The flight was to be flown across a 22 mile channel between islands on an overcast night with no moon and no ground reference lights. The aircraft disappeared from radar and, after extensive searches, could not be found. A three dimensional flight track was reconstructed using recorded radar data. The data indicated that in the last 60 sec the aircraft slowed from 170 to 95 kts, gained 500 ft in altitude, and turned left 190 degrees before abruptly disappearing from radar. Flight tests indicated that the movements of the aircraft in the last 60 sec of the flight were consistent with a loss of the left engine, without compensation by the pilot. The operator's training program did not provide for night or instrument flight conditions. The last documented instrument time for the pilot was 15 months prior during a checkride. The pilot flew sporadic night flights. The pilot had previously flown during the day and was on his thirteenth hour of duty. Aircraft damage and injury index presumed.