What happened
On 16 January 2010, an Asiana Airlines A321-231, registration HL7730, was performing a scheduled international flight from Saipan International Airport to Incheon International Airport via Kansai International Airport. While cruising at 37,000 feet approximately 300 NM south of Kansai, the flight crew received multiple warnings regarding wing leaks and engine bleed faults.
As the crew attempted to address the initial warnings, subsequent alerts indicated faults in both the left and right air conditioning packs. This led to a rapid loss of cabin pressure, with the cabin pressure altitude rising above 8,800 feet. Following established emergency procedures, the crew donned oxygen masks and initiated an emergency descent to 10,000 feet. The aircraft subsequently landed safely at Kansai International Airport with no injuries to the 51 passengers or 8 crew members.
The investigation
The ARAIB investigation focused on the mechanical failure of the air conditioning system and the effectiveness of the aircraft's leak detection capabilities. Investigators examined the maintenance history of the aircraft, specifically regarding a previously issued Service Bulletin (SB 21-1153). This bulletin recommended replacing silicone seals in the pack valve ducts with more durable Teflon synthetic material to improve reliability.
While Asiana Airlines had decided to defer the replacement of these seals until a defect was found, the investigation noted that the aircraft in question, HL7730, was still using the older silicone seals. Furthermore, the investigation looked into the design of the air leak detection loops, noting that both loops are located within a single compartment, which can lead to a single leak being detected by both systems.
Findings
- The primary cause of the incident was a damaged seal in the right air conditioning pack valve duct, which allowed hot air to leak. This leak triggered the automatic closure of both the left and right air conditioning pack valves, resulting in the loss of the cabin's pressure regulation capability.
- A contributing factor was the shared compartment for both air leak detection loops. Because the loops are located in the same space, a leak from one side can be detected by both loops depending on the airflow, potentially masking the specific source of the leak.
- The airline had deferred the implementation of the recommended Teflon seal replacement, opting to replace the silicone seals only upon the occurrence of a defect.
Safety action
Following the investigation, the ARAIB issued several recommendations:
- To Asiana Airlines: Expedite the replacement of existing seals with the new material and implement a more robust scheme for periodic air leak checks during regular maintenance.
- To Airbus Industries: Improve the air leak detection system to ensure better segregation between the left and right pack leak detection loops, and update maintenance manuals to suggest the use of specialized tools or temperature sensors for inspecting difficult-to-access pack ducts.