CHINK IN THE ARMOR: The National Transportation Safety Board (NTSB) has sent this torn piece of the Southwest Airlines Boeing 737-300 that ruptured on April 1 back to its headquarters in Washington, D.C., for further investigation. Image: NATIONAL TRANSPORTATION SAFETY BOARD (NTSB), VIA YOUTUBE
The 1.5-meter-long gash that opened up in the upper cabin of Friday's Southwest Airlines Flight 812 from Phoenix to Sacramento will have a deep impact on the nature and frequency of commercial aircraft maintenance. The Federal Aviation Administration (FAA) issued a directive on Tuesday ordering about 175 Boeing 737 aircraft—80 of which are registered in the U.S., most of those operated by Southwest—to be inspected using an electromagnetic device that can identify metal fatigue.
The FAA is targeting Boeing 737 series 300, 400 and 500 aircraft that have accumulated more than 30,000 flight cycles (takeoffs and landings) in order to prevent a repeat of the April 1 incident. The fuselage of a 15-year-old Southwest Boeing 737–300 ruptured 18 minutes into the flight at an altitude of about 10,670 meters, forcing the pilots to make an emergency landing at Arizona's Marine Corps Air Station Yuma.
The National Transportation Safety Board (NTSB) says its investigators have found cracks in portions of the lap joint running on two lines of riveted joints covering the length of the fuselage of the aircraft involved in the incident. Subsequent Southwest inspections turned up cracks in the lap joints on five other aircraft, grounding them as well. The electromagnetic eddy-current test being performed uses a probe to send high- and low-frequency signals down into the skin of the aircraft. The probe is moved from one rivet to the next. Any crack in the metal changes the current's signal and tips off inspectors to a potential problem.
The riveted joints that failed on Flight 812 were not extensively checked because they were thought not to be susceptible to fatigue, according to the NTSB. "What we saw with Flight 812 was a new and unknown issue," Mike Van de Ven, Southwest's executive vice president and chief operating officer, said in a press release.
Southwest, the largest U.S. domestic carrier with more than 3,400 flights daily, follows a business model that relies exclusively on Boeing 737 aircraft, which for the most part make frequent flights along heavily trafficked regional routes, although the airline has expanded to the Midwest and east coast in recent years. This approach, along with bare-bones service, saves Southwest money but also puts a lot of cycles on its airplanes.
Scientific American spoke with Snorri Gudmundsson, an assistant aerospace engineering professor at Embry–Riddle Aeronautical University in Daytona Beach, Fla., about what causes cracks such as those that may have led to the fuselage rupture, what Flight 812 passengers experienced when their airplane opened up, and how neural networks might be able to someday detect cracks before they become a problem. Prior to joining Embry–Riddle, Gudmundsson worked for 15 years as a flight test engineer, structural engineer and the chief aerodynamicist at Cirrus Aircraft in Duluth, Minn.
[An edited transcript of the interview follows.]
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.
How common are the cracks that were found in the aircraft's fuselage?
Cracks like these are common in aluminum. The longer the aircraft is in operation, the more frequently they begin to appear. Where the crack appears on the aircraft determines whether it is a nuisance or a serious thing. The people who design these aircraft know where the most critical areas are, and they tell the operator which areas to inspect extremely well and which areas to inspect maybe less. The older the aircraft is the more prevalent these cracks are and the harder it is to keep track of them. If you don't have mechanics inspect these locations carefully enough, one or two or three may slip under the radar and something like this may happen.
*Editor's Note (4/08/11): Gudmundsson later clarified that cabin pressure ranges anywhere from seal level to about 1,500 meters.