When the blood supply to the head decreases, the first effect is peripheral vision which starts to deteriorate, followed by a loss of central vision. With the return of the blood supply, recovery of vision may be immediate. However, loss of brain function occurs more quickly than loss of vision. Even after restoration of the blood supply, the brain experiences a period of incapacitation lasting approximately 30 s. During that period, aircrews may experience involuntary limb spasms and narcosis after which they may require extra time (up to two minutes) to fully recover and regain cognition with the capacity to act. The incapacitation and recovery periods are the main reasons why aircrews subject to G-LOC, who lose control of the aircraft cannot respond in time to take corrective action. It is in that hiatus that crashes occur. The incapacitation is characterized by a combination: first of an unconscious period (absolute incapacitation); and then of a period of confusion/ disorientation (relative incapacitation). The sum of the absolute and relative incapacitation periods represents the total incapaci-tation period, and may be equated to the overall length of time during which a pilot would be in uncontrolled flight, should G-LOC occur ( Whinnery, 1988).
The mechanical structures of modern high-performance combat fighters can sustain continuous, high G-forces. The use of a fly-by-wire system amplifies the capability of the aircraft to rapidly generate G-forces (>6 G/s) and receive continuously high G-force (>6 G, >30 s). Although the development of aviation technology has significantly increased the performance aircraft, that perfor-mance is compromised by the physiological limitations of the pilots, and G-LOC is one of the challenges. The capability of the aircraft to rapidly generate high G-forces may render aircrews unconscious (greyout or blackout), without warning. This usually happens to aircrews who are either not fully expecting or prepared to experience G-forces (e.g., rear-seat aircrew), or to aircrews stopping an AGSM while attention is distracted (e.g., while searching for enemies). The centrifuge is, therefore, an ideal ground-based simulator where lessons can be learnt, critical eval-uations of a pilot’s AGSM can be made, and proper techniques can be taught ( Gillingham and Fosdick, 1988).