The United States Air Force realizes there is great benefit to be gained from helmet-mounted display (HMD) technology when compared with traditional head-down displays. HMDs reduce military aircrew workload and improve their performance by mounting information display systems directly on the crew member's helmet. As the crew member concentrates outside the cockpit, information essential for successful mission execution remains within his field-of-view; regardless of his head position. However, mounting a display system on the helmet presents many design and safety related challenges. The Air Force's Interim - Night Integrated Goggle and Head Tracking System (I-NIGHTS) Program identified many of the challenges associated with HMDs. Three of these challenges discussed here are fit, weight/center-of-gravity, and ejection compatibility. Fitting a HMD involves more than just getting a head inside the helmet. The "fit equation" includes comfort, optical accommodation, and helmet stability. The lack of effective design in just one of these factors can negate any tactical advantage the HMD provides. HMDs also add to the weight supported by the crew member's head and neck. This weight generates significant forces during high G maneuvers and emergency situations such as ejection. How much weight and what center-of-gravity can the neck tolerate without injury or fatality? The Air Force I-NIGHTS Program encountered these challenges and serves as a starting point to bound their solutions
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