The most urgent Federal Aviation Administration (FAA) requirement covers about 175 of the most heavily used planes worldwide.
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Checks of those jets must be completed within five days of the airline receiving the order and repeated before they total another 500 takeoffs and landings.
Eventually, the agency wants checks on more of the "classic" 737-300, 400 and 500 planes to undergo the time-consuming electromagnetic metal fatigue inspections before they reach a certain number of takeoffs and landings.
The inspections aim to detect cracks along joints in the fuselage that bond layers of aircraft skin which cannot be seen visually.
Paul Richter, chief project engineer for 737 "classic" at Boeing, told a media briefing on Tuesday the fatigue cracks suspected in the Southwest rupture occurred sooner than Boeing expected in the life of the plane.
Boeing issued a bulletin to its operators outlining the inspection requirements that were narrower and less stringent than the FAA directive.
The aircraft involved in the April 1 event performed more than 39,000 takeoffs and landings, Southwest said. An industry source with knowledge of the Boeing bulletin said that total was higher than average for a plane of its age -- around 15 years.
Flight 812 was heading from Phoenix to Sacramento, California, when a 5-foot (1.52 meters) tear opened up 20 minutes after takeoff along the roof just above the left wing.
The most demanding part of the FAA order was based on an inspection regimen launched by Southwest after last week's incident and completed on Tuesday.
Southwest grounded and inspected more than 70 planes over four days, discovering cracks in five that require repairs, the carrier said.
Boeing believed there was nothing about how Southwest operates planes that may have contributed to the April 1 incident, Richter said.
"It's just a statistical event far more than it has anything to do with Southwest and how they operate the airplane," Richter said.
Safety experts have questioned whether the aggressive way in which Southwest uses its planes -- quick turnarounds and short hops -- may accelerate wear.
Southwest returned to a normal schedule on Tuesday after canceling more than 650 flights over the weekend and on Monday.
Southwest shares fell 26 cents, or 2.1 percent, to close at $12.20, and Boeing was off 72 cents, or about 1 percent, at $72.23.