Southwest Airlines has grounded 79 (not 81 as was previously reported) of its Boeing 737s as it began working with Boeing and the NTSB to determine the cause of a “depressurization event” during a flight from Phoenix to Sacramento on Friday afternoon. The airplane diverted to Yuma, Ariz., for an emergency landing at 5:07 p.m. local time after a hole developed in the top of fuselage. The airline said it has begun an “aggressive inspection effort” with Boeing engineers.

“The safety of our customers and employees is our primary concern, and we are grateful there were no serious injuries,” said Mike Van de Ven, Southwest’s executive vice president and chief operating officer. “We have launched personnel to Yuma to begin the investigation process with the NTSB, FAA, and appropriate parties to determine the cause of the depressurization.” Southwest expected to cancel some 300 flights today to accommodate the inspections.

Flight 812 carried 118 passengers and five Phoenix-based crew members when  the aircraft lost pressure and oxygen masks deployed. After the airplane landed safely in Yuma, the crew confirmed the existence of a three-foot-wide hole in the top of the aircraft, located near the middle of the fuselage. One flight attendant received medical treatment at the scene for a minor injury, as did at least one passenger. No injuries required transport to the hospital. The Company Southwest arranged for a Southwest company Airlines aircraft to transport the Customers passengers from Yuma to Sacramento last night.

Southwest said engineers from Boeing helped the airline assess the damage to the aircraft and develop an inspection plan to look more closely at 79 aircraft that are covered by a set of FAA Airworthiness Directives defining inspections for aircraft skin fatigue. These aircraft will be inspected over the next several days at five locations. said set of airworthiness directives aimed at inspections for aircraft skin fatigue cover the 81 airplanes due to undergo inspection over the course of the next several days.