Southwest Airlines Co. said today that it has completed inspections of its older Boeing 737s and found small cracks in five passenger planes, The New York Times reported.

The airline conducted the inspections after a hole tore open the roof of one of its 737s on April 1 during a flight from Phoenix to Sacramento, forcing the airplane to make an emergency landing at a military base.

The incident prompted the Federal Aviation Administration to say that it would issue a directive requiring all airlines to inspect some older-model Boeing 737s for cracks in the skin that can be caused by pressurization and depressurization of the cabin over thousands of takeoffs and landings. Boeing Co., the manufacturer, said it was recommending that airlines inspect the areas where the skin covers joints on the older 737 models.

Southwest canceled about 670 flights over the weekend and on Monday while inspecting nearly 80 aircraft. A company spokeswoman said the airline has resumed full scheduling of its 3,400 daily flights.

Last week’s incident was at least the third involving metal fatigue in the past few years. The others involved another Southwest 737-300 flight in 2009 and an American Airlines Boeing 757 last year.

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