Flying and Health — Page 100, Lesson 147

I want to walk you through a critical section on flying and health. We'll start with harness protection, then move to G tolerances, and finally into barotrauma — a key physiological hazard in aviation. Let's begin with harnesses. In an impact, if a lap belt is correctly fitted, the body may still jackknife over it, causing the head to strike a forward structure. Knees can be severely damaged, and compression of internal organs may also result. The five-point harness, which includes the anti-G or crutch strap, offers the best protection. The lap and diagonal harnesses, and the four-point harnesses found in most general aviation and some commercial aircraft, do not give complete protection against submarining — that's sliding under the harness, which can occur in some impact situations. Now, a summary of G tolerances. For long-duration G — more than one second — a relaxed subject can tolerate +3.5 G. Using anti-G straining techniques, that tolerance rises to +7 to +8 G. For negative G, the tolerance is -3 G, but only for short periods. For short-duration impact forces, the tolerance is 25 G in the vertical axis and 45 G in the fore/aft axis. Remember: the effects of acceleration are mainly cardiovascular and pulmonary, but acceleration can also produce perceptual disorders and neurosensory illusions. Let's move into barotrauma. Barotrauma is pain caused by the expansion and contraction — due to outside pressure changes — of air trapped in the cavities of the body. Notably, this occurs within the intestines, middle ear, sinuses, or teeth. Barotrauma can cause discomfort or extreme pain sufficient to interfere with the pilot's ability to operate the aircraft. We'll focus on otic barotrauma — that's middle ear barotrauma. Pressure is normally equalized across the eardrum by the eustachian tube, which leads from the middle ear to the back of the mouth and nose. There is seldom any problem in the climb, when air passes from the middle ear to the atmosphere. Most problems occur in the descent, when air is attempting to return to the middle ear. The end of the eustachian tube acts as a flap valve: it allows air to escape with relative ease — that's what's required in the ascent — but it can restrict air entering the middle ear, which is what's required in the descent. With a reduced pressure in the middle ear, the increasing pressure outside will cause a distortion of the eardrum and sometimes extreme pain. The severity of otic barotrauma depends upon the rate of climb or descent. It occurs mainly at lower levels, where pressure changes are the greatest. The problem is increased if the person has a cold or any other condition that has caused the mucous membrane lining the eustachian tube to become inflamed and swell. One or both ears can be affected, and this will cause: pain — gradual or sudden — which can radiate to the temples; temporary deafness; pressure vertigo; tinnitus — that's a ringing in the ears; and in extreme cases, rupture and bleeding of the eardrum, which may cause deafness.