Ingraffea quoted on airplane metal stress
ACSF Faculty Fellow, Prof. Anthony Ingraffea (CEE), was quoted in a Newsweek article on metal fatigue in the Boeing 737. The following is an excerpt:
The plane is America’s most popular model. But aviation experts worry that America’s most popular airplane is prone to cracks in its skin. An investigative report.
by Clive Irving | March 19, 2012 8:37 PM EDT
At 10:56 p.m. on April 1, last year, Southwest Airlines Flight 812, en route from Phoenix to Sacramento with 118 passengers aboard, was completing its climb to its cruise altitude of 36,000 feet, above the small town of Blythe, Calif. An air-traffic controller at Los Angeles Center had just acknowledged a routine call from the pilot. But within a minute or so of that exchange, the controller became aware that Flight 812 was in some kind of trouble. The messages were garbled until, finally, he heard the pilot clearly: “declaring an emergency we lost the cabin.”
One of the most respected authorities on aging airplanes and metal fatigue is Prof. Tony Ingraffea of Cornell’s School of Civil and Environmental Engineering. His extensive investigation of the Aloha 737’s shattered fuselage, part of a long study published in the 1990s, is a classic aviation text. Looking back at the 737’s origins, he explains that the available engines were barely powerful enough for the new model. The designers needed to save weight; to do so, they used an aluminum alloy for the fuselage skin that was only .036 inches thick (the width, for example, of a guitar string). Ingraffea’s investigation focused on the part of the 737’s fuselage design that also figures in the case of Southwest Flight 812, the lap joints, where the two layers of skin are held together with a combination of rivets and adhesive. It is this joining that has proved to be a persistent weakness in the structure—the “Achilles heel of the 737,” as Ingraffea called it.
Back in the 1960s nobody anticipated the coming of budget airlines, when airplanes would be required—as are the 737s of Southwest and other budget carriers—to make five or six flights a day, each flight involving a pressurization cycle. Ingraffea emphasized to me that you can’t measure the aging of airplanes in years; “it’s the total of flight cycles,” he says, referring to a completed flight from takeoff to cruise to landing, in which the full cycle of pressurization has taken place, with all the stresses that that creates. The greater the frequency of flights, the sooner fatigue cracks appear.
Ingraffea believes that everything goes back to the original problem, the thin skin. “The skin thickness remains constant throughout the series. You can’t change that without changing everything that mates with the skin—it would constitute a radical redesign which was never done.”
Here, it turned out, was where every strand in the saga of the 737’s record came together… (read more)