What happened
On April 2, 2019, an IndiGo Airbus A320-271 (NEO), registration VT-ITG, was performing a scheduled flight from Pune to Nagpur. The flight proceeded normally after takeoff until the aircraft reached an altitude of approximately 14,300 feet. At this stage of the climb, the crew heard a loud bang originating from the left side of the aircraft.
Following the noise, the aircraft' and engine's Electronic Centralized Aircraft Monitor (ECAM) triggered several warnings, including an engine stall, high vibration, and Exhaust Gas Temperature (EGT) overlimit. The crew immediately responded by reducing power on Engine No. 1 to idle and executing the required checklists. Due to the abnormal engine parameters, the pilots decided to perform an air turn back. The aircraft landed safely at Pune Airport without any emergency being declared, and all 93 passengers and 4 cabin crew members disembarked without injury.
The investigation
AAIB India conducted an investigation into the serious incident, which included a detailed examination of the engine and metallurgical analysis of the damaged components. A post-flight visual inspection of the engine inlet and exhaust areas revealed that all blades of the Low Pressure Turbine (LPT) 3rd stage were damaged. Further teardown examinations of Engine No. 1 showed significant damage, including burn marks and cracks in the combustion chamber, scoring on the LPT drive shaft, and extensive impact damage to the LPT case and turbine exhaust case. Additionally, the metal chip detector in the accessory gearbox contained over 60% metallic fines.
To understand the nature of the failure, the investigation utilized failure analysis from the National Aerospace Laboratories (NAL) in Bengaluru. The analysis focused on the material properties of the LPT 3rd stage blades, which were constructed from a Gamma-phase Titanium Aluminide (TiAl) intermetallic material.
Findings
- The primary cause of the engine stall and subsequent vibration was the fracture of the LPT 3rd stage blades.
- Metallurgical studies confirmed that the blades failed in a brittle manner, characterized by instantaneous fracture rather than progressive fatigue.
- The TiAl-based material used in the pre-modified blades lacked sufficient crack growth resistance and damage tolerance, meaning that once a crack initiated, it propagated almost instantly.
- The investigation noted that this incident was part of a series of similar occurrences involving the failure of these specific LPT 3rd stage blades in Pratt & Whitney engines.
Safety action
Following the identification of this issue, a Service Bulletin (SB 72-00-0111) was introduced to replace the 3rd stage LPT disk, blades, and shroud segments with a new set made of a different, more impact-resistant material. The DGCA issued mandates requiring airlines to ensure all A320 NEO aircraft equipped with these engines were upgraded. By August 2020, the operator had successfully installed modified engines across its entire NEO fleet.