Kathmandu, Sept. 3 -- The Bhotekoshi floods have changed the landscape that helicopter pilots once relied on to navigate the valleys of Rasuwa and Nuwakot. Roads, riverbanks and familiar landmarks have disappeared, while steep terrain cuts off radio signals from ground-based control towers.

Yet rescue and relief flights have continued. In many of the worst-hit areas, pilots are relying on a system they have largely built among themselves: continuous radio communication, shared information about other aircraft and strict flying discipline to navigate the narrow valleys safely.

The affected areas of Rasuwa and Nuwakot lie in "uncontrolled airspace", according to Debendra Prasad Shrestha, chief air traffic controller at Tribhuvan Internati...