<div class="section abstract"><div class="htmlview paragraph">STEPS in fatigue design of aircraft lower wing surfaces, as presented here, are: <ol class="list nostyle"> <li class="list-item"> <span class="li-label">1.</span> <div class="htmlview paragraph">Selecting a structure that permits easy fatigue-crack detection. X-ray inspection of blind areas is suggested.</div></li> <li class="list-item"> <span class="li-label">2.</span> <div class="htmlview paragraph">Setting a design quality standard. This is the important stress-concentration factor <i>K_t</i>, which is shown to have a lower practical limit.</div></li> <li class="list-item"> <span class="li-label">3.</span> <div class="htmlview paragraph">Choosing a design gross area stress rather than net section stress to give the desired life. The gross area stress is also a better basis than the 1.0<i>g</i> operating stress level for comparing fatigue life in similar aircraft.</div></li> <li class="list-item"> <span class="li-label">4.</span> <div class="htmlview paragraph">Step testing to check design quality. This preferred method of fatigue testing is discussed here.</div></li></ol></div></div>