The flight of a Southwest Airlines Boeing 737 that blew out a panel this month raises the question of whether airplanes are being kept in service too long.
An airplane that wasn't supposed to have metal fatigue problems this soon developed them, sending Boeing, the Federal Aviation Administration and the airlines scrambling to figure out what to do next.
The FAA, operators and Boeing are doing everything they can through inspection, testing and analysis programs to prevent problems, said Melinda Laubach-Hock, director of the National Institute for Aviation Research's Aging Aircraft Laboratory.
"They're being as comprehensive as they can to prevent this from happening," Laubach-Hock said. "That's why we don't see this (problem) on a daily, monthly or weekly basis."
Despite all the rigorous testing, you can't accurately predict the aging process, she said.
"If I asked you to predict when rust is going to occur on your car, it really depends," she said. "Damage doesn't happen on a predictable basis."
Once it starts, however, and "we know it's there, we can predict how it's going to grow," she said.
On April 1, a Southwest Boeing 737-300 with 118 people on board lost cabin pressure just after take-off from Phoenix after the plane's fuselage ruptured, causing a 5-foot rip along a row of fasteners.
Passengers put on oxygen masks and the pilots quickly brought the plane down to an altitude with more breathable air before making an emergency landing at an Arizona military base. Everyone walked away.
While it gains attention and scares the flying public, flyers should remember that flying is very safe, Laubach-Hock said. "It's the safest mode of transportation."
Regulators also do a good job of making sure airlines perform proper maintenance, Laubach-Hock said.
The Southwest jet had been pressurized and depressurized 39,000 times in its 15 years in service. Cracks were subsequently found on five other Southwest jets with more than 30,000 cycles.
The FAA ordered an immediate inspection of 175 older-model 737s 80 of which are flown in the U.S., mostly by Southwest and the eventual inspection of about 570 planes worldwide. The fuselages were made in Wichita by what is now Spirit AeroSystems.
Anytime an airplane has a high number of takeoffs and landings and is pressurized, stress is put on the skins, particularly on the lap joints, Laubach-Hock said.
Aircraft that fly short hops go through more of the cycles.
"That's more times the skin is being stressed," she said.
Southwest planes may take off eight to 10 times a day.
The issue is not just a Boeing problem, Laubach-Hock said.
Other models are susceptible to the same type of damage because of all the short hops, she said.
Boeing says it has reworked the lap joint region on newer 737s.
But FAA officials are reviewing Boeing's manufacturing techniques, Flightglobal reported Wednesday.
"People have leaped to the conclusion that it was fatigue," FAA administrator Randy Babbitt said at the MRO Americas conference in Miami, according to the report. "The airplane didn't have that many cycles on it so we're looking at other things. The manufacturing techniques."
Boeing's chief project engineer for the older 737s, Paul Richter, told reporters that the analysis and testing Boeing had done on those airplanes "convinced us that we would not have an issue with this lap-joint lower row until much, much later in the life of the airplane."
Boeing engineers had forecast that the planes wouldn't need to be inspected for metal fatigue until at least 60,000 cycles, which Richter said, "we felt was a very conservative number."
The rip of an airplane's lap joints where one part of the fuselage meets and overlaps an adjoining section is like ripping a perforated sheet of paper, said NIAR executive director John Tomblin.
One is difficult to start.
"But once you start the unzipping process, it's easier to unzip a whole row of fasteners," Tomblin said.
Once a crack reaches a certain length, dubbed a "critical crack length" the unzipping of the rivets begins quickly.
The hope is that when designing an airplane, problems show up in testing so design changes can be made before they occur, Tomblin said.
"The research we've done shows that they (airplanes) age really well," Tomblin said. "Then you have a situation that crops up like this."
That spurs more research and more frequent inspection intervals, he said.
Are planes staying in service too long?
"I don't know the answer to that," Tomblin said.
But once a "root cause" of the Southwest incident is found, "I'm sure they will be able to inspect for it, and they'll have an inspection program for this type of thing."
A main reason no one was hurt in the Southwest incident was because of improvements made in aviation safety, engineering and design over the years, former National Transportation Safety Board chairman Mark Rosenker said.
"The airplane, although it is not designed to open up in midair, it (a crack) is designed to stop at certain points, and that's what happened here," Rosenker said.
Redundancies are built into the airplane.
The system works, Rosenker said, "even when we don't know what we didn't know."
But Boeing acted responsibly by admitting the company thought cracks wouldn't form until much later, Rosenker said.
Now, changes must be made to the inspection regime.
"We now know that," he said.
The NTSB will make a complete analysis of what happened and why and make recommendations to change.
Boeing's admission that analysis missed the possibility of cracking raises questions about what other risks are not known, said John Goglia, a former NTSB member and airline maintenance expert.
"It begs the question now: If your models are not accurate, then what do we have to ascertain the health of the fleet," Goglia said.
Goglia is calling for a summit of operators, manufacturers, FAA regulators and other experts to re-examine their assumptions about metal fatigue on aging aircraft.
The summit should undertake a detailed inspection using electrical "eddy" current or X-ray technology of the fuselage of an older plane that has accumulated a high number of flight cycles and hours.
"That is the only way we are going to get a baseline that is statistically accurate," Goglia said. "Now that they are admitting the model is wrong that means all of the other things they have done off that model is suspect."
Speaking to business journalists earlier this month, Southwest chairman and CEO Gary Kelly said he didn't agree with those who say the inspection requirements aren't thorough enough.
"The airplane is already subjected to thousands of inspections on a continual basis that are very carefully put together in terms of having a whole program to ensure the safety of the airplane," Kelly said, adding that problems are "extremely, extremely rare."
"There are very extensive, overlapping, conservative inspection and repair procedures that are already in place," he said. "Every time we learn something new and new things are learned every time we learn something new, all that information is fed back to Boeing and then ultimately the FAA. Then there is a reflection on whether changes need to be made to the maintenance program."