WPR24LA122

The pilot reported he was receiving flight instruction and was practicing landings in a tailwheel-equipped airplane. During a power-off approach, the throttle was set to idle and carburetor heat was applied. As the airplane approached short final, both pilots observed an increased sink rate and the flight instructor called for additional power. Although power was increased, the engine briefly hesitated before full power was requested again. The flight instructor adjusted pitch and power, which resulted in a firm touchdown with a minimal bounce. Upon touchdown, the left wheel and axle assembly separated from the left main landing gear leg. The landing gear leg struck the grass surface and the left main landing gear leg fractured at the fuselage end. The airplane slid on the left wing and right main landing gear wheel before it came to rest 180° opposite the direction of landing. Post accident examination of the airplane revealed that the left main landing gear axle assembly had separated; the landing gear attachment bolts remained in place, however, the attachment nuts were not present. Following a search of the accident area, the nuts were not recovered. Metallurgical examination of the fractured landing gear leg revealed features consistent with inboard bending overstress. Although the separation occurred after the wheel and axle assembly detached, the fracture surface exhibited a fatigue crack that had originated from a corrosion pit on the outboard surface of the leg which had been painted over. The corrosion pitting indicated that either the corrosion was not observed or ignored during the last overhaul or landing gear exchange when the floats were removed and the airplane wheels reinstalled. Improper maintenance of the attachment bolts during assembly on the left main landing gear axle likely left them loosely secure, which produced inadequate thread engagement; unable to sustain the stress of landing loads, the bolts failed, which caused the wheel assembly to separate during the landing roll.

On April 4, 2024, at 1445 Pacific daylight time, a Cessna 180A, N5281D, was substantially damaged when it was involved in an accident near Priest Lake United States Forest Service Airport (67S), Nordman, Idaho. 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 pilot had purchased the tailwheel-equipped airplane one month prior to the accident and had been receiving dual instruction from the flight instructor to meet an insurance-mandated tailwheel training requirement. The pilot and flight instructor had spent most of the training accomplishing a dozen or so landings at various local airports on the day of the accident. While conducting a power-off approach the airplane developed a higher-than-normal sink rate. The flight instructor called for additional power, but the engine briefly hesitated as power was applied. The flight instructor managed the power and applied aft yoke input to flare the airplane, which resulted in a firm touchdown with a minimal bounce. During the bounce, the left main wheel axle assembly separated from the landing gear leg. When the exposed landing gear leg contacted the grass surface, it separated from the airframe. The fuselage and left wing then contacted the ground, sliding on the left wing and right main landing gear wheel resulting in substantial damage to the left wing. The airplane came to rest upright, with the nose of the airplane oriented 180° to the landing direction. The associated nuts were not recovered from the accident site. Metallurgical examination of the left main landing gear axle assembly revealed that all 4 bolts remained installed through the flange that was mated with the leg. The bolts exhibited bending and tensile loading near the fracture with a portion of the bolt threads completely sheared off. These features were consistent with the shearing of the nuts from the attachment bolts due to landing forces. The axle surfaces exhibited fretting wear consistent with repeated oscillatory loads between loose joints. Due to the damage of the threads, it may indicate that the nuts were engaged only at the end of the bolts. During the metallurgical examination of the axle assembly, fatigue cracking was identified on the surface of the landing gear leg. The fracture of the landing gear leg exhibited fatigue cracks in multiple locations along the surface of the leg due to a corrosion pit that had propagated upward through the landing gear leg. A review of maintenance records showed that an annual inspection, including brake and wheel work, had been completed on March 20, 2024. Additionally, on June 1, 2023, the main landing gear had been removed for the installation of floats.