Boeing Co. urged airlines operating some of its most-flown models of its best-selling 737 passenger jet to check for metal fatigue after a cabin hole caused a Southwest Airlines safety scare.

The recommendation in a service bulletin to airlines came after a gaping tear in the fuselage of a Southwest Airlines 737-300 forced an emergency landing in Arizona on April 1.

The checks, which are expected to be made mandatory by a Federal Aviation Administration order, apply to older versions of the 737 such as the 737-300 which have carried out at least 30,000 "cycles", Paul Richter, chief project engineer for 737 Classic at Boeing, told a media briefing on Tuesday.

A cycle is one take-off and landing. It is considered the source of most wear and tear on a plane's structure because of stresses like cabin pressurization.

Southwest said the aircraft performed more than 39,000 cycles. 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.

Boeing believed there was nothing about how Southwest operates planes that may have contributed to the April 1 incident, Richter said. Flight 812 was heading from Phoenix to Sacramento, California, when a 5-foot (1.52 meters) tear opened up 20 minutes after takeoff.

"The reality is they (Southwest) operate a large fleet of 737s ... and certainly the largest 737 classic fleet," Richter said. "It's just a statistical event far more than it has anything to do with Southwest and how they operate the airplane," he added.

Southwest said on Tuesday inspections on more than 70 older 737-300 planes found that five had fuselage cracks requiring repairs, and added that flight operations were returning to normal.

The discount carrier grounded 79 planes and cancelled more than 650 flights in recent days as it checked jets for fuselage cracks and fatigue following the April 1 incident.

"We're operating on a full schedule today," spokeswoman Whitney Eichinger said.