What happened
On 25 October 2019, an IndiGo Airbus A320-271 NEO, registration VT-ITA, was operating a scheduled passenger flight from Mumbai to Coimbatore. While climbing through approximately 30,900 feet, the aircraft experienced a sudden malfunction in its number two engine. The crew received multiple warnings on the Engine-Indicating and Crew-Alerting System (ECAM), including "ENG 2 STALL," "ENG 2 HIGH VIB," and "EGT OVERLIMIT."
Engine parameters showed severe instability, with N1 and N2 vibrations reaching levels of 10, and Exhaust Gas Temperature (EGT) climbing beyond 1200°C. Following established emergency procedures, the flight crew reduced power to idle and performed a commanded shutdown of the second engine. Due to the abnormal engine parameters, the crew elected to perform an air turn-back. The aircraft landed safely at Mumbai airport with no injuries to the 183 passengers or 4 cabin crew members on board.
The investigation
AAIB India investigators examined the engine following the incident. A visual inspection of the engine inlet and exhaust areas revealed significant damage. Subsequent teardown examinations of the engine showed that all blades of the Low Pressure Turbine (LPT) 3rd stage had fractured. Additional damage was noted on the LPT 2nd stage blades, the LPT 2nd stage disk, 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 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 resistance to crack growth.
Findings
- The primary cause of the incident was the failure of the LPT 3rd stage blades during flight.
- The material used in the pre-modified 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 failure triggered the engine stall, high vibration, and EGT overlimit conditions.
- The incident was part of a known series of similar occurrences involving this specific engine component, which the manufacturer had addressed via a Service Bulletin.