Abstract : The psychological, physiological, and operational effects of sustained aviation operations in the standard US Army chemical defense ensemble were studied in six male aviators during continuous 6-day sessions in a hot, humid field environment. Standard aviation mission durations and profiles were flown in a specially-equipped JUH-1H helicopter while wearing level IV mission- oriented protective posture (MOPP IV) garments. A single case design was used to investigate the effects of microclimate cooling devices of both air and liquid medium design. Results show that cooling is not physiologically necessary below cockpit temperatures of 29 C WBGT although subjective benefit was reported. Above 29 C WBGT cockpit temperature with closed aircraft windows and doors, microclimate cooling was necessary to avoid adverse effects of body core temperature increases. Keywords: Aircraft protection, Aircrew chemical protection, Helicopter warfare, NBC protection, Aircraft survivability, Heat stress(physiology), Heat stress(psychology), Microclimate cooling system, Chemical warfare.