The Federal Aviation Administration issued an emergency directive yesterday that mandates operators of specific Boeing 737 aircraft to conduct repetitive electromagnetic inspections for fatigue damage. The mandate -- prompted by a Southwest Airlines incident last Friday in which a hole opened up in the main cabin's ceiling during flight -- applies to about 175 planes worldwide, 80 of which are registered in the United States. Southwest operates most of the affected aircraft. One minor physical injury to a flight attendant was sustained during last Friday's flight during its emergency descent. Southwest responded to Friday's incident by voluntarily grounding 79 planes for inspection, and the process was completed yesterday morning. Five of the inspected planes revealed minor subsurface cracks and are being held for repair; the other aircraft have been returned to normal service. According to a statement, the carrier expects that its voluntary, preemptive inspections will meet the FAA directive issued yesterday. The airline returned to its normal flight schedule yesterday. "Last Friday's incident was very serious," said transportation secretary Ray LaHood in a statement, "and could result in additional action depending on the outcome of the investigation." According to the FAA, the only other U.S. carrier that operates any aircraft affected by the mandate is Alaska Airlines, which has a few of the 737s. The FAA has no specific information about international carriers that operate those planes.