U.S. FAA will issue an emergency directive Tuesday that will require operators of "specific" Boeing 737-300/400/500 series aircraft to conduct initial and repetitive electromagnetic inspections for fatigue damage following Friday's midair rupture of the fuselage skin on Southwest Airline Flight 812.

This action, which the agency announced Monday afternoon, will "initially apply to a total of approximately 175 aircraft worldwide, 80 of which are U.S.-registered aircraft," FAA said, noting that most of the U.S.-registered aircraft are operated by SWA.

Transportation Secretary Ray LaHood called the April 1 incident "very serious" adding that it "could result in additional action depending on the outcome of the investigation."

U.S. NTSB said Monday that three more SWA 737s were grounded after inspections revealed small, subsurface cracks similar to the ones suspected in the fuselage tear on another of its planes. In a press briefing Monday afternoon, NTSB Board Member Robert Sumwalt said the agency has concluded the on-scene part of the investigation and found that "all required aircraft inspections were up to date and no discrepancies were found." He went on to say there were "no outstanding maintenance items at the time of the accident" and that "all airworthiness directives were complied with and were up to date."

Sumwalt also said that engineers from Boeing are conducting nondestructive testing of both lap joints on the accident aircraft and are using the same techniques--dual-frequency Eddy current inspections--that will be specified in Boeing's upcoming service bulletin. Sumwalt noted the portion of the aircraft fuselage is en route to Washington so investigators can begin an in-depth analysis in the NTSB materials lab as early as Wednesday morning.

In a statement, Boeing told ATW that it will "continue to provide technical assistance to the U.S. National Transportation Safety Board as it investigates the incident. Boeing also is assisting Southwest Airlines in its own decision to perform additional inspections on a portion of its airplanes. Based on information from the NTSB investigation and Southwest's inspections, Boeing is preparing a service bulletin that will recommend lap-joint inspections on certain 737-300/400/500 airplanes. We also are working closely with the U.S. Federal Aviation Administration. This long-standing process, through which manufacturers, operators and regulators work together, helps ensure that the safety of the world's commercial jetliners continues at the highest levels."

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

The emergency AD "will require initial inspections using electromagnetic, or eddy-current, technology in specific areas of the aircraft fuselage on certain Boeing 737 Classics that have accumulated more than 30,000 flight cycles." It will then require repetitive inspections at regular intervals, it said.

The FAA said that last November, it 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 a plane can operate and be free from fatigue damage... The rule requires aircraft manufacturers to incorporate the limits into their maintenance programs," it said.