Southwest Airlines grounded a portion of its fleet on Saturday after a passenger jet with a hole in its fuselage was forced to make an emergency landing at an Arizona military base on Friday.

The airline grounded about 80 of its planes for inspection, which meant about 300 flights had to be cancelled on Saturday.

Southwest Flight 812 from Phoenix was nearly an hour into the trip to Sacramento, Calif., when the Boeing 737-300 lost cabin pressure.

Oxygen masks dropped in front of all the passengers. One passenger described the scene as "pandemonium." Another watched as a flight attendant and another passenger passed out, apparently for lack of oxygen, their heads striking seats in front of them.

The airline said the flight was immediately diverted to Arizona's Yuma Marine Corps Air Station. Federal Aviation Administration spokesman Ian Gregor said the pilot made a "controlled descent from 36,000 feet to 11,000 feet altitude."

After landing, the crew discovered a gaping hole in the top of the aircraft.

Southwest dispatched another jet to carry the 118 passengers to Sacramento.

Passengers could see the hole and feel the rush of air during the flight after they heard a loud bang.

Southwest operates about 170 of the 737-300 planes. It has replaced the aluminum skin on many of those planes in recent years, but the ones that were grounded Saturday have not had their skin replaced, Southwest Airlines spokeswoman Linda Rutherford said.

"Obviously we're dealing with a skin issue, and we believe that these 80 airplanes are covered by a set of [federal safety rules] that make them candidates to do this additional inspection that Boeing is devising for us," Rutherford said.

Julie O'Donnell, an aviation safety spokeswoman for Chicago-based Boeing Commercial Airplanes, confirmed "a hole in the fuselage and a depressurization event" in the latest incident but declined to speculate on what caused it.

The National Transportation Safety Board said an "in-flight fuselage rupture" led to the drop in cabin pressure. A similar incident on a Southwest plane to Baltimore in July 2009 also forced an emergency landing when a 30-centimetre-long hole opened in the cabin.

The airline, the FAA, and the NTSB are all now investigating the latest incident.