Why Southwest's Airplane Has A Sunroof

By now, you have probably heard that one of the Boeing 737-300 aircraft suffered a rupture of it’s skin in flight, opening up a 5 foot gash in the ceiling and prompting an emergency landing. The cause of the failure has been attributed to metal fatigue. Allow me to try to break this down in layman’s terms.

What Is Metal Fatigue?

Have you ever bent a paper clip or a coat hanger back and forth until it breaks? This is metal fatigue, the use of stress to break down of the molecular bonds that give metal its strength. At the same time, people wonder why a jet liner would fail, as it certainly isn’t being bent back and forth like the paper clip.

Or is it?

Have you ever sat in the back of the plane and noticed the whole cabin bending upon touchdown?  Have you ever looked at the wing and seen it flex up and down in turbulence?  Those are merely the visible indications that the body of an airplane is not a static structure. It is a malleable creation designed to flex rather than be brittle. What you can’t see is the effect of pressurization on the metal structure of the aircraft. Pressurization is what makes it possible for the cabin of an airplane to hold an atmosphere that is breathable when the airplane is flying at altitudes that people cannot survive. People in the business occasionally refer to pressurized planes as ‘inflatable’. Every time the aircraft climbs to high altitude, the cabin is inflated to higher pressures. Like a balloon being inflated, this causes the skin to expand slightly. Do it over and over again, like an airplane that repeatedly makes short flights, and the skin is being flexed just like the paper clip.

What To Do About Metal Fatigue?

Up  until 1988, airlines weren’t particularly concerned about metal fatigue. And then, Aloha Airlines flight 243 suffered an incredible failure of entire sections of the fuselage.  This resulted in the death of a flight attendant and the injury of 65 people.  The cause was traced to corrosion and metal fatigue. The contributing factors were the frequent take-offs and landings made by high frequency, inter-island flights. Corrosion by salt air was also a factor.

Since that landmark incident, airlines and regulators have been scrutinizing airplanes to a much higher extent. Airplanes are regularly stripped of their interiors and examined for cracks at a microscopic level. Cracked portions are either patched or replaced. With proper maintenance, an aircraft should last decades. For example, Delta is still flying 35 year old DC-9 aircraft that they inherited from Northwest.

What About Southwest?

Southwest has suffered a similar incident like this in 1988 when a football sized hole opened up in one f it’s aircraft.  Like the recent incident, there were no fatalities or serious injuries. The aircraft involved in the latest Southwest incident was merely 15 years old. That is a little older than the average age of their fleet, but it is certainly not their oldest plane. Considering that the 737-300 was produced from 1981 until 1999, this plane was one of the newer 737-300s out there.

These facts strongly suggest that there is some sort of maintenance issues that are failing to catch developing cracks in the fuselage of their aircraft.

Should You Fly Southwest?

Despite these troubling maintenance failures, I still remain a fan of Southwest. Air travel in the United States is still posting incredible safety numbers in recent years. The only fatality ever attributed to Southwest airlines in its 38 year history was a person on the ground in this incident.

I have plans to travel with my family on Southwest, and I have not considered changing them. Air travel will never be risk free, but it is by far the safest way to travel. You are several orders of magnitude more likely to die in a traffic accident than you are on a schedule passenger aircraft. This is still a troubling incident, but I expect regulators to take serious measures to ensure that it never happens again.