China Airlines B744 near Taipei on Sep 8th 2013, loss of cabin pressure

A China Airlines Boeing 747-400 freighter, registration B-18716 performing freight flight CI-5621 from Taipei (Taiwan) to Abu Dhabi (United Arab Emirates) with 3 crew, had just reached initial cruise level 300 when the crew donned their oxygen masks and initiated an emergency descent due to the loss of cabin pressure. The aircraft levelled at FL100, dumped fuel and returned to Taipei for a safe landing on runway 05R about one hour after leaving FL300.

A replacement Boeing 747-400 registration B-18718 reached Abu Dhabi with a delay of 6 hours.

Taiwan's ASC have opened an investigation into the occurrence.

On Oct 22nd 2014 Taiwan's ASC released their final report in Chinese (executive summary in English) concluding:

- as maintenance personnel tried to repair the duct assembly, the materials above and below flange were protruded to form a flat geometry of the flange so that the clamp could not be secured on the flat flange of duct assembly after installation.

- the recommended tightening procedure of clamp in Aircraft Maintenance Manual lacked instruction on how to tap the coupling. Therefore the tightness of the coupling might not reach required constant torque value. That caused the clamp could not be secured on the flange of the duct assembly. Under the conditions of the airplane operation and usage of pack the tightness of clamp was insufficient to sustain the interactions of airplane vibrations and the clockwise bending moment resulted from the conditioned air acted on the air condition duct. The air condition duct fell off from the connection of check valve and duct assembly, conditioned air leaked from the opening of air frame continuously and caused excessive cabin pressure altitude warning.

Findings as to risk

- Maintenance personnel did not comply with the Components Repair and Overhaul Procedure to carry out parts receiving inspection, they also did not confirm whether or not China Airlines had repair capability of duct assembly which might increase the risk that the repair of aircraft component might not conform to required quality.

- China Airlines Capability List Manual does not include the repair capability of duct assembly. Maintenance personnel did not comply with the System Engineer Technical Support Procedure to coordinate System Engineering Department to consult with manufacturer. Therefore, no timely technical assistance could be obtained.

- Maintenance personnel did not comply with the S.O.P. of A/C Parts/Components Replacement procedure to hang a tag on the parts removed from airplane which might increase the risk of misuse after parts removal.

- The deletion of the recommended tightening procedure of clamp from the Aircraft Maintenance Manual may increase the risk of air condition duct fall-off.

The ASC reported that following the occurrence flight a post flight examination showed the #3 air condition duct supplying the forward cargo compartment had separated from connection of check valve and duct assembly.

Two safety recommendations were issued to Boeing and 7 safety actions were taken by China Airlines.