The FAA plans to issue an emergency airworthiness directive (AD) tomorrow that will require operators of specific early Boeing 737 models to conduct initial and repetitive electromagnetic inspections for fatigue damage, the agency announced this afternoon.

The action, prompted by the in-flight rupture of the fuselage a Southwest Airlines 737-300 on Friday and subsequent findings of fatigue cracking in three of the airline’s other Boeing 737s, will initially apply to a total of some 175 aircraft worldwide, 80 of them registered in the U.S. Southwest Airlines operates most of the aircraft covered by the order in the U.S.  

“Safety is our number 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.”

The FAA airworthiness directive will require initial inspections using electromagnetic, or eddy-current, technology in specific areas of the aircraft fuselage on certain Boeing 737-300s, -400s and -500s that have accumulated more than 30,000 flight cycles. It will then require repetitive inspections at regular intervals.

Mechanics from Southwest Airlines, under the supervision of NTSB investigators, yesterday removed a section of the ruptured fuselage skin and today sent it to NTSB headquarters in Washington, D.C. for in-depth analysis. Meanwhile, NTSB investigators conducted additional inspections of other portions of the lap joint along the fuselage of the accident airplane and found evidence of additional cracks.

“The FAA has comprehensive programs in place to protect commercial aircraft from structural damage as they age,” said FAA Administrator Randy Babbitt. “This action is designed to detect cracking in a specific part of the aircraft that cannot be spotted with visual inspection.”

Last November, the FAA published a rule designed specifically to address widespread fatigue damage in aging aircraft. The rule requires aircraft manufacturers to establish a number of flight cycles or hours an airplane can operate free from fatigue damage. The rule requires aircraft manufacturers to incorporate the limits into their maintenance programs.