The Catalina was flying over the Greenland Ice Cap at an altitude of 4500 ft. This altitude should have given a clearance of 1000 ft over terrain according to charts of the area. Weather and snow covered terrain resulted in poor depth perception. Without realizing they were close to the ice, as a horizon was visible and were not on instruments, they suddenly made contact on the upslope of a 400 ft hill. The pilot immediately applied throttles in an attempt climb away but he was unsuccessful. Further attempts to swing plane around in order to takeoff downhill were unsuccessful as the plane sank into snow and ice. This crew were stranded on the ice cap for fifteen days before being rescued.