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: http://experimenter.epubxp.com/i/108002

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Page 31 of 44

S a f e t y W ir e be considerably higher due to investigators failing to adequately document the use of restraints or the condition of them after the crash in a significant number of crashes. Many survivable crashes are not survived because of something as simple as the failure of a shoulder harness attachment point. Paying attention to what you are attaching your harness to should be taken very seriously. Figure 2. Strike envelope of a 95th percentile adult man restrained by a lap belt during a 4g deceleration. the anatomical vernacular) and as snugly as possible to minimize the chance of this. However, the best way to prevent submarining is the use of a tiedown strap. Not only does it provide a direct barrier to the seat occupant sliding down, but also more importantly it keeps the lap belt down where it belongs (hence the name). This minimizes the chance of lower abdominal injuries, especially if used in conjunction with a wider than normal (greater than 2 inches) and padded lap belt. John Paul Stapp, the subject of some of the famous "rocket sled" tests and one of the pioneers in my career field, reported that many subjects found these wider and softer belts to be more comfortable than a simple narrow nylon strap. It is definitely something to consider. Often many homebuilders treat the fabric as the primary part of restraint design, and while it is important, as the saying goes, a chain is only as strong as its weakest link. This is why it is important to have the anchorages for your restraints attached in a way that minimize the chances of a failure. While it may seem that it would take a very devastating crash that would be utterly non-survivable to produce failures of a restraint system, this is not the case despite it being widely repeated when the need to improve restraint design is brought up. In the database that I maintain for my research, there were restraint failures in over a third of all crashes. The NTSB has reported similar findings, even though the actual numbers may 32 Vol.2 No.2 / February 2 013 In fact, even if you are building one of the well-established designs, you should strongly consider "running the numbers" yourself when it comes to design anchorages and consider making them stronger. It is important to remember that many of these designs were done with the FAA design guidelines in mind. These guidelines massively underestimate the ability of a well-restrained and protected pilot or passenger to withstand the deceleration forces involved with real-world crashes. The bar having been set so low by the government probably contributes to as many deaths as it prevents annually; we have the technology, the legal freedom, and know-how to do better. The only thing that seems to be wanting in this matter is the personal motivation. To paraphrase Isaac Newton (or more accurately Bernard of Chartres), if we want to see farther in the name of safety, we need to stand on the shoulders of the giants who have come before us. Assuming that those before us have reached the pinnacle of design and their designs cannot be improved upon is to sell our own abilities short and to abandon the mindset that drove them to improve the designs of those who came before them. Anchorages for the shoulder harness should be above the level of top of the shoulders. If they are angled downward, you wind up compressing the spine, and this can lead to fractures. (We will get into the subject of spinal compression injury in another part of this series.) The attachment points of the lap belt should be to the frame of the aircraft and not to the seat itself. This provides a more direct energy path and minimizes the number of potential failures. The design specifics of how to define the appropriate angle for the lap belt across the hips are well defined and can be seen in Figure 3. While anchorages are important, I did not mean to imply that it is not vital to treat the fabric of the restraints as important. The fabric should be inspected and replaced if it shows any signs of wear, fading, or damage. Take a walk around the ramp of any busy general aviation airport and you will find many

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