Phoenix ? Federal aviation officials readied an order Monday for emergency inspections on 80 U.S.-registered Boeing 737 jetliners like the one on which a piece of fuselage tore open more than 30,000 feet above Arizona last week.

The order, to be issued today, is aimed at finding weaknesses in the metal in the fuselage, but virtually all of the affected aircraft will have already been inspected by the time the order takes effect.

A 5-foot-long hole opened up in the roof of the Southwest Airlines plane soon after takeoff Friday from Phoenix, causing a loss of pressure and forcing pilots to make an emergency landing 125 miles to the southwest in Yuma, Ariz. No one was seriously hurt.

The safety directive applies to about 175 aircraft worldwide, including 80 planes registered in the U.S., the Federal Aviation Administration said. Of those 80, nearly all are operated by Southwest. Two belong to Alaska Airlines.

After the midair incident, Southwest grounded nearly 80 Boeing 737-300s for inspections. By Monday evening, 64 were cleared to return to the skies, but three were found with cracks similar to those found on the Arizona plane.

Friday’s incident, however, raised questions about the impact that frequent takeoffs and landings by short-haul carriers like Southwest put on their aluminum-skinned aircraft and the adequacy of the inspections.

Cracks can develop from the constant cycle of pressurizing the cabin for flight, then releasing the pressure upon landing.

Since there had been no previous accidents or major incidents involving metal fatigue in the middle part of the fuselage, Boeing maintenance procedures called only for airlines to perform a visual inspection.

But airlines, manufacturers and federal regulators have known since at least 1988 that planes can suffer microscopic fractures. That year, an 18-foot section of the upper cabin of an Aloha Airlines 737-200 peeled away in flight, sucking out a flight attendant.

The order is “certainly a step in the right direction,” said National Transportation Safety Board member Robert Sumwalt, who is in Yuma with the board’s accident investigation team.

The FAA’s emergency order will require initial inspections using electromagnetic devices on some Boeing 737 aircraft in the -300, -400 and -500 series that have accumulated more than 30,000 takeoffs and landings. It will require repetitive inspections at regular intervals.

Southwest’s jet was 15 years old and had logged 39,000 pressurization cycles, a measurement of the number of takeoffs and landings. That’s 7.2 cycles every day for every year it has been in service.