The Federal Aviation Administration will issue a safety directive Tuesday requiring that older Boeing 737s be checked for the type of cracking that caused a Southwest Airlines flight to make an emergency landing in Arizona last week.
FAA officials said the order would affect about 175 aircraft worldwide, including 80 in the United States, that have accumulated more than 30,000 flight cycles, which stands for a takeoff and a landing.
Most of the aircraft in the United States are operated by Southwest Airlines, which has been conducting its own inspections since a fuselage rupture left a 5-feet by 1-foot hole in a plan that had just left Phoenix en route to Sacramento.
"Safety is our No. 1 one priority," said Transportation Secretary Ray LaHood. "Last Friday's incident was very serious and could result in additional action depending on the outcome of the investigation."
Flight 812 was about 18 minutes into its trip when the section of the fuselage skin burst open. On board, the 118 passengers scrambled for oxygen masks after the cabin lost pressure. One passenger used her Twitter account to send out photos of the hole and the scene inside the aircraft. A flight attendant sustained minor injuries.
Since the incident, inspections uncovered small cracks on other Southwest planes and the airline has canceled hundreds of flights.
National Transportation Safety Board member Robert Sumwalt said Sunday that an inspection of the Flight 812 aircraft found "persistent fatigue along the entire fracture surface."
The cracks would not be immediately apparent from a visual inspection, a fact that prompted air safety officials to question whether new inspection criteria may be needed for the 737s and other aircraft. Sumwalt, who was in Yuma, Ariz., with a team that included investigators from the NTSB, Federal Aviation Administration and the Boeing Co., cautioned Sunday that there were "no existing criteria to perform inspections for this type of failure."
Still, analysts said the incident was not as severe compared to other recent safety scares involving other airlines. Southwest canceled about 300 flights each day over the weekend -- or about 9% all daily flights -- to conduct inspections.
American Airlines, for example, grounded nearly half its fleet in 2008 and canceled 3,300 flights because of missed inspections.
The action involved the airline's oldest model, the Boeing 737-300s, which represent about 30% of the airline's fleet of 548.
By Monday, the airline said it had discovered cracks in three other planes and was making repairs. Southwest said it was putting aircraft in which no defects were found back into service. The airline said it expects to complete all inspections and return to normal operations by Tuesday.
The crack in the fuselage also drew attention to the way low-cost airlines like Southwest, JetBlue Airways and others put heavy use on older planes to keep costs and fares low.
Southwest typically puts more hours and more flights on the 737-300s than most other airlines in the industry, said Scott Hamilton, an aviation industry consultant in Issaquah, Wash.
"As long as the aircraft is getting routine maintenance work, it should operate just fine," he said. "A 15-year-old aircraft is a middle-aged by aviation standards."
The 737-300 is predominantly flown on shorter domestic routes, and is extremely popular among carriers around the world, Hamilton said.
Of the 1,113 737-300 planes built by Boeing, about 900 are still in service, according to the Fairfax, Va.-based aerospace research firm Teal Group Corp.
"This is a common aircraft type," said Richard Aboulafia, aviation analyst at Teal. "Southwest works them hard, but it is a robust aircraft. They've been built for this sort of job."
Times staff writers W. J. Hennigan, Catherine Saillant and Carla Rivera contributed to this report