India's aviation regulator gave a deadline to modify all problematic engines.
One hundred and seventy six passengers, including three infants, were aboard IndiGO flight 6E-6129, a flight between Pune and Jaipur, when the pilot declared a full emergency because of an engine stall. The plane was diverted to Mumbai.
This is the latest engine-related incident to plague the airline, India's largest, and 21st such case since 2018.
GoAir, which also operates the same engine on its substantially smaller fleet of A-320 NEOs, also has had three such engine failures.
Today's incident comes just days after India's aviation regulator, the Directorate General of Civil Aviation (DGCA), gave the airline an extension till May 31 to replace or modify all problematic engines across its fleet of Airbus A-320 and A-321 NEO aircraft.
At the beginning of this month, IndiGo operated 106 such aircraft.
Though the process of modifying or replacing all problematic engines is on, it is understood that IndiGo continues to operate dozens of flights with at least one unmodified engine.
The engine, which failed on today's flight, was one such unit and is believed to have suffered damage to the third stage blades of its Low Pressure Turbine (LPT), a recurring problem across the fleet.
Despite repeated queries, IndiGo has not provided NDTV information on the number of A-320 NEO or A-321 NEO aircraft it presently operates with one unmodified engine.
In November, the DGCA had warned of catastrophic consequences if problematic third stage blades in the engines of A-320 NEOs being operated were not replaced.
''These conditions, if not addressed, could result in uncontained release of the LPT 3rd stage blades, failure of one of more engines, loss of thrust control and loss of aircraft," it had said.
Despite this, the regulator extended its own January 31 deadline for the airline to modify all its engines. The DGCA has been worried at the prospect of dozens of IndiGo flights being grounded if the ban was enforced as per the original deadline, a move that would have impacted innumerable passengers across the country.
NDTV has learnt that one of the two engines on the Airbus A-320 NEO jetliner, which stalled this morning, also developed high vibrations while crossing an altitude of 36,000 feet.
The pilot reduced thrust (power) on the malfunctioning engine, bringing it to idle before landing at Chhatrapati Shivaji International Airport in Mumbai. The damaged engine had flown a total of 3,373 hours and was last inspected on 9 December.
In a press release, IndiGo has said: ''During the flight, the pilot observed an engine vibration message and followed the laid [down] standard operating procedures. The flight landed in Mumbai. The aircraft is currently under inspection at Mumbai.''
NEO stands for New Engine Option - Airbus, the manufacturer of the A-320 series aircraft, had offered options to airlines to fit aircraft being inducted with a new generation of engines which were 15 per cent more fuel efficient, more environmentally friendly and significantly quieter.
IndiGo and GoAir selected the Pratt & Whitney 1100 series engines which have proven to be deeply problematic. Air India and Vistara went with CFM LEAP-1A engines which have not had frequent safety concerns.