Cessna T337 N357 accident near Casper, Wyoming

Probable cause

The pilot's failure to maintain airspeed and airplane control while maneuvering in low visibility and icing conditions.

NTSB analysis

The pilot received two predeparture weather briefings, both of which reported that snow showers were expected over most of the state. During the second briefing, the pilot was advised that adverse weather conditions, including mountain obscuration, moderate icing, moderate turbulence, and low-level wind shear, existed throughout the planned route of flight. The pilot still decided to depart. As the airplane neared its destination, the approach controller issued the pilot vectors to the final instrument landing system approach course. The approach controller subsequently saw that the airplane was at 6,900 feet mean sea level (msl), which was 300 feet below the minimum vectoring altitude for the approach (7,200 feet msl). The approach controller issued the pilot a low altitude alert, and the pilot climbed the airplane back to the minimum vectoring altitude of 7,200 feet msl. About 1 minute after the approach controller transferred control of the flight over to the tower air traffic controller, the controller saw that the airplane was about 1/4 mile right of the inbound approach course. The controller then issued the pilot missed approach instructions, advising him to fly the runway heading and to climb and maintain 8,000 feet msl. The tower controller then transferred the pilot back to the approach controller, who advised the pilot that the runway visual range had decreased below that required for the approach. The pilot then elected to be vectored back to the final approach course to hold on the localizer until the weather improved.

While he was issuing the pilot vectors to the localizer, the approach controller observed the airplane flying an inappropriate heading. After he advised the pilot of the irregularity, the approach controller issued a revised heading toward the localizer. The approach controller observed the airplane begin the right turn toward the assigned heading, and shortly thereafter, observed that the airplane was 1,200 feet below its assigned altitude. The approach controller issued the pilot another low altitude alert. There were no further radio communications with the pilot. The airplane impacted terrain about 8 nautical miles northeast of the airport, slightly left of the missed approach course, in a left-wing-low, steep nose-down attitude.

About 2 minutes before the accident, the destination airport reported 1/2-mile visibility in moderate snow and freezing fog, with scattered clouds at 800 feet above ground level (agl) and an overcast cloud layer at 1,300 feet agl. Postaccident review of the weather conditions in the area at the time of the accident indicated that the pilot was operating in instrument meteorological conditions and that conditions were favorable for structural icing at a moderate to severe level. Postaccident examination revealed no evidence of preaccident mechanical malfunctions or failures that would have precluded normal operation. Based on the weather conditions that the pilot encountered while maneuvering near the destination airport, it is likely that the pilot failed to maintain adequate airspeed due to structural icing, which resulted in a loss of airplane control and subsequent impact with terrain.

Text from the NTSB. Open the NTSB docket for WPR12FA040 for the full record.