The Federal Aviation Administration plans to order immediate inspections of older-model Boeing 737 jetliners that are the workhorse of domestic air travel, a precaution after a hole opened in the hull of a Southwest Airlines plane flying at 34,400 feet on Friday.

There are more than 1,200 Boeing 737s in use by American air carriers, providing for much of the nation’s intercity air travel. The inspection order will apply to about 80 planes that were delivered before 1990, the FAA said.

Most of those planes — in the -300/-400/-500 series — are operated by Southwest, the FAA said.

Southwest grounded scores of planes after the incident, causing hundreds of flight cancellations. Subsurface cracks were discovered after Friday’s incident in three other Southwest 737-300s. By late Monday, the airline said 90 percent of its 79 737-300s had been inspected. Inspections of the rest were scheduled to be completed by Tuesday.

The FAA ordered inspections of planes from those three series — years of production that Boeing calls “the classics” — that have accumulated more than 30,000 takeoff and landing cycles.

Initial inspections will use electromagnetic, or eddy-current, technology in specific areas of the fuselage, the FAA said. Regular follow-up inspections will be required.

After a 5-by-1-foot hole opened Friday in the plane’s hull, the pilot made a controlled descent to 11,000 feet and landed safely at the Yuma Marine Corps Air Station in Arizona.

Boeing joined investigators from the National Transportation Safety Board in examining the “lap joint” where the hole appeared. The aircraft company was preparing an advisory to airlines directing that those joints on older 737s undergo testing.

NTSB board member Robert L. Sumwalt flew to Yuma to oversee the investigation. Inspections on the Southwest plane were up-to-date, he said. The section of the fuselage where the hole appeared was removed and sent to NTSB laboratories in Washington for additional testing, he added.

“This accident indicates that maybe the way we handle older aircraft needs to be examined,” Sumwalt said in an interview. “And that’s why Boeing is going to issue its advisory and the FAA will follow up by making that a directive.”

Sumwalt said that metal fatigue in the flight cycle is caused more by repeated pressurization and depressurization of the airplane’s interior than the physical impact of landing and takeoff.

“That’s no different than you taking a paper clip and bending it back and forth until it breaks,” he said. “That’s why we build in redundancies and require regular inspections of aircraft.”

All of Southwest’s 548 planes are models of the 737, and they accumulate flight cycles more rapidly than some other carriers as they shuttle among several cities each day.

By contrast, the other two U.S. carriers with sizable 737 fleets — American Airlines and United-Continental Airlines — generally use those planes on longer flights and accumulate flight cycles more slowly.

All 152 of American’s 737s are later-model 800s, according to spokesman Ned Raynolds. Mary Clark, spokeswoman for United-Continental, said the recently merged air carrier had 32 737-500s, all with fewer than 25,000 flight cycles.

“The FAA has comprehensive programs in place to protect commercial aircraft from structural damage as they age,” FAA Administrator J. Randolph Babbitt said in a statement. “This action is designed to detect cracking in a specific part of the aircraft that cannot be spotted with visual inspection.”

The FAA will then require repetitive inspections at regular intervals.

Investigators blamed metal fatigue when a Southwest flight bound from Nashville to Baltimore-Washington International Marshall Airport lost a chunk of its skin in July 2009 and made an emergency landing in West Virginia.

Last year, the FAA required aircraft manufacturers to establish the number of flight cycles or hours a plane can operate before metal fatigue becomes an issue. The agency required aircraft manufacturers to incorporate the limits into their maintenance programs.