What happened
On 23 May 2015, a Singapore Airlines Airbus A330-300, registration 9V-SSF, was operating a scheduled passenger flight from Singapore to Shanghai. While cruising at 39,000 feet in international waters approximately 130 NM southeast of Hong Kong, the aircraft encountered adverse weather conditions.
As the aircraft navigated through a weather cell, the flight crew received an ECAM alert indicating a surge in the right engine. Before the crew could react, the right engine self-recovered, but a subsequent alert appeared for the left engine. Believing the left engine was still in a surge state, the crew followed the engine stall checklist, declared a Mayday, and shut down the left engine. To maintain safe flight with a single engine, the crew performed a controlled descent to 26,000 feet. During the descent, the crew successfully restarted the left engine, and the flight continued to Shanghai without further incident.
The investigation
The investigation focused on the cause of the engine surges and the crew's decision-making process. Post-flight inspections of the engines in Shanghai showed no mechanical damage or metal particles in the magnetic chip detectors. However, a subsequent teardown of the left engine in Singapore revealed a significant deposit of grey dust, composed of aluminum and silicon, on various internal components. This dust was identified as part of the IPC rotor path abradable lining material.
Investigators examined several potential triggers for the heavy rubbing of the engine's internal linings, including engine casing contraction due to the ingestion of ice crystals or super-cooled water droplets, rotor dilation, and axial load reversal. Data also suggested that vibration in the Stage 3 IPC blades likely contributed to the contact between the blade tips and the engine casing.
Findings
- The primary cause of the surges was the release of IPC rotor path abragable lining material into the airflow.
- Heavy rubbing at Stages 3 and 6 of the IPC caused the aluminum and silicon-based lining to erode into small particles.
- These particles entered the combustion section and ignited, disrupting the airflow and triggering the surges.
- The flight crew's decision to shut down the engine was influenced by the Electronic Engine Control (EEC) logic, which was programmed to display the engine stall message for 60 seconds regardless of whether the engine had already self-recovered.
Safety action
Several corrective measures were implemented following the investigation:
- The aircraft manufacturer updated flight crew training manuals to provide better guidance on engine stall symptoms and procedures.
- The engine manufacturer increased rotor tip clearances at the affected IPC stages.
- A service bulletin was initiated to modify the EEC logic, reducing the engine surge message display time from 60 seconds to less than 10 seconds.
- The operator adopted a policy of not pairing two brand-new engines on the same A330, instead using one relatively older engine to ensure more stable clearances.