut. The corresponding steel nut with a non-metallic insert, MS21083N5, had a specified height range of 0.235 inch to 0.266 inch.
An inflight separation of the right wing as a result of the manufacturer’s application of a bolt and nut in the upper wing strut attachment point that was inconsistent with the nut design intent and usage limitations.
Source: NTSB Aviation Accident Database
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