The US Federal Aviation Administration is readying an emergency airworthiness directive mandating fuselage lap-joint inspection for approximately 175 737 Classic aircraft, in the wake of the 1 April decompression event caused by a fuselage structural failure aboard a Southwest 737-300, forcing it to land in Yuma, Arizona.
The FAA says it will require operators of early Boeing 737-300, -400 and -500 aircraft to conduct "initial and repetitive electromagnetic inspections for fatigue damage".
The action will "initially apply" to approximately 175 of the roughly 1,800 737 Classics worldwide, though the FAA only has regulatory jurisdiction of 80 US-registered aircraft, most of which the regulator says are operated by Southwest Airlines.
According to Flightglobal's ACAS database, in addition to Southwest, US Airways operates 59 737-300s and -400s and Continental Airlines operates 32 737-500s, though it appears only a fraction of both fleets would be impacted by the airworthiness directive, if at all.
US Transportation Secretary Ray LaHood says the 1 April Southwest "incident was very serious and could result in additional action depending on the outcome of the investigation".
Southwest says with its knowledge of what the FAA has planned, "we believe the 79 aircraft already identified for inspection will accomplish this directive", for the airline. The carrier adds none of its -500 series aircraft will fall under the airworthiness directive.
The FAA emergency airworthiness directive "will require initial inspections using electromagnetic, or eddy-current, technology in specific areas of the aircraft fuselage" on certain -300, -400 and -500 series aircraft that have flown more than 30,000 flight cycles.
The FAA adds that repetitive inspections will be required at regular intervals.
Boeing has not yet disclosed what distinguishes these 737 Classics from newer models, though a statement by Southwest indicates fatigue life is not the only guide for which aircraft require inspection, with the carrier saying the 79 aircraft under scrutiny were "designed differently in the manufacturing process."
As of 22:00 GMT on 4 April, Southwest had inspected and returned to service 64 of 79 Boeing 737-300 aircraft, while three were being held for repairs after additional cracks were found. Those aircraft, says Southwest, will remain out of service until Boeing "recommends an appropriate repair".
Further, Boeing is preparing a service bulletin of its own to cover the operators both inside and outside the US.