What happened
On September 20, 2007, a China Airlines Boeing 737-800, registration B-16805, operated flight CI7552 from Taoyuan International Airport, Taiwan, to Saga Airport, Japan. The flight proceeded normally with no experienced turbulence. Upon arrival at Saga Airport at 13:26 local time, a mechanic performing a transit check discovered a 30-inch (7 .7 cm) through-crack in the fuselage skin. The crack was located on the lower belly, specifically below the area of the after cargo door. Following the discovery, the operator communicated with the maintenance base in Taipei and cancelled the scheduled return flight.
The investigation
The investigation, conducted by the Taiwan Transportation Safety Board (TTSB), focused on determining the origin of the structural failure. Investigators examined the waste water system, specifically the outlet flanges of the plastic waste tank. The investigation involved analyzing the chemical composition of the leaked fluid, which was found to contain high concentrations of Chlorine. Metallurgical examinations revealed that the corrosion had consumed approximately 94% of the skin thickness in certain areas, reducing the original 0.063-inch thickness to just 0.0037 inches. The investigation also reviewed the airline's Aircraft Maintenance Program (AMP) and the effectiveness of existing zonal inspection protocols regarding the detection of damage beneath insulation blankets.
Findings
- The primary cause of the failure was corrosion induced by leaking waste tank fluid, which accumulated in the lower fuselage structure.
- The plastic waste tank outlet flanges were unable to withstand the complex stresses caused by the installation of the waste water system coupling tubes.
- As the leaked fluid evaporated, the concentration of Chlorine increased, leading to severe corrosion of the fuselage skin.
- The residual strength of the compromised skin was insufficient to withstand the hoop-wise stresses experienced during normal flight operations, resulting in the 30-inch fracture.
- Existing inspection protocols were unable to detect the issue because the structural damage was hidden beneath insulation blankets, and the current maintenance intervals had not yet reached the threshold for mandatory removal of these blankets.
- Maintenance tasks for the waste tank compartment and the area below the aft cargo compartment were performed in different zones and at different times, preventing the detection of the fluid path.