A senior Boeing Co. engineer said Tuesday that Southwest Airlines Co., which works its planes longer each day than other airlines, wasn't to blame for metal fatigue that led to a hole in the roof of a jet last week.
Paul Richter said the aircraft maker was surprised by Friday's rupture of the Boeing 737-300 because the company didn't expect cracking in the aluminum skin of such planes for many more years.
Southwest planes make frequent short and medium-length hops. They spend an average of 11.7 hours a day in the air _ a full hour more than the airline industry average, according to government figures.
That pattern of use prompted speculation that Southwest's operations had something to do with tiny cracks forming below the surface of the fuselage on older planes where metal panels are riveted together.
A Southwest jet was forced to make an emergency landing Friday after suffering a 5-foot tear as it cruised 34,000 feet above Arizona. A similar incident happened to a Southwest jet in 2009, and five of Southwest's other Boeing 737-300 aircraft were found to have tiny cracks after they were grounded this weekend for emergency inspections.
Richter, Boeing's chief project engineer for models that are no longer in production, told reporters that Southwest was not at fault.
"I think it's just a statistical event ... far more than it has anything to do with Southwest and how they operate the airplane," Richter said.
Federal officials ordered emergency inspections of about 175 older Boeing 737s, including 80 in the U.S. _ 78 belonging to Southwest and two at Alaska Airlines. Southwest said it had already complied with the order by grounding and inspecting the planes after Friday's incident.
Separately, Boeing said it will tell Southwest and other airlines that own about 560 of the older planes to conduct electromagnetic inspections of a 50-foot section of roof panels and rivets called the lap joint once the jets make 30,000 flights, and then every 500 flights after that _ an unusually aggressive inspection schedule.
Metal fatigue has been an issue in aviation since at least 1988, when an 18-foot section of an Aloha Airlines jet peeled back in flight and a flight attendant was killed. Airline construction was changed, with steps taken to prevent small holes from becoming big ones.
Boeing redesigned the lap joint on 737s in the early 1990s and thought airlines wouldn't need to inspect them closely until 60,000 flights. But the 15-year-old Southwest jet that ripped open on Friday had flown fewer than 40,000 flights.
As for the Southwest jets found to have cracks, Boeing said Southwest will have to replace an 18-inch section of overlapping aluminum panels that are riveted together.
The Arizona plane will face "obviously a much larger repair," Richter said.
Boeing said the new inspections could be done in a day. Jerry Glass, an aviation consultant in Washington, said the real cost to airlines will be felt if cracks are detected, requiring potentially costly repairs on an older aircraft.
"It is really a decision as to whether the cost to actually have to repair the plane is worth it, versus parking it (mothballing the plane) and replacing it with another aircraft," Glass said.
Mike Boyd, an aviation consultant in Colorado, said Southwest was already moving as fast as it could to phase out the oldest 737s in its fleet. He said Southwest is buying AirTran Airways _ a $1.4 billion deal _ partly to grab AirTran's place in the order line for new planes.
Southwest said operations were returning to normal Tuesday after nearly 700 flights were canceled Saturday through Monday.