On September 14, 2008 at 1614 Taipei local time Cathay Pacific Airways Flight number CX521, an Airbus A330-300 aircraft with registration number B-HLH, flew from Narita International Airport, Japan to Taipei/Taiwan Taoyuan International Airport, Taiwan, Republic of China. The flight departed with 72 occupants on board including 59 passengers, 11 cabin crew members and 2 flight crew members. The aircraft encountered interruptions of the bleed air system supply at 38,544 ft during descent from flight level FL400. Flight crew members conducted an emergency descent and landed safely at Taipei international airport at approximately 1929. The aircraft was not damaged and none of the 72 occupants were injured.

According to the Aviation Occurrence Investigation Act and Annex 13 to the Convention on the International Civil Aviation, the Aviation Safety Council (ASC) of ROC immediately launched an investigation of this occurrence. The state of manufacture, represented by the France BEA, the liaison officer of operator, represented by Hong Kong CAD and the Civil Aeronautics Administration (CAA) of CAD were invited to participate in the investigation. On March 13, 2009, ASC held the Factual Data Verification Meeting in Taipei. On November 26th, 2009, ASC held the 1st Technical Review Meeting and followed by the 2nd Technical Review Meeting by BEA and Airbus’s request. All party members of the investigation team were invited to attend the meeting. The ASC completed its investigation report, which was approved by the ASC Council Members on July 27, 2010, at the 135th Council Meeting.

The following findings are presented in three categories: findings related to probable causes, findings related to the risk, and other findings.

Finding related to the probable cause: Due to the THC’s grid filter contaminated from which to reduce the muscle air pressure to control fan air valve that resulted in the fan air valve could not open properly to provide sufficient cooling air to pre-cooler. The no.2 engine bleed air valve was shut down automatically due to bleed air overheat. Both air conditioning systems lost the compressed air source and thereby aircraft lost its pressurization capability.

Findings related to the risk include maintenance related as: The repeated defects of the numerous dual bleed air system and number one engine bleed air defects prior to the occurrence revealed the deficiency of the bleed air system’ reliability and potential operation risk.

Findings related to flight operation which include: The flight crew premature change of frequency might have caused by distracted by the system failure, confused the similar call signs on the same control frequency and did not adhere to company communication procedures by inadvertently omitting the CX521 flight number at the end of one of the transmissions and that the transmission was stepped on, thus resulted in a lost opportunity for the pilot and the controller to correct the mistake and prevent the premature change of frequency.

Finding related to ATC include: Approach controller should be aware the existing similar call sign situation and follow the ATMP regulation for pilot’ distinguishing when the CX521 acknowledged instruction and read back frequency change incorrectly for other aircraft, did not acknowledge the CX521 distress message immediately on Guard frequency until one minute latter, controller did not follow the ATMP request to provide maximum assistance and first priority to distress aircraft; lack of coordination and information exchange internally from both the TPE Tower and the Approach controllers plus TACC controllers failed to receive the CX521 “Mayday” call at 1859:56 on 121.5 Frequency until 1900:52. Other risk findings include Datum Mt and Mekong. Emergency frequencies unable to cover each other due to the 140NM distance and geographic influence, TACC North Sector guard frequency test omitted the occurrence neighbor area waypoint SALMI. (The omitted way point test may have resulted in TACC controllers missing Mayday call from CX521) and the ATMP English version and Chinese version 2-4-15 regarding emphasizing to aid in distinguishing between similar sounding aircraft are inconsistent.

Finding related to survival factors include: Some cabin crew members whose oxygen mask did not drop down, did not try to open their access panels or using portable oxygen bottle around their seats, Some cabin crew members may not be familiar with the cabin masks design features and operation with regard to pulling down on the cord to activate oxygen flow and not be fully aware of the normal operation of the cabin masks and some cabin crew members who were not to or not able to use their oxygen masks may have misled passengers into thinking that wearing the mask was not required plus the side effects of the chemical oxygen generators did not list in any cabin related manual and training course.

There are 13 other findings presented.

The Aviation Safety Council issued following 4 Safety Recommendations to Hong Kong CAD and Cathay Pacific Airways: Require Cathay Pacific Airways to consider evaluating or revising the MEL procedure to reduce the depressurization risk under one engine bleed air fail, and recover the cabin pressurization capability with APU in a timely manner when the second engine bleed air system also failed, require Cathay Pacific Airways to consider evaluating the maintenance program for ThC shop-in service or overhaul interval before the new grid filter design or modification come to effect, require Cathay Pacific Airways to consider evaluating the MEL restriction regarding aircraft been dispatched from home base with an inoperative system to lower the dual bleed system failure risk ,require Cathay Pacific Airways to review air dual bleed fault and emergency descent procedures and revise related inconsistent procedures accordingly and finally require Cathay Pacific Airways cabin crew members to review cabin depressurization related procedures including: provide oxygen bottle side effect information, manually opening the oxygen cover panel to initiate oxygen flow; enhance cabin crew depressurization training.

3 safety recommendations were issued to the DGAC France that require manufacturer to modify or redesign the ThC grid filter to reduce the risk of A330 dual bleed system failure, recommend the manufacturer to evaluate the maintenance program for ThC shop-in service or overhaul interval before the new design or modification come to effect and require manufacturer to review air dual bleed fault and emergency descent procedures and revise related inconsistent procedures accordingly and require manufacturer considering to take the in-service fleet events and family fleet problem solving experiences into Product Safety Process account and form the problem solving task force in an earlier time as proactive risk mitigation measure.

The Aviation Safety Council issued 7 recommendations to CAA as follow:

Require controller followed ATMP procedures, enhance controller emergency response and situation awareness when handling the distress aircraft in accordance with the ATMP procedure , enhance ATC internal coordination, communication during emergency situation includes the training, checking and handling of distress aircraft and carefully selected appropriate radio communication stations as backup system to avoid communication performance degrade and finally to revise the TACC Guard frequency radio test inclusive at SALMI waypoint and review and revise the ATMP Chinese version 2-4-15 word meaning in accordance with the English version 1-2-1.

The report could be down load at www.asc.gov.tw