What happened
On April 1, 2019, an IndiGo Airbus A320-271 (NEO), registration VT-IVB, was performing a scheduled flight from Jaipur to Hyderabad. While climbing through 14,600 feet, the crew heard a loud bang, followed immediately by an engine stall alert on the Engine Centralized Aircraft Monitor (ECAM). The aircraft's first engine exhibited high vibration and Exhaust Gas Temperature (EGT) levels exceeding limits.
Following established emergency checklists, the crew monitored the abnormal engine parameters, which included N1 and N2 vibrations reaching a level of 10. Due to the instability of the engine, the pilots decided to divert the aircraft back to its point of departure. The aircraft landed safely at Jaipur airport with no injuries to the 128 passengers or the crew.
The investigation
An investigation by the AAIB examined the engine components following the incident. During a post-flight inspection, technicians discovered that all blades in the third stage of the Low Pressure Turbine (LPT) had fractured. Further examination of the engine revealed significant damage, including scoring on the LPT drive shaft, cracked stator insulation segments, and heavy deformation of the LPT case and turbine exhaust case.
Technical analysis conducted by the National Aerospace Laboratories (NAL) in Bengaluru focused on the material properties of the LPT blades. The study examined blades from the involved engine as well as other engines that had previously experienced similar failures. The investigation looked into the metallurgical composition of the blades, specifically the gamma-phase Titanium Aluminide (TiAl) used in the construction of the third-stage components.
Findings
- The primary cause of the incident was the brittle fracture of the LPT 3rd stage blades.
- The blades were constructed from a Titanium Aluminide (TiAl) intermetallic material which, while advanced, possesses low fracture toughness and inadequate damage tolerance.
- Metallurgical testing confirmed that the blades failed instantaneously in a brittle manner rather than through progressive fatigue.
- The material lacked sufficient crack growth resistance; once a crack initiated, it propagated rapidly, leading to the structural failure of the blades.
- The engine was equipped with pre-modified blades that were susceptible to failure upon impact from upstream material liberation.