The occurrence of the vection illusion among helicopter pilots while flying over water.

U.S. Coast Guard helicopter pilots were questioned on the occurrence of the vection illusion while flying over water under different light and sea conditions. A total of 267 (79.9%) pilots completed the study questionnaire. The illusion of vection was experienced by 248 (92.5%) of these pilots. The majority of the pilots, 209 (84.6%), reported that dark rather than light visual conditions increased the likelihood of experiencing vection. Vection was considered likely to occur over rough seas by more pilots [114 (46.2%)] then over smooth seas [81 (37.8%)]. Several pilots commented that they had responded to the illusion with aircraft movement. The vection illusion is a common experience among helicopter pilots while flying over open water. Low light conditions and rough sea states are conducive to experiencing the vection illusion. Pilots may respond to the illusion with aircraft control movements, which raises flight safety concerns.