Shall We Take the SUV or the Convertible?

Within the past few weeks, a couple of friends have approached me to ask (and I’m paraphrasing), “It’s okay to fly Southwest, right? They’ve got the planes fixed?”

Anyone except a bird can understand the motivation behind the question. When you have to rely on a man-made machine to be safe up there, you’d prefer that it remain in one piece, and that all the pieces stay where they’re supposed to. Then you read about a Southwest Boeing 737 that lost a portion of the upper fuselage, and a photo taken by a passenger shows a seat back with sky above it. Not a comforting sight.

Okay, so what caused this disturbing incident? From the comfort of this writing desk, and not privy to details of the investigation, here’s an informed opinion.

Assuming adequate design, materials, and construction, airplanes can remain structurally sound indefinitely. And from the standpoint of the historical record, the Boeing 737 has a stellar reputation during many years of extensive service with airlines all over the world. I’ve not read anything to suggest that this or any similar instance in the past has been caused by potential flaws that arrived with the airplane when it was delivered brand-new to the user.

The issue is structural fatigue, and in this case, the integrity of the fuselage was compromised by the loss of a section of “skin.” You can think of the fuselage as an aluminum tube that serves as a “pressure vessel” to provide an environment where humans can exist in relative comfort when conditions on the other side of the skin are hazardous to health.

And in terms of the fuselage, the most significant cause of fatigue is the expansion and contraction that occurs on every flight when the aircraft is pressurized during the takeoff sequence and depressurized on landing. “Short-haul” aircraft like the 737 accumulate many more such “cycles” per hour of flight time than larger jets, and Southwest’s unique business model and route structure increase the ratio even further.

Commercial airliners are governed by the most restrictive regulatory burden of any airplane type when it comes to inspections and maintenance. You should be pleased to know, if you didn’t already, that the vast majority of time, effort, and money spent on maintaining a commercial jet is scheduled and preventative in nature, in which well-trained mechanics go looking for problems by conducting inspections. Unscheduled maintenance occurs, of course, but it’s nothing compared to the far more common actions that airlines have to perform on a regular and continuing basis.

Depending on the airplane, scheduled maintenance can be driven by either the calendar, the number of flight hours since the last inspection, or both (with the phrase, “whichever occurs first”). And since design engineers understand well the role of fatigue in compromising the strength and integrity of aluminum, the inspection interval typically decreases with increasing total number of flight hours. In some cases, inspections can also be driven by the number of cycles. In plain language, the more “used” an aircraft is, the more frequently it’s inspected.

That said, if we make the assumption that the evidence of metal fatigue in the section of skin that separated from the fuselage existed at some point prior to the incident, why wasn’t it found? Southwest Airlines, the Federal Aviation Administration, and Boeing Aircraft Company are all intensely interested in answering that question. Immediately after the incident, The FAA ordered Southwest and other carriers to immediately inspect about 175 of a total of 1,800 older B-737-300 jets for evidence of fuselage cracks. The number of airplanes affected by the directive was determined by an arbitrary (yet carefully considered) number of takeoffs and landings: 30,000. Southwest “grounded” about 79 aircraft until they could be inspected and found cracks in three more jets.

Here’s a partial quote from an April 6, 2011 article in Popular Mechanics by Matt Molnar:

A trip from Phoenix to Sacramento turned horrifying for 115 passengers aboard Southwest Airlines Flight 812 on April 1, when the cabin shuddered with a loud bang, a violent woosh of air and the sudden release of the overhead oxygen masks. A 5-foot hole ruptured in the Boeing 737-300’s aluminum skin as the jet reached 34,000 feet, instantly sucking the breathable oxygen out of the plane. The pilots quickly descended to a lower altitude with breathable air and landed safely in Yuma, Ariz., but not before at least two passengers passed out, a flight attendant suffered a minor injury and at least one terrified flier sent his wife a text message saying, “Plane is going down, I love you.”

Despite the happy ending, Southwest took a public relations beating for several days after, with speculative blame for the accident’s cause ranging from the airline’s use of older planes to cutting corners on maintenance. But now the focus of federal investigators has shifted to a problem in the design and testing of this 737 model: Boeing has acknowledged that a particular joint failed much earlier than its engineers expected. And because they didn’t anticipate trouble, the joint was not reviewed during the 737’s inspections, so any cracks in the fuselage went unnoticed.

Friday’s fuselage rupture occurred along one of the plane’s lap joints along the top of the plane, a spot where two pieces of metal overlap and are joined with rivets and adhesives to form an airtight and watertight seal that allows the cabin to be pressurized. Each time any aircraft pressurizes before takeoff or depressurizes after landing, that takes a toll on these joints.

Robert W. Mann, Jr., a Port Washington, N.Y.–based airline industry analyst and former airline executive, compares the tension exerted on the fuselage by pressurization to that of the surface of a balloon. The thin metal skin is attached to the “stringers” that form the plane’s internal skeleton, he explains. “Every time the cabin is pressurized, there is air pushing to get out,” Mann says. “This creates stress on the metal, especially at points where metal is attached to other parts of the structure.”

Each Boeing 737 has dozens of fuselage joints. Based on stresses measured during initial testing and subsequent use in service, the manufacturer, under the watchful eye of the FAA, sets guidelines for inspections and maintenance of these joints.

According to the FAA and National Transportation Safety Board (NTSB), Southwest followed all the prescribed inspection and maintenance intervals for the 737 that ruptured. But, crucially, those inspections did not check out the lap joint near the top of the fuselage that failed. Boeing engineers expected these joints to endure 60,000 flight cycles of takeoff and landing before they would need inspections, but the Southwest 737 that ruptured had completed only 39,871.

End quote from the article.

My informed guess is that in the aftermath of this incident, relevant inspection criteria will be revised in two ways: reduce the total number of cycles beyond which additional attention must be paid to inspecting for the presence of fuselage cracks, and shorten the interval at which the inspections are performed.

So, the answer to the question posed by my friends, therefore, is, yes. Southwest Airlines has no intention for their original marketing concept of providing an alternative to the family car for shorter trips to become synonymous with leaving the SUV at home and riding in their convertibles.

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