Industry experts believed they knew where to look for crack-inducing metal fatigue on aging airplanes, but the in-flight rupture of a Southwest Airlines Boeing 737 on Friday has raised concerns about part of the fuselage they previously thought wasn’t vulnerable.
A similar hole opened on a Southwest 737 only 21 months ago, and then on an American Airlines 757 last year, raising awareness that metal fatigue can cause the aluminum skin to separate at the so-called lap joints, where panels are spliced together.
Unlike those previous incidents, however, Friday’s 1-by-5-foot hole ripped open at the mid-fuselage.
“It’s a much thicker skin there,” said John Hart-Smith, a world-renowned expert on metal-aircraft structures and a retired high-level Boeing engineer. “Typically, that area has never been susceptible to cracking at the lap splices.”
No one was killed in Friday’s incident, and the limited scope “is a blessing,” Hart-Smith said. “But it’s a warning.”
National Transportation Safety Board (NTSB) officials said the rupture grew from the spread of tiny, invisible cracks in the metal, a well-known phenomenon and one supposed to be addressed through regular maintenance inspections.
Although mechanics cannot see these micro-cracks below the metal skin surface, they can be detected with a handheld electromagnetic testing device.
The Federal Aviation Administration (FAA) said it will issue an emergency directive Tuesday to require operators of certain Boeing 737-300, -400 and -500 models that have accumulated more than 30,000 takeoff-and-landing cycles to conduct electromagnetic inspections for early signs of incipient fatigue damage. Those inspections must be repeated periodically.
Approximately 175 aircraft worldwide must be inspected initially; 80 are U.S.-registered aircraft.
Alaska Airlines said it expects to inspect two of its 737-400s, based on information from the FAA.
Most of the U.S. jets to be inspected belong to Southwest, which said Monday it is almost done checking and will resume normal operations Tuesday after taking three jets out of service.
“Cracking, whether it’s visible or invisible, is just a reality in the airframe life cycle,” Southwest spokesman Brad Hawkins said.
But, because the cracks and then the rupture happened in an unexpected part of the airframe, he said, “what happened Friday on Flight 812 was a new and unknown event.”
In a media briefing Monday in Yuma, Ariz., where the damaged airplane made an emergency landing Friday, NTSB board member Robert Sumwalt said both Boeing and the FAA “have not believed that this particular lap joint on this model airplane was one that warranted attention on an aircraft with this amount of takeoffs and landings.”
“It was not believed that this was an area that could fail, until we see it now,” Sumwalt said.
The earlier Southwest incident on a 737 — in July 2009 — was unexpected at the time. A hole 18 inches by 12 inches appeared in-flight in the crown of the fuselage just in front of the tail.
In response to that occurrence, the FAA ordered fuselage checks for metal fatigue on 135 737-300s, -400s and -500s in the United States, with a requirement that they be rechecked at intervals of not more than 500 cycles. Southwest subsequently replaced the skin panels on the upper fuselage on many of its older 737s.
“Both holes appeared in a part of the fuselage which heretofore had never been [mandated as] a place where we should be looking,” Hawkins said.
The maintenance programs Southwest developed with Boeing and the FAA now “are all being re-evaluated,” he said.
Inspections since Friday found tiny cracks beneath the surface of three of the 79 Boeing 737-300s that had not been reskinned. Hawkins was not able to say on which part of the airplanes those cracks were found.
In the American Airlines incident, in October 2010, a 1-by-2-foot hole opened on a Boeing 757 flying at 31,000 feet.
No one was hurt in the course of an emergency landing, but the incident prompted the FAA in January to require checks on 683 Boeing 757s and 737s.
The NTSB’s Sumwalt confirmed the 737-300 that ripped open Friday was up to date on required inspections and had no outstanding maintenance issues.
Metal fatigue is caused by repeated cycles of stressing and unstressing the plane’s aluminum skin as a plane takes off, cruises and lands.
Southwest planes, flying short routes like most domestic airlines, typically repeat that cycle six or more times a day.
Hart-Smith said the occurrence of metal fatigue generally is predictable and that consequent failures are most likely to occur at high-stress areas along the edges of the lap joints.
“It’s a matter of time,” Hart-Smith said, “If you put enough cycles on the airplane, eventually that will happen.”
He said the latest incident indicates inspections have to be stepped up.
“They should be able to implement a complete inspection and identify where all the starter cracks are,” he said. “The crack doesn’t just appear out of nowhere. It grows progressively.”
Paradoxically, Hart-Smith said, the passengers aboard Southwest Airlines Flight 812 can count themselves lucky that the hole that appeared in the cabin roof was quite large.
The size of the hole ripped in the fuselage must have released all the air from the cabin quickly and immediately eased stress on the airframe, preventing further fuselage damage.
“That’s what saved everybody,” Hart-Smith said. “Once you relieve the pressure, the stress goes away, which means the damage won’t get any worse.”
In contrast, he said, in a much more serious incident on an Aloha Airlines flight in 1988, a failure caused by a combination of metal fatigue and corrosion opened a long, thin tear along the fuselage.
Air leaked slowly, and the stresses then tore away a large section of the upper fuselage, killing a flight attendant who was sucked out of the airplane.
Hans Weber, president of Tecop International in San Diego, said the maintenance procedures to check for fatigue were established after the Aloha Airlines event.
“The whole community worked really hard after Aloha to get full control over this aging-aircraft problem,” Weber said. “And until about last year, we basically had no more problems. This is not good. This is really a surprise.”
The three more recent incidents saw no fatalities.
Still, Hart-Smith said, “it could have been worse. They will have to inspect all these aircraft.”
Boeing said its engineers are preparing a service bulletin that will recommend lap-joint inspections on certain 737-300/400/500 airplanes. The NTSB’s Sumwalt said that bulletin, along with the FAA directive, should be sufficient to make the current fleet of airplanes safe.
“We feel through issuing this service bulletin that will take care of those airplanes that need attention,” Sumwalt said.
“But for the remainder of the fleet, I don’t believe there are structural deficiencies.”
Nonetheless, Transportation Secretary Ray LaHood said in a statement Monday that there could be “additional action depending on the outcome of the investigation.”
Information from Bloomberg News is included in this report.
Dominic Gates: 206-464-2963 or firstname.lastname@example.org
|Operators with more than 10 aircraft. Figures are aircraft in service and include freighters and other uses.|
|China Southern Airlines||25|
|China Eastern Airlines||16|
|Air New Zealand||15|