Effects of helmet weight moments on helicopter pilot fatigue and performance during extended missions were evaluated in a study in which 12 volunteer subjects were exposed to random vibrations in -hour sessions while wearing various helmet configurations. One of the objectives of the study was to investigate an upper limit criterion of 83 N cm for the weight moment of head-worn devices previously proposed by Butler (1992) . In this study, four helmet configurations, having weight moments of 20, 110, 200, and 290 N cm, were tested with each subject. Detection and acquisition of visual targets, a task often performed by AH64 helicopter pilots, were simulated by requiring the subject to aim a light beam, emanating from a helmet-attached source, at light emitting diodes (LED) which were lit at random. Vigilance of the subject was measured by the duration the LED remained lit before it was turned off by the subject. Analysis of this vigilance data did not reveal a consistent relationship between exposure duration and performance. However, the vigilance data clearly demonstrated pilot performance degraded as the weight moment of the helmet increased, and that acquisition times were shortest for weight moments of about 78 N cm. This conclusion, which is based purely on performance consideration, provides an independent confirmation of the Butler criterion which was derived from biomechanical analysis of head pitch accelerations and neck myoelectric activity.