Flying and Health — Page 94, Lesson 141

I want to walk you through the start of the Flying and Health chapter. A career in aviation brings you into situations that aren't usually met in other professions, and we're going to discuss those situations now. Let's begin with acceleration. Acceleration is the rate of change in velocity — that means a change in speed, a change in direction, or both. As a pilot, you're exposed to the forces of acceleration in one form or another almost constantly throughout flight. In aviation medicine, we refer to acceleration in three specific types: linear, radial (which is also called centripetal), and angular. Now, I want to be very clear on the difference between radial and angular acceleration, because they're easy to confuse. Angular acceleration involves rotation about an axis that passes through the pilot's body. Radial, or centripetal, acceleration is where the axis is external to the pilot. So here's how it works in practice: when you initiate a very gentle turn, your body will initially experience angular acceleration. But as the turn develops, you experience both angular and radial accelerations together. We also classify the effects of acceleration on the human body by duration — long duration or short duration. In long duration acceleration, the force acts for longer than one second. In short duration acceleration, which lasts for one second or less, we're mainly concerned with impact forces. That brings us to G-forces. The human body has adapted to live under the force of gravity here on earth — the pull of the earth's gravity gives the body its weight. But acceleration in an aircraft can subject the body to forces much greater than that. For convenience, we measure these forces as multiples of our 1g terrestrial environment — so 1g is the normal force of gravity we feel on the ground. We have specific terms for acceleration in different planes. Acceleration in the fore and aft plane — that's the horizontal plane, front to back — is referred to as Gx. Acceleration in the lateral plane, which is side to side, is known as Gy. However, the usual g-force encountered in aviation is that in the vertical plane, which is termed Gz. Now let's look at the effects of positive G-force on the human body — that's when the force is pulling you down into your seat, increasing your apparent weight. In long-term positive acceleration, the changes in g-force are perceived in stages. At 2g and above, you feel an increase in body weight so that your limbs become harder to move, your head becomes heavy. Mobility is impaired — for example, if you lower your head, it may be impossible to raise it again. At 2.5g, it is impossible to rise from the sitting position. At 3 to 4g and above, internal organs are displaced downwards from their normal positions, and the lower facial area feels "pulled down." There's also an important effect on your circulation. Normally, the blood pressure in your legs and lower body is greater than that at your heart. As positive g-forces increase, that hydrostatic variation — the difference in pressure due to the column of blood — increases. The result is a pooling of blood in the lower body, with a reduced venous return to the heart. That means less blood comes back to the heart to be pumped out, and there is a consequent reduction of blood pressure in the upper body — including the brain. That figure shows the gastro-intestinal tract, and the main problem there is gas in the small intestine — it has no easy exit from the system at either end, which becomes relevant when we talk about how these G-forces affect your body during flight.