What happened
While being positioned to another airport, the crew of Moku 979 observed an airspeed anomaly relative to the power setting and noted that the flap indicator suggested a slight deployment of the flaps. The flying captain requested that a deadheading captain on board inspect the cabin window to verify the flap position; the deadheading captain confirmed the flaps were fully retracted.
Following a request from Air Traffic Control (ATC) to begin a descent, the flight crew reduced power to slow the aircraft. As the deadheading captain was returning to his seat, the Moku 979 encountered severe turbulence lasting approximately 5 to 10 seconds. The impact threw the crewmember about the cabin, resulting in serious injuries. The flying captain declared an emergency, and the aircraft landed without further incident. The injured crewmember was transported to a hospital from the gate.
The investigation
A review of ATC air-ground recordings between Maui approach control (APCON), two other aircraft in the area, and the accident aircraft was conducted. Prior to the arrival of Moku 979 at the approach control, two other aircraft reported moderate turbulence at 4,000 feet and moderate to occasional severe turbulence between 2,000 and 2,500 feet. Both aircraft were located approximately 15 miles east of Maui, near the location of the accident. While the controller contacted Flight Service Station (FSS) to advise them of these pilot reports, the controller did not advise Moku 979 of the reported turbulence when the aircraft checked in or when instructions for descent were issued.