What happened
On September 8, 2019, an IndiGo Airbus A320-271(NEO), registration VT-IZC, was performing a scheduled flight from Hyderabad to Gorakhpur. While cruising at 31,000 feet, the crew heard a loud bang, followed by an Engine 2 stall alert and high vibration warnings on the ECAM. The crew observed abnormal engine parameters, including N1 and N2 vibrations reaching level 10 and Exhaust Gas Temperature (EGT) rising above 900°C.
Due to these unstable parameters, the flight crew declared a 'PAN PAN' and diverted the aircraft to Varanasi. Upon landing at Varanasi, the engine stall and failure alerts triggered again, prompting the crew to shut down Engine 2. There were no injuries to the 148 passengers or 4 crew members on board, and no fire was reported.
The investigation
AAIB India examined the engine components following the incident. A post-flight visual inspection of the engine inlet and exhaust areas revealed significant damage to the Low Pressure Turbine (LPT) 3rd stage blades. Further borescope inspections identified extensive damage throughout the engine, including:
- Damage to the Low Pressure Compressor (LPC) stage 3 blades.
- Dented and heavily rubbed components within the High Pressure Compressor (HPC).
- Damage to the HPC shaft and the High Pressure Turbine (HPT) stage 1 aft wire seal.
- Damage to the LPT shaft and LPT stage 2 disk.
Technical analysis conducted by NAL, Bengaluru, focused on the material properties of the LPT 3rd stage blades. The investigation established that the pre-modification blades were made of a gamma-based Titanium Aluminide (TiAl) alloy, which exhibited low fracture toughness and poor crack growth resistance.
Findings
- The primary cause of the incident was the failure of the LPT 3rd stage blades during flight.
- The material used in these specific blades lacked sufficient damage tolerance to withstand impacts from material liberated upstream in the engine.
- The failure was part of a known series of similar incidents involving this engine type, which the manufacturer (Pratt & Whitney) had addressed through Service Bulletin 72-00-0111.
- The aircraft and crew were fully compliant with all airworthiness directives and operational requirements at the time of the event.