Paul Richter, a senior Boeing engineer, said that the company had thought the jets would not be vulnerable to serious cracks in their skin until “much, much later,” and that it was surprised that its safety projections were so far off the mark.

He said Boeing had expected the aluminum skin and the supporting joints on the planes to last through 60,000 cycles of takeoffs and landings before airlines need to be concerned about cracks. But the Southwest jet had nearly 40,000 cycles, according to federal regulators.

Boeing’s stark admissions underscored how regulators and industry officials were struggling to understand the broader ramifications of the accident.

The Federal Aviation Administration said it would require inspections of at least 175 of the older Boeing planes after every 500 flights until the problems are better understood. And analysts said the problems could eventually lead to more extensive inspections of a wider variety of aging planes. Southwest, meanwhile, said it had found minor subsurface cracks in a total of five other 737s since the accident on Friday forced that jet to make an emergency landing at a military base.

Southwest has been buffeted by questions about how intensely it operates its planes. But in describing how surprised Boeing was by the accident, Mr. Richter came to the defense of the airline, Boeing’s largest customer.

Noting that Southwest operates more 737s than any other airline, Mr. Richter said he thought its involvement in the accident “was just a statistical event far more than anything to do with Southwest and how they operate the aircraft.”

But the new information about how soon the fatigue set in raised concerns among aviation safety experts about how much progress the industry had made on such issues, which burst into view when a large section of the roof of a 737 flown by Aloha Airlines ripped open in 1988 and a flight attendant was sucked out of the plane.

John J. Goglia a member of the National Transportation Safety Board from 1995 to 2004, said the Southwest incident once again highlighted a problem with older aircraft that endured tens of thousands of pressurization cycles.

“We’re talking about the same issues today, and we’re proposing the same fixes,” he said. “We need to take a few of these high-cycle airplanes and run them through a very vigorous inspection program and see where we can identify deficiencies, and see whether there is anything different than what the manufacturer thought there would be. But I don’t see that happening because it is expensive.”

Mr. Richter, the chief engineer for Boeing’s older 737 models, told reporters that Boeing had felt so confident about the joints that it had not planned to tell airlines to inspect that part of the plane until it reached 60,000 cycles.

He also said Boeing had redesigned the joints — where overlapping pieces of the outer skin are riveted together — in 1993 after weaknesses appeared in an earlier version.

After the Aloha accident in 1988, Boeing introduced hundreds of modifications to several of its 737 models. It also recommended that airlines replace joints after 50,000 cycles for the more than 2,000 737s it had already produced.

Mr. Richter said the changes in 1993 were expected to make a row of rivets that fasten the flaps of skin together — also known as lap joints — hold up better against the tensions caused by repeated pressurization and depressurization.

When the joints are stressed by that pressure, “you get a slight rotation of the material that causes a bending in the skin, right at or adjacent to where this row of fasteners is located,” he said. “And it’s a combination of the pressure loads in flight and the bending that promotes fatigue growth at a faster rate.”

Aviation experts said that Boeing might have been too confident about the durability of the new design. “When you model something you make assumptions,” Mr. Goglia said, “and if your assumptions are weak, your outcome is compromised.”

Mr. Richter said that the newest generations of 737s — starting with the 600 series that entered service in 1998 and known as the Next-Generation 737 — incorporated significant design changes intended to reduce the chance of lap joint cracking. These changes reduce the amount of bending.

Large cracks are rare, though they seem to be appearing with more frequency in recent years, including an incident involving a larger Boeing 757 last October in which a 1-foot by 2-foot hole opened up as the plane was flying at 31,000 feet. The F.A.A. issued an airworthiness directive in January mandating that airlines inspect their Boeing 757-200s and 300s, after it received several reports of cracking in the fuselage skin of roof panels.

In July 2009, another Southwest flight between Nashville and Baltimore, also a Boeing 737-300, experienced a rapid decompression when an 18-inch hole opened up while the plane was flying at 35,000 feet. The N.T.S.B., in its report on the incident, said the hole was caused by “continuous fatigue cracks initiated from multiple origins on the inner surface of the skin.” Those occurred near a step formed at the edge of aluminum panels that had been chemically milled.

Mr. Richter said this was fundamentally different from the latest incident on the Southwest flight. In the latest event, the cracks apparently formed inside the holes that fastened two pieces of metal together, he said.

Under an emergency directive issued by the Federal Aviation Administration on Tuesday, all of the planes with more than 35,000 flight cycles must be inspected within five days. Those with 30,000 to 35,000 cycles must be examined within 20 days, and Mr. Richter said that a total of 570 planes, including 737-300s, 400s and 500s built from 1993 and 2000 — could eventually require the inspections as they reached those milestones.

Christine Hauser contributed reporting.

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