The skies over Calcutta are becoming as congested, and probably just as unsafe, as its roads. Multiple flights on one route, skeletal infrastructure and an obsolete airspace management plan have combined to bring down the safety level of air travel to and from the city by several notches, experts say. It's not that the civil aviation ministry is not spending money on modernisation. As much as Rs 2,000 crore has been set aside for Netaji Subhash Chandra Bose International Airport, but most of it is for passenger amenities on the ground rather than technology for safer skies. "In the past few years, there has been a manifold increase in the number of flights to and from Calcutta and those flying over the city. The city's airspace needs to be restructured immediately,' a senior airport official said. An airspace management committee set up by the Airports Authority of India did compile a to-do list three months ago, but there has been no official word on implementing the plans. Pilots and Air Traffic Control (ATC) officials have long been asking for demarcation of air corridors for busy routes like Delhi, Mumbai and the Northeast. Flights on these routes are often forced to wait long for take-off or hover over the airport for permission to land. "Separate arrival and departure corridors are immediately required for certain routes. High traffic increases safety hassles, like aircraft coming dangerously close to each other or mistakes in calculating climbing and descending levels of two aircraft,' an ATC official said. Calcutta is the only metro where one-way air corridors have not been demarcated. Another recommendation of the airspace management committee is to change the procedure for coordination between the control tower and the approach radar. This comes into play whenever a flight lands or takes off. "Personnel manning the control tower have to seek permission from their counterparts monitoring the approach radar before allowing any change in flight altitude. A lot of time is wasted on this,' the official said. The same holds true for coordination between the approach control and area control radar operators. The area control radar takes over surveillance when a flight is 60 nautical miles from the airport and at a minimum height of 14,000 ft. "Some of the air routes are now very close to each other. It wasn't a problem when flights were fewer. But the proximity between planes on some routes is now too close for comfort,' the official said. Delhi, Patna and Kathmandu are the routes closest to each other. They are also among the busiest in the country. The minimum separation between two routes should be 15 degrees, but it is much less on these routes. "On air routes close to one another, near misses happen frequently. The chance of a mid-air collision can be eliminated only through traffic collision avoidance systems in each aircraft,' the official said.