Incident Description
A helicopter operating at an altitude between 150 to 200 feet above ground level and an airspeed of 35 knots during the transition from an out-of-ground-effect hover to forward flight experienced a partial loss of engine power. Approximately 10 seconds after transitioning to forward flight, the pilot sensed the engine turbine spooling down toward idle and observed a red 'GOV' light illuminated on the instrument panel. The helicopter subsequently collided with the ground in a parking lot.
Engine Power Loss
The partial loss of power was caused by an inconsistency greater than 3% between the two N2 (free turbine speed) sensors on the engine. This discrepancy triggered the fuel control system to enter a fixed mode, which froze the fuel flow at the level present when the inconsistency was detected. The fixed fuel flow provided insufficient power to sustain flight and landing at speeds below 35 knots, leading to low rotor RPM and a high rate of descent.
Pilot Actions
Upon recognizing the red 'GOV' light, the pilot did not disengage the flight notch on the collective twist grip to manually control fuel flow to the engine. Instead, he flew the helicopter approximately 1/8 mile toward the parking lot, manipulating the collective in an attempt to maintain altitude above trees and house roofs. He used all available rotor RPM in an effort to arrest the landing.
Outcome
The helicopter landed hard, collapsing the landing skids, and slid approximately 100 feet before coming to rest. After stopping, the main rotor continued rotating at a slower RPM, and the engine operated at what seemed a low idle speed. The pilot was operating within the avoidance zone of the documented height-velocity diagram at 150 to 200 feet and 35 knots, which may have limited the time available to fully respond by selecting an emergency landing site, disengaging the twist grip flight notch, and exercising manual engine control.