What happened
On 21st January 2019, an IndiGo Airbus A320-271 (NEO), registration VT-ITQ, was performing a scheduled flight from Lucknow to Jaipur. While climbing through Flight Level 120, the flight crew heard a loud bang, followed immediately by several critical engine alerts on the Engine-Indicating and Crew-Alerting System (ECAM). These alerts included an engine stall, excessive Exhaust Gas Temperature (EGT), and high vibration on the second engine.
Engine parameters showed significant instability, with N1 and N2 vibrations reaching 7 and 10 respectively, while EGT climbed beyond 1000°C. Following established checklists, the crew elected to divert the aircraft back to its departure point. The aircraft landed safely at Lucknow airport with no injuries to the 120 passengers or the crew. A subsequent inspection of the engine revealed extensive damage to the Low Pressure Turbine (LPT) 3rd stage blades.
The investigation
AAIB India examined the engine components and reviewed technical data from the manufacturer. A borescope inspection of the High-Pressure Compressor (HPC) and High-Pressure Turbine (HPT) revealed various issues, including nicks on rotor blades, worn seals, and a cracked combustion chamber outer seal. Most notably, the LPT 3rd stage blades were found to be fractured from the tips, and the LPT case and shaft showed significant impact damage.
Failure analysis conducted by the National Aerospace Laboratories (NAL) in Bengaluru focused on the material properties of the LPT 3rd stage blades. The study confirmed that the blades, constructed from a Gamma-phase Titanium Aluminide (TiAl) intermetallic material, failed in a brittle manner. The analysis showed that the blades lacked sufficient damage tolerance and crack growth resistance to withstand impacts, leading to instantaneous fracture rather than progressive fatigue.
Findings
- The primary cause of the engine stall and subsequent vibration was the failure of the LPT 3rd stage blades.
- The blades were made of a TiAl-based intermetallic material which, while advanced, is inherently brittle and possesses low fracture toughness.
- The engine was fitted with pre-modification blades that lacked the necessary resistance to withstand material impacts from upstream components.
- The failure of these blades resulted in the observed high vibration and EGT overlimit conditions.