Southwest Airlines' older aircraft plus its famously efficient short-haul operations — requiring planes to fly an average of six times a day — probably contributed to the fuselage rupture that forced a jetliner carrying 118 passengers to make an emergency landing in Arizona last week.
Aviation experts said the aluminum skin of the 15-year-old Boeing 737-300 could have become fatigued from the stress of daily landings and takeoffs as well as frequent changes in cabin pressure.
According to airline officials, some Southwest planes fly even more often as the low-cost carrier hustles to perform in an industry battered by high fuel prices and cutthroat competition.
Late Monday, Federal Aviation Administration officials said the agency would issue an emergency directive requiring inspections of older Boeing 737-300s. The order will apply to about 175 aircraft worldwide, including 80 in the United States, that have at least 30,000 cycles (takeoffs plus landings.)
"The Boeing 737 is very reliable and efficient, but this one is an aging aircraft," said Michel Merluzeau, an expert in commercial and military aviation at G2 Solutions, a consulting firm based in Kirkland, Wash. "In 15 years, it could have 30,000 to 35,000 cycles. It could be making 2,200 flights a year. Maybe more than that."
So far, Southwest Airlines has inspected 57 older planes in its fleet of 548 Boeing 737s and released them back into service, said Chris Mainz, a company spokesman. Three other aircraft will be evaluated further after small cracks were found, he added.
Based on those inspections and the National Transportation Safety Board's ongoing investigation, Boeing officials announced Monday that the company was preparing a service bulletin that would recommend the inspection of lap joints — where sheets of aluminum are riveted together — on certain 737s.
The day's developments also drew comments from Secretary of Transportation Ray LaHood. "Safety is our No. 1 priority," LaHood said. "Last Friday's incident was very serious and could result in additional action depending on the outcome of the investigation."
Since the Arizona incident, Southwest has had to cancel about 670 flights, including 70 on Monday, and make other travel arrangements for its customers. The airline has more than 3,000 flights per day systemwide.
Mainz said that safety is the top priority for Southwest, which has an ongoing program to replace its older aircraft.
The transportation safety board will probably take six months to a year to determine what caused the fuselage gash to open in the plane just after Flight 812 took off from Phoenix and triggered a loss of cabin pressure. The incident caused the oxygen masks to automatically deploy throughout the cabin and cockpit.
The aircraft, which was on its way to Sacramento, made an emergency landing about 4 p.m. Friday at a military base in Yuma, Ariz. A flight attendant and a passenger received minor injuries.
Inspectors later found cracking around rivet holes in two areas of the upper fuselage that overlapped. The section, which measured 5 feet by 1 foot, opened up about two rows behind an emergency exit over the wing.
Jim Hall, a safety consultant and former chairman of the transportation safety board, said that short- and medium-haul aircraft that make many flights a day are more susceptible to metal fatigue than long-haul jetliners that fly one daily trip.
He said the constant pressurizing and depressurizing of aircraft cabins can flex the fuselage skin and lead to cracking. "It's like blowing up a balloon and letting the air out over and over," he noted.
Hall questioned the adequacy of the FAA's current inspection process and Southwest's maintenance programs for older commercial aircraft. He cited two earlier incidents involving cracks found in Boeing 737s operated by the airline.
In March 2009, the airline agreed to pay $7.5 million in penalties after the FAA accused the carrier of operating 47 aircraft in 2007 that had not been properly inspected for metal fatigue. Cracks had been discovered in about half a dozen aircraft.
In July 2009, another Southwest flight made an emergency landing in West Virginia after a football-sized hole opened in the fuselage. The transportation safety board concluded that metal fatigue caused that incident.
"In this situation, we have had three strikes," Hall said. "It is obvious that the inspection and oversight process is going to have to be a very careful subject of this investigation. It is extremely important that the FAA and aircraft manufacturers increase the frequency of inspections of these aircraft."
Hall also said he was concerned that Southwest was outsourcing some of its maintenance to aviation companies in Latin America. Questions have been raised over the years about the quality of the work done by foreign mechanics on commercial planes operated in the U.S. and elsewhere.
Airline spokesman Mainz said Southwest has a strong maintenance program that is certified and overseen by the FAA. "We hold our vendors up to the highest standards," he said.