The Incident
A medical transport flight was en route to pick up a patient on a neighboring island, operating under an instrument flight rules (IFR) flight plan in dark night conditions over the ocean. About 13 minutes after departure, while at 13,000 ft mean sea level (msl), the airplane's vertical gyro failed. This caused the pilot's Electric Attitude Director Indicator (EADI) to fail and the autopilot to disconnect.
The failure required the pilot to manually fly the airplane using the copilot's attitude gyro for horizon information (bank angle and pitch attitude) for the remainder of the flight. The pilot did not declare an emergency and did not inform air traffic control (ATC) about the failed electric attitude indicator or the autopilot disengagement.
Sequence of Events
After the instrumentation failure and autopilot disconnect, the airplane entered a series of right banks before being brought back to level, followed by a left turn, and then subsequent right banks. ATC asked the pilot to change course, and the pilot agreed. The copilot attitude indicator showed that the airplane entered a descending, steep right bank turn.
Over the next 5 minutes, ATC issued varying instructions to the pilot. During this time, the airplane entered several right- and left-hand banks and rolls and descended at 1,000 ft per minute (fpm). The descent rate increased to -3,500 fpm as airspeed increased. Approximately 7 minutes after the instrumentation failure, the airplane was in a 65° bank angle when ATC asked the pilot to verify his heading. As the pilot responded, the bank angle increased to 90° and airspeed exceeded 260 knots. The bank angle and airspeed continued to increase; a loud metallic bang was recorded, consistent with an in-flight separation of the empennage from the fuselage before impact with the water.
Recovery and Examination
After an extensive underwater search, the main wreckage was located on the seabed at a depth of about 6,420 ft. The wreckage was recovered and transported to a facility for examination.