On June 22, 1999, a Hughes 369D (registration N1089Y) was involved in an aviation accident near Shelton, WA. Investigators recorded the probable cause as: The pilot's failure to maintain rotor RPM during an autorotation. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
During a practice autorotation, a helicopter's rotor RPM dropped below 410, leading to a rapid descent and collision with the ground. The aircraft sustained substantial damage, and no mechanical failures were reported.
Accident Description
A pilot was practicing autorotations from an altitude of approximately 200 feet above ground level (AGL) and an airspeed of approximately 50 knots. During the second practice autorotation, the pilot reduced power with the throttle and used collective control to adjust the rotor speed to 410 RPM, which is the minimum rotor speed for practice autorotations. Subsequently, the rotor RPM decreased below 410 RPM, and the helicopter began a rapid descent. Before the pilot could recover, the aircraft collided with the ground and rolled over.
Damage
The accident resulted in substantial damage to the main rotor blades, tail rotor, tail boom, and fuselage of the helicopter.
Pilot Report
The pilot reported that there were no mechanical failures or malfunctions with the aircraft at the time of the accident.