The US Federal Aviation Administration on 23 July will publish a final airworthiness directive (AD) requiring US operators of approximately 46 Boeing 777-200 and -300 series widebodies to perform repetitive inspections for cracks in the fuselage skin lap splices at several locations on the aircraft.
The AD resulted from reports of fatigue cracks in certain lap joints and inspection are directed at Sections 41, 43 and 44 of the aircraft, says the FAA, noting that Boeing issued a service bulletin to address the potential problem in November 2011.
Cracks were found to have started at scribe lines that were made during production "when maskant was removed from the affected skin panels during the chemical milling process," says the FAA.
"We are issuing this AD to detect and correct such fatigue cracking, which could grow large and cause sudden decompression and the inability to sustain limit flight and pressure loads."
The final AD calls for performing initial and repetitive inspections using external phased-array ultrasonic devices, though Boeing and the FAA have not stated publicly when the initial inspection must be completed. The FAA does say however that damaged joints must be repaired before further flights and inspections on the joints must be performed at least every 4,200 flight cycles thereafter. The directive was first published as a proposed AD in February.
FAA last year issued a similar AD for Boeing 737NG models, requiring inspections to look for fatigue cracks related to scribe line damage adjacent to the skin lap joints, decals, and wing-to-body fairings.