Kathmandu valley is ringed by mountains, and does not allow a pilot much room for manoeuvre. Approaching the runway at more than 320km/h, the pilot has only 16km in which to nose his aircraft down from 2,900m, over rows of sharp ridges, to hit the runway. Too sharp a descent and the plane can end up smashed on one of the ridges. Too gradual, and the pilot overshoots the runway.
When the mountains are hidden in cloud, as they were last Monday when a Pakistan International Airlines flight crashed, killing all 167 on board, the pilot has to rely solely on his cockpit instruments and a guiding radio beam from the tower. Nepalese controllers do not have radar. Even sophisticated instruments, such as those in the PIA Airbus cockpit, are not infallible.
One theory as to why the crash occurred is that the ridges rise so steeply from the valley that the instruments could not warn the crew how close the airliner was to the mountain, shrouded in the mist, until it was too late. The plane's instruments can measure the distance from the ground, but not from what is ahead.
Once the plane dips into the sea of cloud, there is no indication how close the mountains are until the pine trees and the farmhouses on the ridges seem to be scraping against the bottom of the fuselage. Peaks give way sharply to deep valleys, terraced like a contour map with rice paddies and cornfields.
Once the pilot has cleared the last range of blue-ish hills, the runway speeds into view. The descent is so steep that the pilot must drop 1,600m within the last 1.5km before the runway. After threading down through the mountains, the pilot faces one last obstacle: at the end of the runway there are young Nepalese boys flying kites.