NTSB Identification: LAX05LA025.
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Accident occurred Tuesday, November 02, 2004 in Sierra Vista, AZ
Probable Cause Approval Date: 03/28/2006
Aircraft: Eurocopter AS350B3, registration: N106LN
Injuries: 3 Uninjured.
NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.
The helicopter experienced a partial loss of power during the transition from an out of ground effect hover to forward flight, and collided with the ground in a parking lot. About 10 seconds after the transition to forward flight, at 35 knots and between 150 to 200 feet above ground level, the pilot sensed the engine turbine start to spool down towards idle. He saw a red 'GOV' light illuminated on the instrument panel. The pilot did not disengage the flight notch on the collective twist grip to manually control the fuel flow to the engine and proceeded to fly the helicopter about 1/8 mile towards the parking lot, working the collective to try to keep the helicopter above the trees and the roofs of the houses. He used all the available rotor rpm in an attempt to arrest the landing. The helicopter landed hard, collapsing the landing skids, and the ship slid about 100 feet. After the helicopter came to rest, the main rotor was still rotating at a slower rpm, and the engine was operating at what seemed like a low idle speed. The partial loss of engine power was initiated by an inconsistency of greater than 3 percent between the two N2 (free turbine speed) sensors on the engine. This inconsistency resulted in the fuel control entering a fixed mode that freezes the fuel flow at the level it was at when the inconsistency was detected, and the illumination of a red 'GOV' light in the cockpit. The power required for forward flight is less than the power required to hover. The fixed fuel flow to the engine provided insufficient power to fly and land the helicopter at speeds below 35 knots, which resulted in low rotor rpm and a high rate of decent prior to the hard landing. In the event of a red 'GOV' light the pilot is to disengage the flight notch and control the fuel manually with the collective twist grip, which would allow the pilot to regain the full power range of the engine. At an altitude of 150 to 200 feet and airspeed of 35 knots, the pilot was operating in the avoidance zone of the documented height-velocity diagram, and therefore, may not have had enough time to fully respond to the partial loss of power by selecting an emergency landing site, disengaging the twist grip flight notch, and exercising manual control of the engine.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: An inconsistency between the engines' two N2 sensor signals caused the fuel control to enter a fixed mode, which limited the power available. Contributing to the accident was the lack of altitude and airspeed available to give the pilot time to select a landing zone, disengage the flight notch, and exercise manual control of the engine after the illumination of the red 'GOV' light. Full narrative available
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