NEW DELHI/BENGALURU (Reuters) – India’s civil aviation regulator has ordered domestic carriers IndiGo and GoAir to modify some of their Airbus A320neo aircraft fitted with Pratt & Whitney engines, linked to in-flight shutdowns, within 15 days to avoid their grounding.

IndiGo planes fitted with Pratt & Whitney 1100 series engines that have clocked more than 2,900 hours must have at least one modified engine, the Directorate General of Civil Aviation (DGCA) said in a statement on Monday.

The DGCA said it has issued a similar directive to GoAir on Tuesday.

The regulator said a total of 29 A320neo planes are affected, of which 16 are operated by IndiGo and 13 by GoAir. The airlines together have more than 100 A320neo planes in their fleet.

For IndiGo, India’s top airline, the restrictions come days after it posted its biggest-ever quarterly loss hurt by higher maintenance costs from leasing A320ceo planes to fill a gap caused by the grounding of A320neo due to engine issues.

The airline, owned by InterGlobe Aviation Ltd (INGL.NS), said “it will cooperate with authorities and will comply with the next course of action.”

GoAir said it has not yet received any such directive from the DGCA.

Pratt & Whitney said in a statement it is working with the airlines to address the issue with minimal disruption.

IndiGo is Airbus’ biggest client for the A320neo planes and while the United Technologies’ (UTX.N) Pratt & Whitney engines are fuel-efficient they have consistently caused issues since they entered into service in 2016, forcing IndiGo to ground its planes several times.

The DGCA said it found three in-flight engine shutdowns on IndiGo’s planes in October following which it reviewed maintenance and safety data at the airline.

Earlier this year, the DGCA had directed airlines to make extra checks on their A320neo aircraft fitted with Pratt & Whitney engines as part of new safety protocols after temporary grounding orders affected the planes last year.