What are some of the reasons that cracks might appear in an aircraft's outer aluminum skin? What may have caused the actual rupture?
In order to provide comfort or actually make it possible for a passenger to live at the altitude where it's efficient to run a jet engine—between 30,000 and 40,000 feet [9,150 and 12,200 meters]—you have to pressurize the cabin, so that the pressure inside the cabin is the same as it is at sea level.* There's an analogy with a balloon—if you blow up a balloon, the pressure inside the balloon is higher than the outside pressure, which is why it expands. On every flight the airplane takes off, flies to those altitudes, and pressurizes the fuselage. When it descends, the fuselage is depressurized. And then you do it again and again and again for subsequent flights. Each of these events is called a cycle. You're basically putting force into the aircraft's aluminum and [then] relieving it. Eventually, the aluminum begins to give in, and that phenomenon is called fatigue. When you pressurize an aircraft tens of thousands of times, the material's properties change—and one day it's flying and just cannot take the next cycle.