SEA01TA143

While cruising at 1,000 feet above the ground, the helicopter encountered an area of strong updrafts and downdrafts. As it passed through one particularly strong downdraft, the aircraft experienced a complete loss of power. The pilot then initiated an autorotation to the only available landing area. Because the area was small and surrounded by trees, the pilot found it necessary to execute an abrupt flare in order to arrest the forward movement of the helicopter prior to colliding with the trees ahead. The abrupt flare resulted in a hard landing, and the main rotor blades flexed downward far enough to sever the tail boom. Post-accident inspections of the engine and aircraft systems did not reveal any malfunctions or anomalies that would have been expected to contribute to a loss of power. Although the flight manual warned of possible fuel starvation associated with large side slip angles of uncoordinated flight with less than 35 pounds of fuel remaining, approximately 115 pounds of fuel were drained from the helicopter's main fuel bladder.

On July 24, 2001, approximately 1600 Pacific daylight time, the tail boom of a Boeing H500D helicopter, N8964T, separated during a power-out autorotational landing about three miles south of Union Gap, Washington. The pilot and his two passengers were not injured, but the aircraft, which is owned and operated by Omega Air, sustained substantial damage. The local Public Use flight, which departed Yakima Air Terminal about 1320, was being operated in visual meteorological conditions. No flight plan had been filed. There was no report of an ELT activation.

According to the pilot, as he passed through Union Gap while returning to Yakima at 1,000 feet above ground level (AGL), at an indicated airspeed of 80 knots, the aircraft encountered a series of strong updrafts and downdrafts. As it flew through one particularly strong downdraft, the helicopter pitched nose down and the engine stopped producing power. Because he was crossing the Yakima River at the time of the occurrence, the pilot immediately initiated an autorotational descent to an island in the middle of the river channel. The only landing area available was small and tightly ringed by trees. During the power-off landing, after clearing the trees and entering the landing area, the pilot found it necessary to flare abruptly in order to arrest the forward movement of the aircraft prior to impacting the trees ahead. As a result of the abrupt flare, the helicopter touched down hard, and the main rotor blades flexed downward to an extent that allowed them to sever the tail boom.

Post accident inspections of the airframe, engine, and aircraft systems did not reveal any malfunctions or anomalies that would have been expected to contribute to a loss of engine power. Although the 500D flight manual warns that fuel starvation can occur if large slip angles and uncoordinated maneuvers are performed with less than 35 pounds of fuel remaining, after the accident, approximately 15 gallons of fuel was drained from the main fuel tank of the aircraft.