ANC25LA030

The pilot reported that, while taxiing for takeoff from a remote, snow-covered lake, the right landing gear leg collapsed, resulting in substantial damage to the landing gear box assembly and door post assembly. A postaccident examination of the right landing gear leg assembly revealed that the gear leg separated from its position in the landing gear box assembly and folded under the airplane. The bolt that secured the gear leg to the landing gear box assembly remained in place; however, the nut was found in the belly of the airplane. Although the hardware installed on the airplane’s main landing gear spring differed from the hardware specified in the airplane manufacturer’s parts catalog, the composition and hardness of the installed bolt were consistent with typical specifications for an AN-type bolt. However, the bolt and nut exhibited worn threads consistent with progressive wear over time rather than a sudden overload. The mechanic who performed a landing gear change about a year and a half before the accident stated that, although he did not recall the hardware used during the gear change, when he changes landing gear, he uses the hardware provided by the owner for installing the landing gear. Based on the condition of the nut and bolt, the worn hardware was likely re-used during the installation of the landing gear assembly. Re-use of the worn bolt and nut then allowed the hardware to fail under load during taxi, resulting in collapse of the landing gear.

On March 7, 2025, about 1400 Alaska standard time, a ski-equipped Cessna 185 airplane, N1718R, sustained substantial damage when it was involved in an accident near Skwentna, Alaska. The pilot and two passengers were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The airplane was taxiing for takeoff from a remote snow-covered lake when the right landing gear leg collapsed, resulting in substantial damage to the landing gear box assembly and door post assembly. According to maintenance records, the airplane had flown about 83 hours between May 2023 and the time of the accident. The owner reported that in October 2023, floats and float rigging were removed from the airplane and the main landing gear and wheels were installed. A review of the maintenance records did not reveal this alteration. Further review of the maintenance records indicated the wheels were removed in December 2023 and skis were installed on the airplane’s main landing gear legs. The mechanic who performed the landing gear change in October 2023 stated that, although he did not recall the hardware used during the landing gear change, when he changes landing gear he uses the hardware provided by the owner for installing the gear. When asked why there was no entry made in the maintenance logbook for the landing gear assembly change, he said the owner never provided the logbooks for him to sign. The hardware found installed on the airplane’s main landing gear spring was an AN7-20 bolt with a cross-drilled hole through the threaded portion of the bolt and an MS20365-720A nylon locking nut. According to the Cessna 180 and 185 Parts Catalog the hardware at the inboard end of the main landing gear spring should have consisted of an AN7-20A bolt that does not have a cross-drilled hole and an MS20365-70C nut, which is a locking nut with an all-metal design, and a spring adjustment shim. The hardware was submitted to the NTSB’s Materials Laboratory for examination. The examination indicated that the bolt exhibited patchy and faint remnants of cadmium plating on the flats of the head. The shank of the bolt did not exhibit any remaining cadmium plating. The composition and hardness of the bolt were consistent with typical specifications for an AN-type bolt. The nut had worn and patchy cadmium plating. Both the bolt and nut exhibited worn threads. The worn threads near the end of the bolt contained bits of the nut’s nylon locking insert. The nylon locking insert was worn down to the depth of the worn threads inside the nut; the wear was consistent with the thread wear occurring over time and not from a sudden instance of overload.