Flying and Health — Page 94, Lesson 143

I want to walk you through the effects of g-forces on the human body during flight. We're starting with positive g-forces — the kind you feel when you pull back on the control column and the aircraft accelerates upward, pushing you down into your seat. Let me define what's happening. Under positive g, blood is forced away from the head and toward the feet. This reduces blood supply to the brain, heart, and eyes, while increasing blood pressure at the feet. That redistribution is the root cause of everything we're about to discuss. Now, the photosensitive cells of the eyes — these are the rods and cones — need a disproportionate amount of oxygen from the blood. Because positive g reduces the amount of oxygen available to them, vision is affected first. At around 3 to 4 g, you experience what's called 'greying out' — your vision dims. This is accompanied by tunnelling of vision, because the eye cells at the edge of the retina are furthest from the blood supply, so they suffer first. If the g-force increases further, above 5 g, the individual will eventually lose consciousness — this is called 'blackout' , now more commonly referred to as G-LOC — G-induced Loss Of Consciousness. The important thing here is that the effects of blacking out disappear almost as soon as the g level is reduced. However, the individual will be confused for a few seconds and may have difficulty focusing their eyes. There are other effects at different g levels. At 4 to 5 g, you get inspiration difficulties — trouble breathing in — because the diaphragm is being pulled downward. Above about 8 g, you start losing sensory functions. You'll also get cramping of the calf muscles. At very high g-forces, haemorrhages — bleeding — can occur around the legs and feet. And at extreme g-forces, you can get fracture of the vertebrae and ultimately death, all due to lack of venous return to the heart. Now let's flip to the opposite — long duration negative g. This happens during inverted flight, outside loops, and some forms of spinning. The symptoms here can be more uncomfortable than those caused by positive g. Under negative g, organs are forced upward and blood is forced into the region of the head, affecting the hydrostatic variation. The individual experiences respiration difficulties, facial pain, and the lower eyelid is pushed up, giving rise to what's called 'redout' vision. Additionally, the upward flow of blood causes a slowing of the heart. With high negative g, the small blood vessels in the face and eye may burst. Next we have short duration g-forces. These are concerned with impact forces — think crash landings or ejection seat firings. The maximum tolerable levels are determined by the strength of various parts of the body. The human body can actually stand short-term g-forces of surprising magnitudes. Finally, let's look at susceptibility and tolerance to g-forces. The ability to withstand even moderate g-forces is reduced by several factors: hypoxia — lack of oxygen; hyperventilation — over-breathing; hypotension — low blood pressure; stress; fatigue; heat; and low blood sugar. Any of these will lower your g-tolerance, so as a pilot, you need to be aware of your own physical state before and during flight.