Mbb BK 117 A-3 N911BK accident near Compton, Illinois

Probable cause

The inadvertent encounter with inclement weather, including snow, freezing rain, and reduced visibility conditions, which led to the pilot’s spatial disorientation and loss of aircraft control.

NTSB analysis

The medical transport helicopter was on a night flight conducted under visual flight rules (VFR) to pick up a patient for transport. The pilot had computer-based weather information available, but it is unknown what information he reviewed before deciding to accept the flight. Weather observation stations along the route of flight were reporting VFR conditions around the time that the pilot accepted the flight. About 17 minutes into the flight, the pilot reported to the receiving hospital's communications center that he was aborting the mission due to encountering inclement weather and was returning to base. Flight track data indicated that the helicopter initiated a right turn at this time, away from a nearby lighted windmill farm toward an area with sparse ground lighting . The flight track then showed a slight descent before the end of the data. The last recorded position was about 0.75 miles east-southeast of the main wreckage site. The helicopter impacted an agricultural field in an inverted, nose-low attitude. Examination of the wreckage revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation. Weather data and reports from first responders indicated that the flight likely encountered areas of snow, freezing drizzle, and supercooled liquid water. The lack of ground lighting combined with the precipitation encountered likely reduced the visibility and outside visual references available to the pilot resulting in spatial disorientation and subsequent loss of control.

Text from the NTSB. Open the NTSB docket for CEN13FA096 for the full record.