What happened
On March 24, 2019, an Airbus A320neo, registration VT-ITJ, operated by IndiGo, was flying a scheduled service from Amritsar to Guwahati. During the flight, the crew observed high N2 vibrations on Engine #2, accompanied by an ECAM warning. To manage the instability, the pilots followed the Quick Reference Handbook (QRH) procedures, reducing both thrust and airspeed to maintain stability. The aircraft landed safely in Guwahati with no injuries to the 160 passengers or 6 crew members.
However, the technical issues persisted. During the following day's first flight, the crew attempted to accelerate the aircraft for takeoff, but Engine #2 remained unresponsive, and vibrations spiked to 10 units. This forced the pilot to abort the takeoff and return to the bay, leading to the decision to ground the aircraft for an engine change.
The investigation
AAIB India examined the maintenance logs and flight data recorder (DFDR) to understand why the engine instability was not resolved during the night halt in Guwahati. The investigation revealed that the aircraft had experienced high N2 vibrations on two consecutive sectors. At the Amritsar stop, maintenance personnel, acting in coordination with the Maintenance Control Center (MCC), performed troubleshooting but did not download the MCDU data to analyze the fault deeper. The aircraft was subsequently cleared for the flight to Guwahati.
Upon arrival in Guwahati, further troubleshooting was conducted. While the aircraft was eventually released for its next flight, the subsequent failure to accelerate during takeoff highlighted that the underlying mechanical issue remained unaddressed.
Findings
- The primary cause of the engine instability was a High Cycle Fatigue (HCF) crack located on the front hub and tie shaft.
- This structural failure caused excessive rotor whirl and tie shaft imbalance, which manifested as high vibrations, particularly at higher thrust settings.
- Maintenance practices were identified as a contributing factor, as the aircraft was released for further flight after the first incident without establishing the root cause of the vibrations.
- The investigation noted that the maintenance engineer performing the rectification did not hold the full scope of authorization required for that specific aircraft type.
Safety action
Following the investigation, it was noted that the Original Equipment Manufacturer (OEM) had already identified the need for improvements in the PW1000G engine design to prevent such fatigue cracks. A short-term plan involving the recoating of tie shaft threads has been implemented to extend component life.