ERA13FA026 · Aerospatiale AS 355 F2

According to the operator, the airline transport-rated pilot sent a text indicating that he was intending to depart on a scheduled repositioning flight from a helipad located behind the pilot's home to an airport located about 25 nautical miles southwest. Data recovered from a handheld GPS device showed that the helicopter lifted off from the helipad and began accelerating forward while turning right and maintaining a relatively constant altitude. During the 27 seconds of recorded flight that followed, the helicopter's right turn rate increased, shortly decreased slightly, and then significantly increased again as the helicopter began to descend. The helicopter subsequently impacted trees and terrain.

Although the weather reporting stations closest to the accident site and at the destination airport generally reported that visual meteorological conditions prevailed, the presence of calm wind, near-coincident temperatures and dew points, and the clear night sky favored the formation of patch radiation fog and/or dew on the surface. Visible satellite imagery captured about 1 hour after the accident depicted a band of low stratiform clouds or fog/mist over the accident site and along the adjacent river valley. Additionally, several witnesses near the helipad reported that the lighting and weather conditions about the time of the accident were "dark" and "foggy." Postaccident examination of the wreckage revealed no evidence of any preimpact mechanical malfunctions or failures of the airframe or engine that would have precluded normal operation.

The helicopter was not equipped to operate in instrument meteorological conditions (IMC). Although the airline transport-rated pilot possessed airplane and helicopter instrument ratings, his most recent instrument proficiency check was completed about 8 months before the accident in an airplane, not a helicopter, and no evidence was found indicating that he was current or proficient in operating a helicopter in IMC. Regardless, the fog should have been an indication to the pilot that IMC existed, and he should not have decided to operate the helicopter in such conditions. The flight profile and the presence of radiation fog during dark night conditions, which would have obscured visual references such as the trees, are consistent with the pilot having experienced spatial disorientation, specifically a vestibular illusion known as the "graveyard spiral."

Verbatim NTSB analysis and probable cause from the NTSB dataset