In-flight magnitude of "the leans" illusion in pilots deprived of visual cues: An exploratory study

"The leans" is a spatial illusion encountered by pilots flying without reliable visual references. It can occur when a pilot promptly returns to straight-and-level flight after unawarely having entered a banked turn. The return to straight-and-level flight is sensed by the semicircular canals and erroneously perceived as banking in the opposite direction. The purpose was to establish (1) the magnitude and interindividual variability of the leans, and (2) if centrifuge-based spatial-orientation (SO) training, improving roll-tilt assessment, would affect the illusion. In a fighter-jet-trainer aircraft, pilots (n = 10) were sitting in darkness and setting a luminous line parallel with their imagined horizon, while being exposed to a gradual entering of a 15-20° banked turn, followed by a prompt return to straight-and-level flight. Measurements were performed before and after a 4-week period, during which five of the pilots undertook a 9 × 1 h SO-training regimen. During the pre-training trial, the pilots perceived on average 47% (range = 0-140%) of the roll-angular displacement encountered during the transition from the turn to level flight, i.e., they perceived being roll-tilted 10 ± 9° in the opposite direction. SO-training did not affect the leans magnitude (p = .84). The results confirm anecdotal accounts of marked interindividual variability of the leans illusion, and suggest that such variability in the canal-induced illusion exists even without perceptual interference from visual input.