What happened
On 24 October 2019, an IndiGo Airbus A320-271(NEO), registration VT-IVX, was performing a scheduled flight from Kolkata to Chennai. While the aircraft was climbing through Flight Level 260, the flight crew heard a loud bang originating from the starboard side engine. This was immediately followed by an Engine Stall ECAM alert and indications of high vibration in the second engine.
Engine parameters showed significant abnormalities, including N1 and N2 vibrations reaching a value of 10 and Exhaust Gas Temperature (EGT) rising beyond 900°C. The crew halted the climb and, following established procedures, performed a commanded In-Flight Shut Down (IFSD) of engine number two. The aircraft then diverted back to Kolkata, landing safely on a single engine. There were no injuries to the 121 passengers or the crew on board.
The investigation
An investigation by the AAIB examined the engine components following the incident. A visual inspection of the engine inlet and exhaust areas revealed extensive damage. Specifically, the Low Pressure Turbine (LPT) 3rd stage blades were found to be damaged, and further examination showed damage to the LPT 1st and 2nd stage blades, the LPT shaft, the LPT case, and the turbine exhaust case.
Technical analysis conducted by NAL, Bengaluru, focused on the material properties of the LPT 3rd stage blades. The study determined that the pre-modification blades were constructed from a gamma-based Titanium Aluminide (TiAl) alloy. While engineered for specific performance, this material was found to be inherently brittle, possessing low fracture toughness and inadequate crack growth resistance.
Findings
- The primary cause of the engine instability was the failure of the LPT 3rd stage blades during flight.
- The material used in the pre-modification blades lacked the necessary damage tolerance to withstand impacts from material liberated upstream in the engine.
- Once a crack initiated, the brittle nature of the alloy caused the crack to propagate instantaneously, leading to the fracture.
- This incident was part of a known series of similar occurrences involving this engine type, which the manufacturer (Pratt & Whitney) had previously identified.
- The aircraft was fully compliant with all applicable Airworthiness Directives and Service Bulletins at the time of the event.