PHOENIX, AZ --A senior Boeing engineer said the company was surprised by Friday's rupture of the roof of a Southwest Airlines Co. jet because Boeing didn't expect cracking in the aluminum skin of such planes for many more years. Paul Richter said Tuesday that metal fatigue that led to the hole in the plane's roof had nothing to do with Southwest's heavy use of its planes.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 in the aluminum skin of older planes, resulting in the 5-foot tear in the roof of a Southwest plane as it cruised 34,000 feet above Arizona on Friday.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.Southwest said operations were returning to normal Tuesday after nearly 700 flights were canceled Saturday through Monday.