The Aviation Safety Council (ASC) published an investigation report of a China Airlines B747-400F cargo aircraft occurrence. On September 8, 2013, the number 3 air condition duct fell off from the connection of check valve and duct assembly, conditioned air leaked from the opening of airframe continuously and caused excessive cabin pressure altitude warning during cruising. Therefore, the flight crew declared emergency situation, donned oxygen mask, executed emergency descent procedure, broadcasted to cabin and announced emergency situation to Air Traffic Control. The cabin pressure warning was disappeared when the airplane descended to cross about altitude 19,400 ft. When the airplane descended to about altitude 10,000 ft, flight crew checked cabin pressure altitude and it was decreased to about 5,000 ft. The pilot cancelled emergency declaration and judged that the aircraft was already unable to fly to the destination. After crew discussion and communicated with China Airlines' Operation Control Center, the pilot decided to fly back to Taoyuan International Airport. The aircraft had no damage and people on board were all safe.

The ASC launched investigation according to the Aviation Occurrence Investigation Act after the occurrence. Parties to the investigation included the Civil Aeronautics Administration (CAA), China Airlines, National Transportation Safety Board (NTSB) and Boeing Company. Investigation report was approved in the ASC 25th Board meeting on August 26, 2014.

Findings related to probable causes：As the maintenance personnel tried to repair the duct assembly, the materials above and below flange were protruded to form a flat geometry of the flange such 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 coupling might not reach required constant torque value. That caused the clamp could not be secured on the flange of duct assembly. Under the condition of airplane operation and the usage of pack, the tightness of clamp was insufficient to sustain the interactions of airplane vibration 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 airframe continuously and caused excessive cabin pressure altitude warning.

Findings related to risks：

In total there are 4findings:
1.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.
2.Structural Maintenance Section lacks of 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.
3.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.
4.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 Aviation Safety Council issued a total of 2 Safety Recommendations to Boeing Company

Recommendations to Boeing Company include: Re-evaluate the air condition duct design of the #3 pack to prevent fall-off of air condition duct from the coupling of check valve and duct assembly. A recommended tightening procedure for the clamp should be set up in the 747-400 Aircraft Maintenance Manual. The procedure should include the instruction on how to tap the coupling, specify the material, size and weight of the mallet and quantify the range of hitting force in the clamp tightening works.

The China Airlines had been accomplished seven safety actions during the progress of occurrence investigation. Therefore, the ASC will not issue safety recommendations to the China Airlines. Those accomplished safety actions included:

1.Issued 5 work orders for aircraft B-18716, replaced cabin pressure controller and carried out repeated inspection on the area where the number 3 air condition duct fell off.
2.Issued 2 engineering orders and carried out one time inspection of all 747-400 cargo aircrafts.
3.Finished announcement of the example case to maintenance personnel, totally 77 man times; Finished work relay and work report related safety announcements, totally 76 man times; Performed on job training of air condition duct related removal and installation as specified in the Aircraft Maintenance Manual Chapter 21, classroom study first then practical training. Practical training included the tightening of clamp fixed nut, hitting techniques and precautions of wood mallet application.
4.Incorporated the inspections of the number 3 air condition duct area and check valves of forward/rear cargo compartments into C check inspection items.
5.Finished announcement and education to maintenance personnel of the ERI Maintenance Section; Requested to fill out maintenance record and make sure its correctness when performing maintenance work; Reinforced the double check mechanism of supervisory personnel before and after the installation of air condition duct.
6.Finished announcement and education to maintenance personnel of the Structure Maintenance Section; Requested to comply with the 「System Engineer Technical Support Procedure」 to carry out maintenance support.
7.Finished announcement and education to section managers of the Base Maintenance Department; Requested to supervise maintenance personnel to comply with the「Components Repair and Overhaul Procedure」, the 「System Engineer Technical Support Procedure」 and the  procedure of 「S.O.P. of A/C Parts/Components Replacement」to carry out maintenance work; Requested section managers of the Base Maintenance Department to have situational awareness and risk sense and to comply with related operational quality procedures.

Full investigation report is available for download at ASC website: www.asc.gov.tw/

Contact: Sherry Liu, Engineer
Tel: 89127388-330
Email: Sherry@asc.gov.tw