What happened
A U.S. Government flight for the Department of Interior, National Parks Service, departed for a base camp on the Kahiltna glacier at 7,200 feet MSL to transport three park rangers for mountain patrol operations on Mt. McKinley, Alaska. The flight was conducted as a VFR, on-demand air taxi operation using a wheel/ski equipped airplane.
Upon departure, the pilot filed a VFR flight plan and received advisories regarding an active AIRMET for turbulence and mountain obscuration. Approximately 45 minutes into the flight, the pilot communicated with another aircraft at the Kahiltna base camp and learned that low clouds had closed the upper portion of the glacier. The pilot noted that weather conditions had also closed the lower portion of the glacier and stated he was diverting west toward a different glacier drainage. During the flight, a ranger on board contacted their station via handheld radio to report the flight was returning; however, the aircraft failed to arrive at its destination and was reported overdue.
The wreckage was located the following day, scattered across an area of approximately one-half mile. The engine struck the ground below the left wing, causing the propeller to separate before the engine tumbled downhill. The fuselage came to rest roughly one-quarter mile below the left wing and was destroyed by a postcrash fire.
The investigation
Physical evidence indicated that the airplane broke apart in-flight. Components including the left wing, left wing lift strut, left landing gear strut, left door, and windshield were found on the upper portion of a steep hillside.
Structural analysis revealed that the airplane's wing spars and both horizontal stabilizers exhibited negative bending signatures. The flap handle was found in the 20-degree extended position. Notably, the aircraft manufacturer does not publish any published negative structural load factors for a configuration with flaps extended.
A review of meteorological data showed an occluded front characterized by convective activity had moved northward toward the accident area. This system brought heavy rain and hail to the vicinity of the aircraft at the time the pilot was attempting to divert away from the mountain.