This report describes a study on the effects of low frequency vibration on pilot performance. Army pilots acted as subjects and flew simulated helicopter missions in a realistic fixed base simulator environment. While flying the two-hour missions pilots were exposed to vibration stimuli varying in frequency from 6 to 12 Hz and in amplitude from ±0·1 to ±0·3g (measured at the floor). Measurements were taken of the vibration tramsmissibility of the pilot's seat so that the vibration actually felt by the subject could be determined. Missions involved transporting external loads in a simulated logistics environment. Performance was evaluated by measuring flight path deviations from prescribed en route, approach, and hover parameters. The vibration stimuli used did not degrade performance. In fact, performance tended to improve with increased stress. It is hypothesized that this trend was due to motivation, i.e. as subjects felt the onset of fatigue they compensated by working harder and thus tended to improve...