---

Ultrasonic testing found it was too porous -- too many bubbles were in the composite material after the fuselage section came out of an autoclave in April, Bair said. It took engineers more than a month to fully understand the issue.

---

---

engineers discovered that worrisome bubbles were developing in the skin of the fuselage during the process of baking the plastic composite tape.

Think of the operation as a massive wallpapering exercise. The "paper" is wide tape that's loosely woven from superstrong carbon fibers, then soaked in a honey-thick mixture of polymers. The gooey tapes are plastered on the inside of molds, and then cooked. The heat triggers a chemical reaction that turns the polymers into a hard, super-sturdy structure. The advantages of a carbon-fiber fuselage are enormous in saving weight, reducing costly inspection checks, and simplifying assembly of the plane.

But the main challenge is the sheer size of the fuselage sections. These require multiple layers of carbon-fiber tape to assure structural integrity. However, each added layer increases the likelihood of variations or flaws, say composite experts, such as bubbles on the skin. Bubbles could weaken the material and eventually cause cracks by allowing water to seep under the surface, then freeze up and expand at high altitudes, raising the possibility that the fuselage could crack under extreme conditions. Boeing says a defective mandrel -- or large mold that holds the shape during the taping process -- contributed to the failure of the barrel while it was baking. "Engineers are confident the problem will be fixed," Bair says.

---