Background
An air ambulance flight departed Steamboat Springs, Colorado (SBS) to transport a patient in serious condition from Rawlins Municipal Airport/Harvey Field (RWL) to Casper, Wyoming. Before departure, the pilot received a weather briefing from Denver Flight Service. The briefer informed the pilot of a band of light to moderate snow shower activity between Rock Springs and Rawlins, spreading northeast, and of adverse conditions and flight precautions for occasional mountain or terrain obscurations. The pilot stated his intention to fly under instrument flight rules for the entire flight.
Accident Sequence
Approaching RWL, the pilot initiated a right turn outbound to maneuver for the final approach course of the VOR/GPS approach to runway 22. On the inbound course to the airport, the airplane impacted mountainous terrain approximately 2.5 nautical miles east-northeast of the airport. The airplane, configured for landing, struck the terrain wings level in a 45-degree nose-down dive. This impact signature was consistent with an aerodynamic stall.
Weather Information
About five minutes before the accident, RWL reported broken ceilings at 1,100 and 1,800 feet above ground level (agl), 3,100 feet agl overcast, visibility 2.5 statute miles in light snow and mist, temperature 33°F, dew point 30°F, winds from 240° at 3 knots, and altimeter 29.35 inches of mercury. The National Weather Service surface analysis indicated a north-south stationary front along the Rocky Mountains from the Wyoming/Montana border into north-central Colorado, with station plots showing patchy snow over western Colorado and Wyoming. An AIRMET reported occasional moderate rime or mixed icing in clouds and precipitation between the freezing level and flight level 220, with the freezing level at the surface along the route. Witnesses near RWL reported surface conditions ranging from freezing rain to heavy snow.
Aircraft Examination
Post-accident examination revealed clear ice up to 1.5 inches thick on the vertical stabilizer, left and right wings, right main landing gear tire, and right propeller. The ice contamination degraded the airplane's aerodynamic performance, leading to a stall. No anomalies were found in the airplane's systems.
Human Factors
A review of interviews and other materials found insufficient evidence that the company placed pressure on the pilot to take the flight, but the review did not rule out the possibility that the pilot induced pressure on himself. An FAA advisory circular (AC 135-15) addresses several subject areas not practiced by the operator, including advice to avoid flight in icing weather whenever possible.