What happened
On 30 August 2019, a Saab 3D4B, registration VH-RXX, experienced an engine failure and in-flight shutdown. The incident began when heavy coking within the B-Sump oil cavity caused oil to leak from the component. This leakage led to an internal engine fire. The intense heat from this over-temperature condition caused the PT stage 3 disk posts to weaken, allowing several turbine blades to detach. This loss of blades created an imbalance that resulted in the fracture of the PT shaft. The flight crew received an engine fire indication after hot gases escaped through burn holes in the PT case, which triggered the fire-detection system.
The investigation
Investigators examined the B-Sump component, which had only been in service for 1,481 flight hours since its installation in May 2018. This usage represented only approximately 10 per cent of the standard interval between major work scopes. The investigation focused on why such significant coking accumulated in such a short period. While the component had been cleaned according to manufacturer recommendations, the internal oil cavity could not be visually inspected to verify the total removal of all deposits. The investigation considered whether the coking was caused by residual deposits left after the previous major work scope, an accelerated formation of coke, or a combination of both factors.
Findings
- The excessive coking in the B-Sump oil cavity led to the oil leak and subsequent internal fire.
- The fire and resulting heat caused the PT shaft to fracture and the C-Sump assembly to separate.
- The high level of coking was likely due to the component not being entirely clean upon installation or an accelerated formation of coke within the part.
- There was no evidence that the engine was operated outside of the manufacturer's recommended parameters.
- Significant coking leading to engine failure in the CT7 engine type is an extremely rare occurrence, with only two known instances in 38 million flight hours.