February 2013

Experimenter is a magazine created by EAA for people who build airplanes. We will report on amateur-built aircraft as well as ultralights and other light aircraft.

Issue link: https://experimenter.epubxp.com/i/108002

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Page 30 of 44

top of the instrument panel. It is an almost certainty that in anything beyond a couple of g deceleration that the occupants will be thrown forward and suffer serious facial injuries. While this may seem to be a rather cosmetic concern, one must remember that the face is connected to the skull, and the brain is rather poorly tolerant to abrupt changes in direction or speed. Being slung forward and then bouncing your face off the glare shield is an excellent way to induce serious or fatal head injuries. Even in the absence of a skull fracture, the differing density of the human brain causes it to slow down at slightly differing rates. A good way to visualize this is to imagine a plastic container of Jell-O with bits of fruit in it. If you grab it and throw it really fast to a friend and then look at the areas around the fruit, you will notice something interesting; the Jell-O around the fruit will often show small tears. This is caused by the denser fruit taking longer to decelerate and pushing and pulling its way through the Jell-O. The same thing happens in the brain in something called a diffuse axonal injury. Those tears cause bleeding and swelling, which is often what causes the coma and sometimes death that we see in folks who initially survive crashes in either aircraft or cars. While you do not have to hit anything to suffer this type of injury, an impact will increase the severity of it as well as add the risk of even more severe injuries. Thus, it is important to avoid gaining any more of an intimate knowledge of your instrument panel than you developed during its construction. This is where shoulder harnesses come into play. The difference in the strike envelope a shoulder harness makes is quite dramatic (see Figure 2) from what we saw in the lap belt case. This allows us to produce a safer cockpit environment while maintaining the clean and narrower lines that lend to a better performing and more aesthetically pleasing aircraft. Looking at the strike envelope when viewed from the front (the upper right drawing in Figure 2), you see it is easier to envision a fuselage being designed to accommodate the flailing distances so that it minimizes the likelihood of a head impact. That should be a step in the design of any light aircraft if there is any concern for the safety of the occupants. There is an additional option that is often overlooked outside of aerobatic and military aircraft, and that is the tiedown or "crotch strap." To understand why this is needed in a broader application, one must keep in mind the ways a human body reacts in a crash. There are two predominant "paths" a body can take when restrained. The first is the "up and over" where Te frst is to keep the seat occupant from being thrown either completely out of the aircraf or being failed forward or laterally into the structure of the cockpit. Figure 1. Strike envelope of a 95th percentile adult man restrained by a lap belt during a 4g deceleration. the body moves upward and toward the front of the aircraft (the direction of travel at the moment of impact). This is the classic flailing impact previously described that results in the face or chest impacting the controls or instrument panel. The second is what is known as "submarining" and happens as the mass of the pelvis and legs pulls the body down and under the lap belt. This is why in some crashes one finds the victim partially in the floorboard of the aircraft. You can also get a combination of the two, especially in cases where the restraints are worn loosely. This is one way you can get injuries to both the upper and lower body. Submarining often leads to leg injuries along with more serious abdominal injuries. The latter results from the lap belt sliding further up on the abdominal wall. As the body moves forward, the abdomen and its contents (the intestines and the major blood vessels located in front of the spine such as the aorta and inferior vena cava) are compressed, potentially causing serious or life-threatening injuries. It is vital that lap belts be worn low on the bony part of the pelvis (the iliac crests for those of you interested in EAA Experimenter 31

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