New Delhi: IndiGo and Go Airlines India Ltd use the same type of engine made by Pratt & Whitney that’s susceptible to mid-flight shutdowns. Yet IndiGo, one of Airbus SE’s biggest customers, is the only one to encounter turbine failures this year, drawing heavy scrutiny from the aviation regulator.

The reason could be linked to how the budget airline flies. India’s Directorate General of Civil Aviation told IndiGo’s operator, InterGlobe Aviation Ltd, that its practice of revving A320neo jets at full thrust right after take-off could wear down the engines, people familiar with the matter said. By contrast, Go Air — India’s fourth-largest carrier by market share — typically uses a so-called alt-climb approach that applies less thrust, the people said, asking not to be identified discussing a private matter.

Climbing at full thrust can help planes burn less fuel, two of the people said. IndiGo has suffered 13 engine shutdowns related to low-pressure turbines during climbs this year, according to one of the people who was directly involved in an investigation where the DGCA ran a comparative analysis on how both airlines operate.

The issue has been costly. The DGCA this week said every time a new plane joins IndiGo’s fleet, it must ground one A320neo that hasn’t had its engines modified. That essentially prevents Asia’s biggest budget airline by market value from adding new flights until the issue is fixed. IndiGo has 730 of the latest model on order — making it the world’s top A320neo customer — and wants to expand its network beyond cities such as Istanbul to destinations including London. InterGlobe’s shares rose 1 per cent Friday.

Pratt, a unit of United Technologies Corp, invested $10 billion to develop its fuel-efficient geared-turbofan engine for single-aisle jets like the A320neo, but it’s suffered repeated setbacks since its commercial introduction in 2016, including a cooling problem, durability issues and delivery delays. IndiGo shifted away from the engines in June with a $20 billion order from CFM International Inc, a joint venture between General Electric Co. and France’s Safran SA, although those deliveries have yet to take place.

An IndiGo spokeswoman said the matter is “strictly between the airline and the concerned authorities.” The US Federal Aviation Administration hasn’t established any connection between the climb procedure and engine problems, she wrote in a text message, adding that the safety of passengers, crew and aircraft remains the utmost priority.

An Airbus spokesman said the planes are designed to handle full thrust, but it is “established best practice” for pilots to lower the thrust while climbing to reduce stress on the engine.

Pratt declined to comment on whether the climbing procedure can impact engines. A representative for the aviation ministry, which oversees the DGCA, referred queries to the regulator, saying it was a “technical” question. A Go Air spokesman didn’t immediately comment.

India originally asked IndiGo to replace all its faulty engines by Jan. 31, but the DGCA said Monday that the airline’s efforts to meet the deadline didn’t “instill enough confidence.” In a meeting with the regulator Monday, IndiGo offered to replace all unmodified engines by January 2021, but the request was denied, one of the people said, adding that the initial deadline remains.

IndiGo still needs to replace 110 engines out of 196 that were affected, Civil Aviation Minister Hardeep Singh Puri said Wednesday. He said Go Air hasn’t had the problems IndiGo encountered on its A320neo jets, without providing a reason. After the DGCA informed IndiGo of its findings, the company started taking steps to employ a climbing procedure similar to Go Air, the people said.

Given the number of replacements required and Airbus delivery delays, IndiGo may not able to update all the engines by the fiscal year ending in March, according to Motilal Oswal Financial Services Ltd In a note Tuesday, the brokerage cut its capacity growth estimate for IndiGo to 14 per cent from 25 per cent this year, and to 6 per cent from 10 per cent for next year.