Background
A twin-engine turboprop airplane was conducting a night cargo flight under instrument flight rules (IFR). The crew planned to execute a localizer DME back course approach to runway 36. The minimum visibility for the approach was one mile, and the minimum descent altitude (MDA) was 460 feet mean sea level (msl), equivalent to 338 feet above ground level (agl).
Approach
Before leaving cruise altitude, the first officer listened to the Automatic Terminal Information Service (ATIS), which provided an altimeter setting of 29.30 inches of mercury (inHg). No other altimeter information was received until the crew reported inbound on the approach. At that time, tower personnel informed the crew that visibility was one mile in light snow, wind was from 040 degrees at 22 knots, and the altimeter setting was 29.22 inHg. The crew did not reset the airplane's altimeters from 29.30 to 29.22.
At the final approach fix, located 5 miles from the runway, the captain began a descent to the MDA. Thirty-six seconds before impact, the first officer cautioned the captain about the airplane's high airspeed. Due to strong crosswinds, the captain disconnected the autopilot 22 seconds before impact. He stated that he pushed the altitude hold feature on the flight director at the MDA.
Accident
Eighteen seconds before impact, the airplane leveled off at about 471 feet indicated altitude, but then began descending again 9 seconds later. The descent continued until the airplane collided with the ground, 3.5 miles from the runway. The crew reported that neither the airport nor the snow-covered terrain was observed before impact. The crew also stated that the landing lights were off. The airplane was not equipped with a ground proximity warning system.
Findings
The official probable cause was the captain's continued descent below the minimum descent altitude, which resulted in impact with terrain during an instrument landing approach. Contributing factors included the flightcrew's failure to reset the altimeters to the correct altimeter setting, and meteorological conditions consisting of snow obscuration that limited visibility, and the ambient night light conditions.