Acceleration and Its Effects
The axes of acceleration
Acceleration is described along three axes: Gz (head-to-foot, the usual seating axis), Gy (back-to-chest) and Gx (chest-to-back). Positive Gz pushes blood away from the head; negative Gz pushes it toward the head. Magnitude and duration matter, as does the rate of onset.
Physiological effects
Positive Gz causes greyout, tunnel vision and then G-LOC as blood pressure at the eye and brain falls. Negative Gz causes redout, facial congestion and discomfort. Sustained high Gz also causes fatigue and reduced performance. Tolerance is improved by the anti-G suit, the straining manoeuvre (tensing muscles and breathing against a partially closed glottis), physical fitness and hydration, and is reduced by fatigue, dehydration, illness and alcohol.
Traps in questions
Candidates confuse greyout (positive Gz, vision dimming) with redout (negative Gz). They also forget that the anti-G straining manoeuvre must be practised and that relaxation after a high-G turn can precipitate G-LOC. Another trap is assuming that fitness alone protects; hydration and rest matter too.
Worked example
During a sustained 5 G turn, a pilot notices tunnel vision. This is the early stage of G-LOC from positive Gz. The correct response is to reduce G, tense the muscles and breathe the straining manoeuvre, and to report the episode. Rule of thumb: greyout means get the G off and strain.