In 1902, the Wright brothers flew the most sophisticated aircraft of the day -- a one-person glider featuring muslin "skin" stretched over a spruce frame. Over time, wood and fabric gave way to laminated wood monocoque, an aircraft structure in which the plane's skin bears some or all of the stresses. Monocoque fuselages allowed for stronger, more streamlined planes, leading to a number of speed records in the early 1900s. Unfortunately, the wood used in these aircraft required constant maintenance and deteriorated when exposed to the elements.
By the 1930s, almost all aviation designers preferred all-metal construction over laminated wood. Steel was an obvious candidate, but it was too heavy to make a practical airplane. Aluminum, on the other hand, was lightweight, strong and easy to shape into various components. Fuselages bearing brushed aluminum panels, held together by rivets, became a symbol of the modern aviation era. But the material came with its own problems, the most serious being metal fatigue. As a result, manufacturers devised new techniques to detect problem areas in an aircraft's metal parts. Maintenance crews use ultrasound scanning today to detect cracks and stress fractures, even small defects that might not be visible on the surface.