New Delhi, Sep 7 : A Vistara airline Mumbai-bound flight returned midway to Delhi after a "whistling" sound was heard on the right side of the Boeing 737 aircraft cockpit. Aviation regulator, the Directorate General of Civil Aviation has ordered a probe into the incident even as no structural deficiency was observed during preliminary ground inspection of the plane. "Vistara operated (Delhi-Mumbai) flight UK 951 was involved in an air turn back to Delhi as a whistling sound was heard from the right side in the cockpit. Aircraft has landed safely," the official said. A Vistara airlines spokesperson said that the aircraft returned midway due to a technical snag and alternate aircraft was immediately arranged. "A minor technical snag was detected shortly after take-off on Vistara flight UK 951 operating Delhi to Mumbai on 5 September 2022. As a precautionary step, pilots decided to turn-back and the aircraft landed safely at the IGI Airport, Delhi. "An alternate aircraft was immediately arranged, which departed shortly. Inconvenience caused to customers is deeply regretted. As always, safety of our customers and crew is most important to Vistara," said the airline's spokesperson. /IANS