During the inspection of China Air Lines flight CI7752, a Boeing 737-800, registration B-16805, a 77 cm crack in the fuselage skin was found below the aft cargo door during a transit check after landing at Saga Airport, Japan. The flight had originated in Taoyuan International Airport, Taipei, Taiwan. Findings Related to Probable Causes 1. The plastic waste tank outlet flanges could not resist complex stresses resulting from the installation of coupling tubes of waste water system. 2. The consistence leakage of waste tank fluid was trapped in the lower level of affected area, and the concentration of Chlorine was increased by evaporating of water. It induced corrosion to the detriment of the fuselage skin. The residual strength of the skin was not of sufficient to endure the hoop-wise stress resulted from flight operation. Finally the fuselage skin fractured to a 30 in (77 cm) crack due to the overstress.