ANC13LA032

On March 8, 2013, about 1330 Alaska standard time, a wheel/ski-equipped Cessna 185 airplane, N185FK, sustained substantial damage while taxiing on a remote, frozen, snow-covered lake near Talkeetna, Alaska. The certificated commercial pilot and three passengers were not injured. The airplane was registered to and operated by Talkeetna Air Taxi, under the provisions of Title 14 Code of Federal regulations Part 135, as a visual flight rules (VFR) sightseeing flight. Visual meteorological conditions prevailed, and a VFR flight plan had been filed. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on March 8, the pilot stated he departed the Talkeetna Airport on a scenic flight tour, which included an off-airport landing on skis. After landing, while taxiing in a left turn on the frozen, snow-covered lake, he felt the weight of the airplane transfer to the right ski. He said that during the turn, the airplane’s wheel/skis began to sink into the soft snow, and it subsequently nosed over, sustaining substantial damage to the wings, rudder and vertical stabilizer. After exiting the airplane, the pilot discovered water overflow in the ski tracks. In a written statement provided to the NTSB IIC, a passenger that was seated in the right front seat reported that the touchdown was soft with plenty of space for a long gentle rollout. At low speed, the pilot attempted to make a 180 degree turn to the left, and just after initiating the turn, the right ski began sinking in the soft snow, causing the aircraft to pitch down and roll to the right. The pilot attempted to prevent the nose down by pulling aft on the control yoke and retarding the throttle, but the airplane continued to nose over coming to rest inverted. A postaccident examination, by the operator, revealed that the ski arm on the right hand ski separated at the weld. The ski arm was sent to the NTSB’s Materials Laboratory in Washington, D.C. for examination. A Senior Safety Board Metallurgist reported that magnified optical examination of the fractured surfaces revealed features consistent with overstress separations of the welds. The examinations uncovered small amounts of gas porosity in the fractured angled tube weld, but no indications of corrosion or preexisting cracking in either separation. Details in the weld fractures were consistent with an inboard to outboard side load applied to the cross tube of the ski arm. (A synopsis of the NTSB Metallurgist's case review is included in the public docket for this accident.)