FORT WORTH, Texas — Federal authorities are requiring airlines to check more Boeing 737 airplanes for metal fatigue in the wake of Friday’s incident, in which a 5-foot hole opened in a fuselage of a Southwest Airlines jet in midair.

The Federal Aviation Administration said it will issue an emergency directive on Tuesday, requiring checks of older Boeing 737-300, 737-400 and 737-500 models that have more than 30,000 flight cycles.

The directive will initially apply to about 175 aircraft worldwide, including 80 in the U.S. The agency said most of the aircraft in the U.S. are operated by Southwest Airlines.

“”This action is designed to detect cracking in a specific part of the aircraft that cannot be spotted with visual inspection,”” said Randy Babbitt, FAA administrator.

Southwest said it believes it has already complied with the directive, since it grounded 79 aircraft over the weekend for inspections following the emergency landing of Southwest Flight 812 in Yuma, Ariz.

The Dallas-based carrier added that the 737-500s mentioned in the directive do not include the 25 Boeing 737-500s in Southwest’s fleet.

As of Monday, Southwest said it had found cracks on three other aircraft following inspections of its older Boeing 737-300s.

Southwest had returned 64 inspected aircraft into service by Monday, said Ashley Dillon, spokeswoman. The carrier had cancelled about 70 flights on Monday out of its schedule of 3,400 daily flights.

“”We are hoping to have all of the inspections completed by Tuesday,”” Dillon said with the carrier able to run a full schedule on Tuesday.

Three aircraft remain out of service as the carrier said there were “”small, subsurface cracks,”” found in those three aircraft. The three planes will be evaluated and potential repairs will be made before the aircraft is returned to service.

The carrier said it was using a “”non-destructive test in the form of Low-Frequency Eddy current of the aircraft skin,”” to detect any metal fatigue or cracks in the skin that are not visible to the eye.

According to its annual report filed in February, Southwest said it has 548 aircraft in its fleet, with more than half of its fleet Boeing 737-700s with an average age of seven years. The carrier also has 171 Boeing 737-300s with an average age of 19 years and 25 Boeing 737-500s with an average age of 20 years.

Aging aircraft and issues with metal fatigue rose to national attention in 1988 when an Aloha Airlines Boeing 737-200 lost an 18-foot section of its fuselage, killing one flight attendant. Since then aircraft manufacturers and the FAA have issued a number of airworthiness directives aimed at checking for fuselage cracks, said John Eakin, who owns Air Data Research in Helotes, near San Antonio.

“”It goes far beyond just the -300 or just the 737,”” Eakin said about metal fatigue. “”This is an issue that everyone is looking at on all makes and models.””

Last year, American Airlines performed additional inspections on 87 of its Boeing 757 aircraft after a one-foot-by-two-foot hole in an American Boeing 757-200 aircraft opened just above the passenger loading door near the front of the aircraft while the plane was at 31,000 feet.

And in 2009, a hole opened up mid-flight on a Southwest Boeing 737-300. Federal investigators concluded that metal fatigue contributed to the incident.

Since Southwest uses its aircraft to fly shorter routes, some industry analysts have speculated that the carrier’s planes endure more stress, making the occurrence of metal fatigue more likely.

However, aviation consultant Michael Boyd said Southwest has one of the youngest fleets in the industry, with the average age of its planes around 11 years old. The aircraft fleets at Delta Air Lines, United Airlines and American Airlines have an average age of 15 years old.

And since airworthiness directives require maintenance and inspections based on the number of landing and take-off cycles, it means that Southwest may be performing maintenance and inspections more frequently on its aircraft, he said.

“”If you land the plane five times in a day or 5 times in a month, it is the same wear and tear on the airplane,”” Boyd said.

The Southwest aircraft involved in Friday’s incident was about 15 years old and had around 40,000 cycles.

———

(c) 2011, Fort Worth Star-Telegram.

Visit the Star-Telegram on the World Wide Web at http://www.star-telegram.com.

Distributed by McClatchy-Tribune Information Services.

—————

GRAPHIC (from MCT Graphics, 202-383-6064): 20110404 SOUTHWEST 737