WPR09FA176 · Maule M-4-220C

The private pilot departed for a short local flight while visual meteorological conditions existed in the immediate vicinity of the departure airport. He proceeded to climb to the northeast, through a mountain pass, and then to the south, paralleling the shore of a frozen, snow-covered reservoir. The flight continued for about 20 miles. Analysis of radar and recorded GPS data showed that the airplane then experienced large fluctuations in ground speed while still on the same approximate track. Shortly thereafter, the airplane began a 180-degree left turn and collided with the reservoir surface. GPS data, airplane instrumentation, and ground scars indicated that the airplane was in a descending left turn when it struck the ground. All major sections of airplane were accounted for at the accident site, and post accident examination of the engine and airframe revealed no obvious anomalies that would have precluded normal operation. Mountain obscuration and occasional precipitation were forecast for the area of the accident flight. Additionally, weather observation stations and local pilot reports indicated that moderate snow showers were in the vicinity of the site at the time of the accident. The white surface of the frozen lake, in conjunction with the snow and limited visibility, would have provided the pilot limited external visual references, and as such could have resulted in him becoming spatially disoriented or affected by a visual illusion. The pilot's logbooks revealed that he had a history of flying in marginal weather conditions, and had performed multiple Special VFR takeoffs and landings. The pilot did not possess an instrument rating.

Verbatim NTSB analysis and probable cause from the NTSB dataset