The value of training has been well established during the decades since the 1970's and has been the subject of much research and literature, and training has contributed to extending pilots' G tolerance in the areas of both magnitude and duration. Certainly, this training has allowed pilots and crews to more safely exploit the capabilities of high performance aircraft. This training will become more important as new weapons systems are fielded with even higher performance capabilities.
Training includes Academic training on acceleration physiology and the Anti G Straining Maneuver (AGSM), and Centrifuge training.
The human body has different tolerances for g-forces depending on the acceleration direction. Humans can withstand a positive acceleration forward at a higher g-forces than they can withstand a positive acceleration upwards at the same g-forces. This is because when the body is moving up at such high speeds the blood rushes from the brain which causes loss of consciousness.
A further increase in g-forces will cause g-LOC where consciousness is lost. This is doubly dangerous because, on recovery as g is reduced, a period of several seconds of disorientation occurs, during which the aircraft can dive into the ground. Dreams are reported to follow G-LOC which are brief and vivid.
The g thresholds at which these effects occur depend on the training, age and fitness of the individual. An un-trained individual not used to the g-straining manoevre, can black out between 4 and 6 g, particularly if this is pulled suddenly. Roller coasters typically do not expose the occupants to much more than about 3 g. A hard slap on the face may impose hundreds of g-s locally but not produce any real damage: a constant 15 g-s for a minute, however, may be deadly. A trained, fit individual wearing a g suit and practising the straining manoeuvre, can, with some difficulty, sustain up to 9g without loss of consciousness.
The human body is considerably more able to survive g-forces that are perpendicular to the spine. In general when the g-force pushes the body backwards (colloquially known as 'eyeballs in') a much higher tolerance is shown than when g-force is pushing the body forwards ('eyeballs out') since blood vessels in the retina appear more sensitive to that direction.
Early experiments showed that untrained humans were able to tolerate 17 g eyeballs-in (compared to 12 g eyeballs-out) for several minutes without loss of consciousness or apparent long-term harm.
Positive-pressure ventilators work by increasing the patient's airway pressure through an endotracheal or tracheostomy tube. The positive pressure allows air to flow into the airway until the ventilator breath is terminated. Subsequently, the airway pressure drops to zero, and the elastic recoil of the chest wall and lungs push the tidal volume -- the breath -- out through passive exhalation.
The US Air Force at Holloman Air Force Base, NM operates a human centrifuge. The centrifuge at Holloman AFB is operated by the aerospace physiology department for the purpose of training and evaluating prospective fighter pilots for high-g flight in Air Force fighter aircraft.
The use of large centrifuges to simulate a feeling of gravity has been proposed for future long-duration space missions. Exposure to this simulated gravity would prevent or reduce the bone decalcification and muscle atrophy that affect individuals exposed to long periods of freefall. An example of this can be seen in the film 2001: A Space Odyssey.
Man-rated centrifuges are made by AMST Systemtechnik in Austria (Austria Metall SystemTechnik), Latacoere in France, the Environmental Tectonics Corporation (EYC) and Wyle Laboratories in the USA.