On October 8, 1999, a Mcdonnell Douglas 369E (registration N991SD) was involved in an aviation accident near Moreno Valley, CA. Investigators recorded the probable cause as: The failure of the pilot to maintain sufficient forward airspeed to preclude an inadvertent encounter with loss of tail rotor effectiveness, which resulted in a loss of yaw control. The pilot's failure to initiate timely remedial action is a factor. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A Sheriff's Department helicopter experienced a loss of control during a night operation, leading to a hard landing that severed the tail boom.
Incident Details
A helicopter operated by a Sheriff's Department was conducting an orbit over ground-based law enforcement activity on a dark night. Winds aloft were reported at 20 to 25 knots at 500 feet above ground level.
Sequence of Events
The pilot was attempting to direct the helicopter's spotlight onto a vehicle. During the first two orbits, the wind caused the helicopter to drift past the target location. On the third orbit, the pilot inadvertently allowed the helicopter to slow below translational lift speed. This resulted in an uncontrollable 180-degree right yaw. The pilot momentarily regained directional control, but the right yaw resumed. Despite several further recovery attempts, the pilot observed decreasing altitude and proceeded to a pre-selected landing zone.
Landing and Damage
The helicopter landed hard while in a right yawing spin. During the landing, the main rotor blades struck and severed the tail boom. The helicopter subsequently came to rest on its right side.