Accident Overview
On the day of the accident, a twin-engine airplane was in the initial climb phase after departure. Witnesses reported that the airplane rolled into a steep right bank and entered a spin at an altitude of less than 700 feet. The airplane then descended and impacted terrain approximately 1.5 miles from the end of the departure runway. Day visual meteorological conditions prevailed at the time.
Witness Observations
Several witnesses stated that they observed the airplane's unusual maneuver. Some witnesses reported hearing an unusual engine noise just before the airplane began to roll and spin. No further details regarding the nature of the noise were provided.
Engine Examination
Examination of the right engine revealed that the ring gear support of the engine/propeller gearbox had fractured in flight. The fracture was due to high cycle fatigue originating from the corner radii of the high-speed pinion cutout. The specific reason for the fatigue could not be determined. The failure of the ring gear support caused it to disengage from the ring gear, resulting in a disconnection of power transfer from the engine power section to the propeller.
Propeller and Drag Reduction Systems
The engine, a Honeywell TPE-331, includes systems for manual and automatic feathering of the propeller, as well as a Negative Torque Sensing (NTS) system. The NTS system is designed to automatically respond to a typical failed engine condition where the propeller is being driven by the engine. Feathering the propeller reduces drag and asymmetric yawing caused by the failed engine. All Federal Aviation Administration (FAA) certification evaluations for one-engine-inoperative handling qualities for this airplane type were conducted with the NTS system operational.
According to the airplane manufacturer, the NTS system was intended to automatically reduce drag on the affected engine, providing a margin of safety until the pilot could shut down the engine using the condition lever. However, if a drive train disconnect occurs at the ring gear support, the NTS system becomes inoperable. In such a scenario, if the pilot subsequently retards the power lever on the affected engine, the propeller can come out of feather on its own. Once the fuel flow setting is reduced below the point required to maintain 100% (takeoff) rpm, the propeller governor senses an underspeed condition and attempts to increase engine rpm by unloading the propeller. This action drives the propeller out of feather toward the low pitch stop, resulting in a flat pitch condition. This flat pitch condition causes a significant increase in aerodynamic drag on one side of the airplane, leading to unanticipated control difficulty due to asymmetry.