What happened
On September 8, 2013, a China Airlines Boeing 747-400F freighter, registration B-18716, was operating flight CI 5621 from Taoyuan International Airport to Abu Dhabi International Airport. The flight crew, consisting of one captain and two first officers, departed Taoyuan using runway 05R under instrument departure procedures.
While cruising at 30,000 feet, the aircraft's EICAS system issued a high cabin altitude warning. The crew observed that the cabin pressure altitude was rising at a rate of approximately 400 to 500 feet per minute, reaching 10,400 feet. Upon identifying the pressure anomaly, the captain declared an emergency, donned oxygen masks, and initiated an emergency descent procedure. The crew notified air traffic control and passengers of the situation. As the aircraft descended through 19,400 feet, the warning cleared, and by 10,000 feet, the cabin pressure altitude had stabilized at approximately 5,000 feet. After consulting with the airline's operations center, the crew decided to return to Taoyuan, landing safely on runway 05R at 05:51 local time. No injuries or damage were reported.
The investigation
Following the incident, the TTSB conducted a pressure test on the aircraft's cargo hold. The investigation established that a component of the number 3 air conditioning duct had separated from its connection point at the fuselage pressure bulkhead. Specifically, the investigation focused on the repair history of the inner air conditioning duct inlet. It was discovered that a recent repair to the inlet flange had caused the metal to protrude outward, flattening the original flange profile. This deformation prevented the clamp from properly securing the flange.
Further examination of maintenance practices revealed that the aircraft maintenance manual (AMM) lacked specific instructions for the proper torqueing procedure of the clamp, such as a recommended tapping method to ensure stability. The investigation also reviewed the maintenance facility's procedures regarding part receiving and technical support, noting that the repair was performed without formal verification of the facility's capability to handle such specific component repairs.