MUMBAI/LUCKNOW: A Vistara flight from Mumbai to Delhi with 153 passengers on board was airborne for close to four hours on Monday during which the aircraft circled over Delhi, diverted to Lucknow, then headed to Allahabad only to return to Lucknow where the pilots, who were trying to avoid a low-visibility landing, radioed a Mayday call and carried out a providential touchdown with almost empty fuel tanks.The weather gods and Lucknow air traffic controllers saved the day because the aircraft didn’t have enough fuel to fly the approximate 200-km distance from Lucknow to Prayagraj airport, Allahabad. Flightradar24 data showed that about seven minutes en route to Allahabad, the aircraft turned to Lucknow where it landed in 20 minutes. The pilots had returned after the Lucknow ATC informed them that weather over Lucknow had cleared. “When it touched down, the A320Neo had only 200kg or 5 minutes of flying time left. Allahabad runway is about 30 minutes away from Lucknow,” said a source.The A320Neo aircraft, operating flight UK 944, departed Mumbai at 2.40pm with about 8,500kg of fuel, said a source.The flying time between Mumbai and Delhi is less than two hours. Directorate General of Civil Aviation, which is probing the matter, has grounded the two pilots.Said a senior pilot, requesting anonymity: “It’s a miracle they landed. The big question on everyone’s mind is: Why did the pilots not opt to auto land (fully automated landing) in Lucknow after they diverted from Delhi? It’s the standard practice to follow in a fuel emergency, low-visibility situation. To divert from Lucknow to Allahabad was sacrilegious.”Vistara statement corroborated that the pilots had indeed decided to divert to Allahabad. “En route Prayagraj (Allahabad), Lucknow ATC ( air traffic control ) informed the crew that the weather in Lucknow had improved significantly following which the crew decided to return to Lucknow due to better passenger and aircraft support there.”A source said: “The airline seems to have accounted for bad weather in Delhi as it carried extra fuel.” At Delhi, tail wind and low visibility had the aircraft circle for almost 75 minutes, after which the pilots diverted to Lucknow, the “destination alternate” listed in the flight plan. Destination alternate is the airport pilots divert to if it’s not possible to land at the scheduled destination. Every flight carries enough fuel to fly up to the destination, then to fly from destination to destination alternate airport, apart from fuel for holding, taxing etc.“But diverting again from destination alternate after having circled over Delhi for more than an hour was a dangerous decision. The day was saved only because Vistara allowed its pilots to carry over and above the minimum flight plan fuel. The pilot-in-command was an experienced training Captain. This should be a lesson to other airlines that strictly upload only minimum fuel needed according to the flight plan,” the source said.Vistara, in its statement, said: “Flight UK944 ... initiated a diversion to Lucknow due to bad weather over Delhi. However, over Lucknow, the visibility suddenly dropped and a safe landing was not possible. The crew then considered alternative airfields, including Kanpur and Prayagraj, to land in comparatively better weather condition.” The airline said the unexpected drop in visibility at Lucknow was the main reason why the aircraft ended up in a low-fuel situation despite carrying excess fuel over and above the required flight plan fuel as per regulations. “Safety of passengers and crew was kept at the highest priority throughout the flight,” Vistara said. “The flight finally touched down safely in Delhi around 1.35am after refuelling at Amausi airport,” said Rashmi Soni, PRO, Vistara Airlines.Air safety expert Capt Mohan Ranganathan said: “This would have been similar to the Indian Airlines A300 aircraft in November 1993 that landed in a paddy field near Tirupati after it diverted from Hyderabad due to low-visibility and then ran out of fuel. The aircraft should have carried out an auto land in Lucknow after diverting from Delhi.”