AIR TRANSPORT
Aging,
Again
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.
L
ess 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
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."
Southwest does have a recent history
fuselage of an Aloha Airlines Boeing 737 ofWFD-related incidents and issues. In
separated from the aircraft in flight. That 2008, the FAA fined the carrier $10.2 milaircraft had logged 90,000 takeoffs and lion for failing to correctly inspect some
landings. The accident resulted in the 737s for fuselage cracks the previous
death of a flight attendant and injuries year; Southwest protested the amount
to eight passengers and became the of the fine and eventually negotiated it
launching point for conversation, debate, down to $7.5 million.
research and action related to the threat
In July 2009, a section of the fuselage
as well as regulatory needs and mainte- skin ruptured near the crown on a Southnance requirements for aging aircraft west 737-300, creating a 17 X 12-in. openand widespread fatigue damage (WFD). ing that depressurized the plane. That
In 1988, Congress passed the Aviation aircraft was 15 years old and had accumuSafety Research Act and, in 1991, the Ag- lated approximately 42,500 cycles, and
ing Aircraft Safety Act. The FAA, NASA, the incident resulted in a new inspection
U.S. Air Force and aerospace industry mandate for certain 737 Classics. Howdevoted millions of dollars and many ever, the NTSB concluded its investigayears to research-generating uncounted tion without assigning any blame.
Thus far, the NTSB is not assigning
numbers of research papers, proceedings
and conferences-and the FAA created any blame in the April 1 accident, either.
an aging aircraft working group, issued NTSB officials say their review to date
about 100 airworthiness directives aimed shows that Southwest complied with all
at preventing WFD and published some of the airworthiness directives and intargeted regulations. But the FAA's No- spection requirements.
So what went wrong?
vember 2010 rule making on WFD was
seen as the culmination.
The FAA says the April 1 rupture was
"This rule is a comprehensive solution in the lower skin at the lower row of fas-
36 AVIATION WEEK & SPACE TECHNOLOGY/APRIL 11, 2011
AviationWeek.com/awst
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 industry, 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
AviationWeek.com/awst
whether or not they are appropriate."
The most immediate impact on carriers 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
espite 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-
D
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
AVIATION WEEK & SPACE TECHNOLOGY/APRIL 11, 2011 37