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The Federal Aviation Administration announced Monday that it would require extensive inspections of some older-model Boeing 737s for cracks in the planes’ fragile skin that can be caused by pressurization and depressurization of the cabin over tens of thousands of takeoffs and landings.

Three days earlier, undetected cracks widened into a 5-foot hole in the roof of a Southwest Airlines flight, forcing the 737-300 to make an emergency landing at a military base. The FAA announcement applies to 175 aircraft worldwide, including 80 based in the United States. Most are operated by Southwest, which started inspections over the weekend and has found three more planes with small cracks.

The airline also canceled 70 flights from its schedule of 3,400 departures Monday. About 300 flights were canceled both Saturday and Sunday.

Southwest insisted that it had done all the required inspections of its aircraft. But the latest incident focused attention on how the carrier uses its planes on up to 12 flight segments a day. Other airlines, which often fly longer routes, typically have six to eight segments for their planes. The plane involved in the incident Friday had logged 39,000 takeoffs and landings, a relatively high number for a 15-year-old aircraft.

The FAA directive focused on planes that had accumulated a large number of takeoff and landing cycles. It applies not only to the 737-300 model but also to similar 737-400 and 500 models, a design that dates to the early 1980s and is known as the 737 Classic series.

“This was a very serious failure,” said William R. Voss, president and chief executive of the Flight Safety Foundation, an independent nonprofit group. “Is there something wrong with the inspections we’ve been using in the past 20 years, or was there something wrong on the inspection with this one plane?”

The FAA directive is intended to detect cracks in places where the skins overlap and other structural weaknesses, J. Randolph Babbitt, the FAA administrator, said in a statement. The directive came shortly after Boeing, the manufacturer, said it was also recommending that airlines inspect the areas where the skin covers joints on the older 737 models.

The National Transportation Safety Board, which is conducting an investigation into Friday’s incident, said that it was probably the result of fatigue cracks in these joints. Southwest began flying the plane, which carried 118 passengers, in 1996; it is among the oldest in its fleet.