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On March 7, 2009, about 1000 Pacific standard time, a MD Helicopter 369D, N501EF, collided with terrain near Trona, California. Airwest Helicopters, Inc., was operating the helicopter under the provisions of 14 Code of Federal Regulations (CFR) Part 135. The airline transport pilot and two passengers sustained minor injuries; the helicopter sustained substantial damage to the airframe. The cross-country, on-demand, air taxi flight departed the Wildrose National Park’s heliport near Inyokern, California, about 0945. The purpose of the flight was to transport maintenance personnel to a radio tower. Visual meteorological conditions (VFR) prevailed, and a company VFR flight plan had been filed.
The pilot departed Redlands, California, and stopped to refuel at Inyokern. He departed Inyokern, and landed at the heliport to load equipment. He then flew 6 miles east to a radio facility on top of Rogers Peak, elevation 9,993 feet. He noted that the peak was at the intersection of two ridge lines; one ran north/south, and the other ran east/west. The north-to-east quadrant consisted of very steep terrain; the other quadrants formed a large ice bowl with grades varying from 15 to 30 degrees.
The pilot made one high reconnoiter right-hand turn around the summit. He assessed the wind as light and variable. He continued the circling approach from north to south, east of the ridge line and about 100 feet higher. He slowed from 60 knots to 40 knots so that he could assess his available power during the landing.
The pilot felt that the helicopter was performing adequately with the torque level in the top of the green operating range on the torque gage. The airspeed suddenly decreased to 20 knots, and the helicopter yawed 90 degrees to the right. He held the new heading, lowered the nose to gain airspeed, and held the same torque input as he attempted to go over the ridge in order to abort the landing.
The situation appeared stable until the helicopter went into a violent and uncontrollable right spin. The pilot indicated that he felt no response to his control inputs. He did not want to land in the ice bowl, and proceeded over the east/west ridge. He was unable to get forward airspeed and stop the spin. The helicopter spun down vertically, and he pulled all remaining pitch to cushion the impact.
The helicopter was on a southwesterly heading, and the main rotor blades made first contact followed by the skids. It rolled onto its right side, and slid about 75 yards before coming to a stop facing uphill. The right front passenger exited through where the bubble had been, and helped the rear passenger out. The pilot turned off the battery and fuel. He exited with the fire extinguisher, and put out a fire in the tailpipe.
One of the passengers held a rotorcraft/helicopter pilot certificate. He indicated that on the first landing attempt, the pilot appeared to be running out of left pedal. The second attempt was at a different angle to the landing zone, but once again the pilot seemed to run out of pedal, and the helicopter began to spin. The pilot couldn’t recover this time, and it made three turns before colliding with the terrain.
TESTS AND RESEARCH
Post accident examination of the wreckage revealed no anomalies with the engine that would have precluded normal operation.
Examination of the airframe revealed a fracture of a component of the tail rotor control rod. Metallurgical examination determined that the fracture was a result of ductile overload during impact. No anomalies were discovered that would have precluded normal operation.
The Rotorcraft Flying Handbook describes unanticipated yaw as an uncommanded yaw rate that does not subside on its own. It notes that, if not corrected, this can lead to a loss of control of the helicopter. This is referred to as a loss of tail rotor effectiveness. It indicates that this condition is not related to equipment or maintenance, and may occur in all single-rotor helicopters at airspeeds less than 30 knots. The tail rotor does not provide adequate thrust to maintain directional control. Common causes are certain wind directions during hover, or insufficient tail rotor thrust for a given power setting at higher altitudes, where tail rotor thrust and efficiency are reduced.