Report: A330 emergency descent due to bleed air system interruption (Taiwan)

The Aviation Safety Council (ASC) of Taiwan issued several safety recommendations as a result of their investigation into an  occurrence when an Airbus A330 made an emergency descent due to temporary interruption in the bleed air system supply.

On September 14, 2008 an Airbus A330-300, registered B-HLH, operated by Cathhay Pacific as regularly scheduled Flight CX521, took off from Tokyo-Narita Airport (NRT/RJAA), Japan on a flight to Hong Kong. Due to the presence of a typhoon the flight was rescheduled to Taipei-Taiwan Taoyuan International Airport (TPE/RCTP). There were 72 persons on board. The airplane was dispatched with the nr.1 engine bleed air system inoperative per MEL.

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 19:29. The aircraft was not damaged and none of the 72 occupants were injured.

The investigators concluded:

Giving the de-activated status of the No.1 engine bleed air valve per MEL 36-11-02, the no.2 engine bleed air was the only one compressed air source for the two air conditioning systems.
The no.2 engine bleed air valve operated in a high demand status. During aircraft descent, the compressed air automatically bled from high pressure stage which provided the compressed air with higher pressure and higher temperature.
This led the pre-cooler downstream temperature air getting higher. 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.

The Aviation Safety Council issued following 4 Safety Recommendations to Hong Kong CAD and Cathay Pacific Airways:

1. 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;
2. 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;
3. 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;
4. require Cathay Pacific Airways to review air dual bleed fault and emergency descent procedures and revise related inconsistent procedures accordingly; and 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.

Three safety recommendations were issued to the DGAC France to:

1. require manufacturer to modify or redesign the ThC grid filter to reduce the risk of A330 dual bleed system failure;
2. 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
3. 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  recommendations to CAA Taiwan as follows:

1. require controller followed ATMP procedures;
2. enhance controller emergency response and situation awareness when handling the distress aircraft in accordance with the ATMP procedure;
3. 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
4. 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.