Is this a case where there is a problem specifically with the Boeing 737 aircraft or would closer inspections across all different commercial aircraft yield similar fuselage problems?
All airplanes are subject to metal fatigue. The only way to catch it is by proper maintenance procedures. Every day, many airplanes have patches and skin plates put on them to prevent further fatigue, and we never hear about it. It only becomes a problem if the maintenance is inadequate.
Southwest has replaced the aluminum skin on many of its 737–300 airplanes in recent years, according to a spokeswoman. The planes that the company has grounded over the past few days had not had their skin replaced. What does this tell you?
They are aware of this problem and are trying to prevent it from becoming too serious. I don't want to say anything that's unfair, and I don't know how they operate their maintenance program, but it is not unreasonable for the flying public to question two incidents such as this occurring from at the same airline. [On July 13, 2009, Southwest Flight 2294 from Nashville to Baltimore was forced to divert to West Virginia after a hole formed on the top of the Boeing 737's fuselage near the tail, resulting in depressurization of the cabin and deployment of the oxygen masks.] Southwest is known for short flights, which means their airplanes accumulate a lot of cycles over a short period of time. Perhaps because of that they should change their maintenance procedures so they have a better chance of catching these cracks before they become failures.
Does this encourage the airline industry to look into new types of composites and other materials that might be used to build their airplanes?
The industry has been moving toward the use of composites for a while. But it doesn't matter whether it's composites or aluminum—all materials have their shortcomings. Aluminum is a fantastic material. The problem with aluminum, though, is that it doesn't have what we call an endurance limit. Steel, for example, has a known stress endurance limit. This means that as long as the stress levels in the material are kept below a certain value, you can cycle it endlessly. In the case of aluminum it doesn't matter if you apply high stresses or low stresses, eventually you're going to break the material. Of course it will take you a lot longer if your stress levels are low. Moving away from aluminum and toward other lesser-known materials, however, might be opening a different can of worms.
Instead, it would be better to incorporate into the airplane a system that would monitor crack growth. One way of doing this that is being developed at Embry–Riddle is to put a microphone on the skin of the airplane that picks up noise. You would then use a neural network, basically artificial intelligence to break the sound into constituent components and identify the sources of different types of sounds. For example, you create a mathematical model that can estimate the characteristics of a crack generating a particular sound. This system would be on every flight, and when it determines too much noise is coming from a particular direction it would warn the pilot.