What happened
On 29 June 2019, an IndiGo Airbus A320-271 NEO, registration VT-ITP, was performing a scheduled flight from Dehradun to Bengaluru. While climbing through 12,000 feet, the crew heard a loud bang, followed by an Engine Indicating and Crew Alerting System (ECAM) warning. The engine parameters showed significant instability, with N1 vibrations reaching 10 and N2 vibrations at 7, while the Exhaust Gas Temperature (EGT) climbed beyond 1000°C.
The flight crew responded by reducing power on engine number one to idle and following the required checklists. Due to the abnormal engine parameters, the crew elected to divert the flight to Delhi. The aircraft landed safely at Delhi International Airport with no injuries to the 186 passengers or 4 cabin crew members on board.
The investigation
An investigation by AAIB India focused on the physical damage found during post-flight inspections. A visual examination of the engine inlet and exhaust areas revealed that all blades in the third stage of the Low-Pressure Turbine (LPT) had been damaged or fractured. Further technical teardown of the engine revealed additional issues, including scoring on the LPT drive shaft, damage to LPT Stage 1 and Stage 2 blades, and impact damage to the LPT case and turbine exhaust case.
To understand the failure mechanism, the AAIB engaged the National Aerospace Laboratories (NAL) in Bengaluru to perform a failure analysis on fractured blades from the incident engine and others from similar recent events. The metallurgical study focused on the Gamma-based Titanium Aluminide (TiAl) material used in the LPT 3rd stage blades.
Findings
- The primary cause of the incident was the fracture of the LPT 3rd stage blades, which occurred instantaneously due to the material's inherent brittleness.
- Metallurgical analysis confirmed that the TiAl-based intermetallic material used in the blades lacked sufficient crack growth resistance and damage tolerance. Once a crack initiated, it propagated through the material without signs of progressive fatigue.
- The investigation established that the failure was part of a known series of similar occurrences involving this specific engine configuration.
- The engine was equipped with pre-modified blades that were susceptible to fracturing upon even minor impacts or stress, leading to the high vibration and EGT overlimit observed by the crew.