THE PILOT WAS NEARING HIS DESTINATION WHEN HE NOTICED THE SKY WAS DARKENING NORTH OF THE AIRPORT. THE PILOT DECIDED TO TURN AROUND AND HEAD BACK TO WHERE HE DEPARTED. HE SAID THAT AFTERWARDS HE ENCOUNTERED A DOWNDRAFT AND HE WAS UNABLE TO MAINTAIN ALTITUDE. CONSEQUENTLY, THE AIRCRAFT DESCENDED AND STRUCK THE EAST SIDE OF A MOUNTAIN.