The pilot's misjudgment of distance and altitude from the runway, and his subsequent failure to maintain clearance from the terrain. Factors contributing to the accident were dark night conditions, the pilot's improper decision to conduct the flight at night, his lack of recent experience in the type of operation (night flying), his lack of familiarity with the geographical area, the inoperative airport visual approach slope indicator, and the improper dispatch of the airplane by the operator.