Occurrence
On September 20, 2007, a Boeing 737-800 aircraft, registration B-16805, operated by China Airlines as flight CI7552, landed at Saga Airport, Japan, at 13:26 local time. The flight originated from Taoyuan International Airport, Taiwan, and proceeded without incident. During the post-flight transit check, a mechanic discovered a 30-inch (77 cm) through crack in the lower belly fuselage skin, located below the aft cargo door. The aircraft was subsequently grounded and the return flight was canceled after communication with the Taipei maintenance base.
Investigation Findings
The investigation, conducted by the Aviation Safety Council (ASC) of Taiwan under delegation from Japan, identified two key findings related to the probable cause of the crack. First, the plastic waste tank outlet flanges were unable to withstand complex stresses arising from the installation of coupling tubes in the waste water system. Second, consistent leakage of waste tank fluid accumulated in the lower affected area. As water evaporated, the concentration of chlorine increased, inducing corrosion that degraded the fuselage skin. The residual strength of the skin became insufficient to endure the hoop-wise stress from flight operations, leading to an overstress fracture resulting in the 30-inch crack.
Maintenance and Inspection Issues
The report noted that the aircraft's structural inspection program, based on FAA MRBR and Boeing MPD, required removal of insulation blankets to detect structural failure, but this was not mandated until the aircraft reached eight years of service. Since the aircraft was newer, corrosion could not be detected early. Zonal inspections were performed without removing insulation blankets, so existing damage may have gone unnoticed. Specifically, inspection tasks for the waste tank compartment (zone 141) and the area below the aft cargo compartment (zone 143) were conducted separately, and leakage from zone 141 accumulated in zone 143, where corrosion occurred. The inspection program did not require blanket removal during general visual inspections, preventing detection of the structural abnormality.
Additional Observations
Other findings included that an installation quality check of the 737-800 fleet revealed unmatched centerlines between waste tank outlets and adjacent tubes. The damaged flanges showed no definite failure modes, indicating multiple stress factors. Some gap dimensions met a "clamp installation" criterion but not Boeing's specification, though no evidence linked this to the flange cracks. Chemical analysis confirmed that leaked waste tank fluid was the primary corrosive agent. The belly skin panel had been chemically milled during manufacturing, removing the pure aluminum cladding and reducing corrosion resistance despite protective coatings. No abnormal maintenance records were found, and scheduled zonal inspections were completed on time. The flight crew were properly certified. The occurrence was unrelated to flight operations or weather. Ground video showed no evidence of ramp damage at Taoyuan.
Recommendations
The ASC issued an interim flight safety bulletin and made recommendations to China Airlines, including performing immediate structural inspection of zone 143 if stains are found in zone 141, and applying direct visual inspection of the skin structure on the lower surface. The report also urged review of maintenance policies to prevent recurrence.