On December 14, 1998, a Bell 47-G5 (registration N8103J) was involved in an aviation accident near Essex Junction, VT. Investigators recorded the probable cause as: the pilot's improper entry into the autorotation, which resulted in the loss of power and a low rotor rpm condition. Also causal was the pilot's improper execution of the autorotation which resulted in the hard landing. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
During a maintenance test flight after an engine overhaul, a helicopter experienced a hard landing when the pilot executed an autorotation and the engine stopped. No anomalies were found in the engine after examination.
Flight Sequence
The pilot and mechanic conducted a maintenance test flight after completing the installation of an overhauled engine. They performed ground engine runs and two short legs of the test flight. After each leg, the helicopter was shut down and the engine was checked for leaks. The pilot departed on the third leg after official sunset. After climbing to approximately 1,700 feet, the pilot closed the throttle and entered autorotation. The mechanic reported that the engine and rotor rpm decayed simultaneously, after which the engine stopped running. During the descent, the pilot attempted to restart the engine without success. The autorotation was terminated with a hard landing.
Engine Examination
Examination of the engine revealed no anomalies. The engine ran to rated power in a test cell. Additionally, while operating at full rpm, the throttle was moved rapidly to the flight idle position. The engine exhibited a smooth reduction in rpm to the idle power setting with no interruption. An FAA Helicopter Handbook stated that to begin an autorotation, the collective should be placed full down, maintaining cruise rpm with the throttle, then decreasing the throttle and adjusting it to keep the engine rpm well above normal idling speed.