Aging, Again

AIR TRANSPORT

Southwest Airlines fuselage tear reopens concerns about metal fatigue inspections

ANDREW COMPARTIWASHINGTON

to ensure the structural safety of today's airliners and the airplanes of tomorrow," FAA Administrator Randy Babbitt said last November. The FAA expected that the rule's primary benefit for airlines would be "the near elimination of emer­gency airworthiness directives pertain­ing to widespread fatigue damage." The agency called the rule the "last element of the overall Aging Aircraft Program."

Now, in light of the April 1 incident, Babbitt says his staff will review the FAA's Aging Aircraft Program "to ensure we are asking the right questions and taking full advantage of all available data . . . . I want to make absolutely certain that what we learn from this accident gets incorporated into our requirements for reviewing aging aircraft."

Having been manufactured in 1996, the Southwest aircraft barely even fits into the legal definitions of the Aging Aircraft Program. Both the code and the Aging

Lap jOint cracks on the Southwest 737 that led to the fuselage rupture, displayed at NTSB headquarters, caught Boeing by surprise.

Aircraft Safety Act stipulate that aircraft be inspected after their 14th year in ser­vice and at specified intervals after that.

"This [Southwest] aircraft is 15 years old," NTSB Chairman Deborah Hers­man says. "By many industry standards, that would not be considered an aging

_ aircraft."

Less than two weeks ago, the issue of aging aircraft and metal fatigue seemed to be settled, with a final rulemaking last November that culminated decades of research

and regulation spurred by a stunning ac­cident in Hawaii. Then a Boeing 737-300 fuselage ruptured, potentially exposing cracks in the accident prevention efforts.

The rupture of that Southwest Air­lines fuselage in flight on April 1, creating a 5 X I-ft. hole, could compel the National Transportation Safety Board to rethink its positions on when aging aircraft should undergo non-visual inspections for cracks, as well as which locations on the fuselage require the deeper checks. It also could raise questions about how aircraft manufacturers, and Boeing in particular, determine when certain parts of the aircraft must be more thoroughly inspected for fatigue.

The final rule the FAA issued last No­vember aimed to prevent accidents like this and one that occurred in April 1988, when an 18-ft.-long section of the upper

fuselage of an Aloha Airlines Boeing 737 separated from the aircraft in flight. That aircraft had logged 90,000 takeoffs and landings. The accident resulted in the death of a flight attendant and injuries to eight passengers and became the launching point for conversation, debate, research and action related to the threat as well as regulatory needs and mainte­nance requirements for aging aircraft and widespread fatigue damage (WFD).

In 1988, Congress passed the Aviation Safety Research Act and, in 1991, the Ag­ing Aircraft Safety Act. The FAA, NASA, U.S. Air Force and aerospace industry devoted millions of dollars and many years to research-generating uncounted numbers of research papers, proceedings and conferences-and the FAA created an aging aircraft working group, issued about 100 airworthiness directives aimed at preventing WFD and published some targeted regulations. But the FAA's No­vember 2010 rule making on WFD was seen as the culmination.

"This rule is a comprehensive solution

36 AVIATION WEEK & SPACE TECHNOLOGY/APRIL 11, 2011

Southwest does have a recent history ofWFD-related incidents and issues. In 2008, the FAA fined the carrier $10.2 mil­lion for failing to correctly inspect some 737s for fuselage cracks the previous year; Southwest protested the amount of the fine and eventually negotiated it down to $7.5 million.

In July 2009, a section of the fuselage skin ruptured near the crown on a South­west 737-300, creating a 17 X 12-in. open­ing that depressurized the plane. That aircraft was 15 years old and had accumu­lated approximately 42,500 cycles, and the incident resulted in a new inspection mandate for certain 737 Classics. How­ever, the NTSB concluded its investiga­tion without assigning any blame.

Thus far, the NTSB is not assigning any blame in the April 1 accident, either. NTSB officials say their review to date shows that Southwest complied with all of the airworthiness directives and in­spection requirements.

So what went wrong? The FAA says the April 1 rupture was

in the lower skin at the lower row of fas-

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teners, along the lap joint. The location is not altogether surprising. When air­craft are pressurized, the metal rotates slightly, right where the fasteners are lo­cated, and this bending is what promotes fatigue. There was a history of similar lap joint cracks on earlier model 737s.

Boeing, however, redesigned the lap joint area to beef it up and introduced the first of these into service in 1993. The Southwest 737 on which the lap joint area ruptured was manufactured in 1996. Boeing's analysis predicted skin cracks in the redesigned lap joint area could start to occur at around 60,000 cycles; the Southwest 737 in question had 39,78l. In post-incident inspections of about 78 of its similar 737 aircraft, Southwest dis­covered cracks in five more.

Tony Ingraffea, a Cornell University engineering professor, director of the Cornell Fracture Group, and principal investigator on numerous aging aircraft research projects in the 1990s, says the gap between Boeing's prediction and ap­parent results should raise some red flags about the tests done to recertify the new lap joint design. "Clearly, something's wrong," he says.

The level of confidence in Boeing is important when considering the newer 737s. Paul Richter, Boeing chief project engineer for the 737 Classic, says the 737 Next Generation model is designed with different structural characteristics that reduce the potential for cyclic fatigue with each cabin pressurization, so Boe­ing does not anticipate similar issues with them. But questioned about the engineering analysis that predicted the 60,000 cycles, he deferred to the NTSB.

The NTSB has no answers yet. "The manufacturer, the FAA, the in­

dustry, have not believed that this par­ticular lap joint on this model airplane was one that warranted attention on air­craft that only had this amount of takeoffs and landings on it," NTSB board member Robert Sumwalt notes. "It was not be­lieved that this was an area that could fail until we see it now."

Asked about aging aircraft in general, Sumwalt says, "we have no reason to be­lieve there are significant difficulties or deficiencies in the fleet of airplanes fly­ing," and he has confidence in the steps taken since the incident. But he adds that "we will be looking to see if this issue does relate to other airplanes in the fleet."

Hersman says the NTSB also "will be looking at some of the assumptions that are being made with respect to the in­tervals between these inspections and

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whether or not they are appropriate." The most immediate impact on carri­

ers worldwide is the need to thoroughly inspect the lap joints for cracks much earlier than expected on 737-300, 737-400 and 737-500 aircraft with the same lap joint and tear strap spacing configuration as the one in the April 1 incident.

The carriers will use eddy current methodology, proven effective over two decades, to identify cracks. Eddy current inspection is a small probe that sends high-frequency and low-frequency elec­trical signals into the skin of the aircraft.

The impact on the airlines should not be great, unless cracks are found that force them to ground considerable num­bers of aircraft, though the inspections do take time and money.

Under the Boeing service directive and an emergency airworthiness directive issued by the FAA, and assuming other countries follow suit as is the usual prac­tice, about 175 aircraft will be inspected right away because they have more than 30,000 cycles. Nearly another 400 world­wide will need to be inspected as they ap­proach the 30,000-cycle mark.

Roughly a third of them are operated by European carriers, according to an Aviation Week assessment using the As­cend database. Another third are oper-

ated by airlines in Asia and a quarter by North American carriers.

Southwest has the most, with 78, but that is still a relatively small fraction of its 548 aircraft. This is also the case for United Continental, the next highest on the list, with 32.

More burdensome, perhaps, could be the FAA order-at least in the emer­gency directive-that the inspection be repeated every 500 cycles. Boeing says the FAA is mandating the short re­inspection period as a precaution until Boeing can verify root causes, final fixes and inspection periods.

Caution could be the watchword for now, and the potential for more strin­gent regulations to come may be the biggest worry for carriers.

Ingraffea says he is not surprised about what happened to the Southwest aircraft. There is "well-known variability in metal fatigue life," especially in alloys and riveted and bonded joints, because of variability in material, manufacturing differences and human error, he says. "Maybe the surprise here is fundamen­tally technical," he says. "Maybe we weren't conservative enough." 0

- With Guy Norris in Los Angeles and Lee Ann Tegtmeier in Washington.

Diving into the Wreck Investigators hope to tap AF447 cockpit voice, flight data recorders and other devices

ROBERT WALULONDON

Despite more than 18 months at deep-sea levels, the wreckage of Air France Flight AF447 promises

to yield important clues to unraveling the mystery of what brought down the Airbus A330-200 in midflight.

The circumstances surrounding the June 1, 2009, crash of the wide body en route from Rio de Janeiro to Paris have confounded industry officials, in llart be­cause the little information gleaned until now has provided no cohesive picture of the sequence of events. That informa­tion was gathered from a set of Aircraft Communication Addressing and Re­porting System (Acars) error messages broadcast by the aircraft before all con­tact was lost and from wreckage recov-

ered immediately after the accident. What is more, the inability to find the

bulk of the wreckage for over a year exac­erbated industry anxiety that the safety reputations of Air France and the A330 could sustain lasting damage if a prob­able cause could not be established. That fear caused the airline and Airbus to help finance the last two search phases.

But now that large parts of the wreckage of AF447 have been discov­ered, safety officials are optimistic they will be able to unlock more clues about what occurred. The wreckage was found about 10 km (6 mi.) north of the last known position of the aircraft by Remus 6000 submersibles operat­ing off the Alucia vessel. It lies 4,000

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