NEW DELHI: An IndiGo A320 neo operating on Ahmedabad-Kolkata route on Friday with 174 passengers onboard had to return to the origin city following yet another snag in a Pratt & Whitney (PW) engine According to sources, the unmodified engine — awaiting to be replaced by a modified one — had high vibrations. "As per procedure, the pilots reduced thrust on the affected engine and the vibrations became normal. When thrust was again increased on the affected engine, vibrations reappeared and were accompanied by the light airframe vibration," said sources. After this the pilots decided to return to Ahmedabad and the Neo (VT-IVY) operating as 6E-125 landed safely.Confirming that the flight had returned to Ahmedabad, an IndiGo spokesperson said, "During flight, the pilot momentarily observed a caution message and followed the laid down standard operating procedure. Taking a precautionary measure, the pilot landed back at the Ahmedabad airport. The aircraft was back in operation after inspection."Comments from PW have been sought and are awaited.