Southwest Airlines canceled 300 flights Sunday as it began inspecting 79 of its Boeing 737-300 passenger jets.

What Southwest says will be “an aggressive inspection effort in cooperation with Boeing engineers” follows an emergency Friday when a flight from Phoenix to Sacramento, Calif., had to make an emergency landing at the Marine Corps Air Station in Yuma, Arizona.

Shortly after Flight 812 had reached an altitude of 36,000 feet, a five-foot section of the overhead fuselage ripped open, depressurizing the cabin and slightly injuring a flight attendant and one passenger.

Inspectors will be looking for evidence of “aircraft skin fatigue” in the grounded jetliners. Flight data recordings and maintenance records for the damaged aircraft will be inspected as well.

"Our mission is to determine not only what happened, but figure out why it happened, so that we can prevent things like this from happening in the future," National Transportation Safety Board (NTSB) member Robert Sumwalt told reporters Saturday. “If we find deficiencies that need addressing… we can issue an urgent safety recommendation at any point during the investigation.”

According to the Federal Aviation Administration (FAA), the US fleet includes 288 Boeing 737-300s (about 170 of them flown by Southwest), and 931 operate worldwide.

The first 737-700 flew in 1984, and Boeing built more than 1,000 of the aircraft until 1999. Since then, the aircraft – which Southwest is gradually replacing with newer models – have undergone regular mandatory inspections, including the airframe and skin for any evidence of metal fatigue.

The aircraft in question is about 15 years old. Federal records show cracks in the airframe were found and repaired a year ago, according to the Associated Press.

FAA records of maintenance problems for the aircraft showed that in March 2010 at least eight instances were found of cracking in the aircraft frame, reports the AP. The records showed that those cracks were repaired.

According to Southwest, the airline over the years has replaced the skin on most of its 737-300s, which is the oldest aircraft in Southwest’s fleet. Those grounded for inspection this weekend had not yet had their skin replaced.

Friday’s incident was not the first.

In July, 2009, Southwest Flight 2294 flying from Nashville to Baltimore at 34,000 feet had a football-sized hole in its fuselage near the tail, which caused rapid decompression and a forced landing in Charleston, West Virginia. No one was injured. The NTSB determined that metal fatigue had been the cause.

According to the Dallas-based airline, Southwest’s fleet aircraft average 11 years old and fly six times a day for nearly 11 hours on relatively short trips averaging 648 miles and just under two hours.