Alpine Air flight 253 departed Honolulu (HNL) at 04:43 carrying about 4,200 pounds of mail on a regularly scheduled flight to Kauai, Lihue (LIH). It was scheduled to arrive at Lihue at 05:15. The pilot had routine contact with air traffic control, and was advised by the controller to maintain 6,000 feet at 05:01 when the airplane was 11 miles from the destination airport. Two minutes later the flight was cleared for a visual approach to follow a preceding Boeing 737 (Aloha Airlines Flight 917) and advised to switch to the common traffic advisory frequency at the airport. The pilot altered his flight course to the west, most likely for spacing from the airplane ahead, and descended into the water as he began a turn back toward the airport. The majority of the wreckage sank in 4,800 feet of water. The pilot most likely descended into the ocean because he became spatially disoriented. Although visual meteorological conditions prevailed, no natural horizon and few external visual references were available during the visual approach. This increased the importance of monitoring flight instruments to maintain awareness of the airplane attitude and altitude. The pilot’s tasks during the approach, however, included maintaining visual separation from the airplane ahead and lining up with the destination runway. These tasks required visual attention outside the cockpit. These competing tasks probably created shifting visual frames of reference, left the pilot vulnerable to common visual and vestibular illusions, and reduced his awareness of the airplane’s attitude, altitude and trajectory. PROBABLE CAUSE: “The pilot’s spatial disorientation and loss of situational awareness. Contributing to the accident were the dark night and the task requirements of simultaneously monitoring the cockpit instruments and the other airplane.”