FAA to order more inspections of 737s

The order comes after Southwest canceled nearly 700 flights nationwide — including 16 of the 130 flights in and out of Boston — in the three days since a 5-foot gash opened up in the fuselage of a Boeing 737-300 shortly after takeoff from Phoenix on Friday.

Southwest, which has since found cracks in three additional planes, said it expects to complete inspections today of 79 Boeing 737-300s — about half its fleet of that aircraft — that it grounded after the incident.

“This is a completely new and unknown issue,’’ Southwest spokesman Chris Mainz said of the plane that was damaged last week.

But this isn’t the first incident involving a Southwest 737-300. In July 2009, a flight from Nashville to Baltimore made an emergency landing after a hole opened up in the top of the plane. In 2008, the FAA fined Southwest $10.2 million, later reduced to $7.5 million, for flying 46 jets overdue for fuselage inspections.

No one was seriously injured in Friday’s incident, but several passengers waiting for Southwest flights at Logan yesterday expressed concern about the damaged planes.

“I’m not one that enjoys flying as it is, so this adds to the anxiety,’’ said Mareen Johnson, 61, who was flying to Chicago with her husband.

Others were able to put it in perspective. “I feel like it’s still a lot safer than driving,’’ said Mary Farbman, 34, a chemist who was flying back to Washington, D.C., with her husband and 1-year-old daughter.

A 5-foot gash opening up midflight is rare, aviation analysts say, but it’s crucial that Southwest and Boeing get to the bottom of it.

“These planes are largely US airplanes,’’ said Andy Golub, head of risk advisory for Ascend Worldwide Ltd., an aviation research and consulting firm. “As this aircraft continues to fly around the world, this will really set the standard for required maintenance going forward.’’

Planes are being flown more than they used to as airlines try to save money, Golub said. The average lifespan of a jet is 25 years, he said; the Southwest 737-300 with the gash in the roof was 15 years old.

“Since the financial crisis, what we’ve seen is an increase in utilization,’’ he said. “Metal fatigue is a real issue.’’

The 737, various models of which have been sold since the 1966, is the most widely used jetliner in the world. Boeing produced more than 1,100 of the 737-300s from 1984 to 1999. About 760 remain in use, including 195 in the United States, according to Ascend Worldwide.

The FAA’s directive will require operators of early Boeing 737s, including the -400 and -500 series, as well as the -300, to conduct electromagnetic inspections. Of the 175 Boeings that the FAA is ordering inspections for, about 80 are registered in the United States, the majority of them operated by Southwest.

Southwest spokesman Mainz said he did not anticipate that the airline would have to inspect more aircraft than it had already grounded.

Southwest is the biggest operator worldwide of the Boeing 737-300s, which account for 169 of the airline’s 550 planes in service. Lufthansa, the second biggest carrier of 737-300s, said it has no plans to ground its 33 737-300s. US Airways, the only other major US carrier that flies the 737-300s, has 18 in service and also said that it has no plans to pull them for inspection because they have a different configuration, according to a spokeswoman.

Aviation analyst Michael Boyd doesn’t expect the incident to affect Southwest’s market share or reputation. “It could be that somebody turned a screw the wrong way,’’ he said. “This will pass.

Katie Johnston Chase can be reached at johnstonchase@globe.com. Allison Knothe can be reached at aknothe@globe.com.