Incident Summary
On 8 September 2013, China Airlines flight CI 5621, a Boeing 747-400F freighter with registration B-18716, operated a cargo flight from Taoyuan International Airport to Abu Dhabi International Airport. The aircraft was crewed by a captain and two first officers. The aircraft departed from runway 05R at approximately 03:25 local time. During climb, cabin pressure was normal. The aircraft reached 30,000 feet at 03:46:54 and leveled off. At 03:51:56, a cabin altitude warning activated. The crew observed the outflow valve in the closed position and cabin altitude rising at 400–500 feet per minute. The captain declared an emergency, donned an oxygen mask, and initiated an emergency descent. The warning ceased at approximately 19,400 feet. After descending to 10,000 feet, the cabin altitude reduced to 5,000 feet. The captain canceled the emergency and decided to return to Taoyuan. The aircraft landed safely on runway 05R at 05:51. No injuries were reported.
Post-Incident Inspection
A cabin pressurization test revealed that the air conditioning duct from pack 3 to the forward cargo compartment had become disconnected at the coupling between the check valve and the pressurized fuselage.
Investigation Findings
The investigation identified two findings related to the probable cause. First, after the inner air conditioning duct inlet was removed for repair, the flange material bulged outward, causing the flange's original protrusion to become flattened, preventing the clamp from securing the inlet flange. Second, the aircraft maintenance manual's clamp torque procedure lacked instructions for tapping the pipe wall, which may have resulted in insufficient clamp torque. Under flight operations and air conditioning use, the clamp tightness was inadequate to withstand the torque from fluid pressure and aircraft vibrations, leading to the duct separation. The resulting air leak from the forward cargo compartment triggered the cabin altitude warning.
Other Findings
Additional findings noted that maintenance personnel did not follow procedures for incoming inspection of repaired parts, and that the maintenance shop lacked the capability to repair the inner duct inlet. The aircraft maintenance manual had deleted a recommended tapping procedure for clamp installation, increasing the risk of duct separation. The aircraft had experienced a similar duct separation on 13 August 2013, but without a cabin pressure anomaly, likely because the collapsed duct wall blocked the opening.
Safety Recommendations
The Aviation Safety Council issued recommendations to Boeing to review the design of the pack 3 duct system and to reinstate the tapping procedure in the aircraft maintenance manual. China Airlines implemented corrective actions, including replacing the cabin pressure controller, conducting fleet-wide inspections, and providing training on proper clamp installation.