What happened
During a maintenance or inspection period, damage was identified on the aft upper side of the aircraft fuselage, located forward of the inlet for the #2 engine. The fuselage skin had "flapped" open between body stations 1090 and 1110, peeling downward from stringer #4 left to stringer #5 left.
The investigation
Investigators examined the area of damage and discovered a longitudinal crack along stringer #4. This crack was co-located with a scratch or scribe mark on the metal surface. The investigation determined that the crack had initiated at multiple points along this score mark.
Further analysis of the material showed that the crack had progressed through the thickness of the skin due to fatigue. By performing a striation count, investigators estimated that between 3,360 and 5,040 cycles of fatigue progression had occurred. The source of the initial damage was traced back to the use of marking tools during a prior repair process, which created the score mark that served as the crack's origin.