What happened
During the transition from an out-of-ground-effect hover to forward flight, the helicopter experienced a partial loss of power. Approximately 10 seconds after transitioning to forward flight, while traveling at 35 knots and at an altitude between 150 and 200 feet above ground level, the pilot observed the engine turbine spooling down toward idle. 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 fuel flow. Instead, the pilot continued flying the aircraft approximately 1/8 mile toward a parking lot, adjusting the collective to attempt to maintain altitude above trees and rooftops. Utilizing all available rotor rpm to arrest the descent, the aircraft performed a hard landing that collapsed the landing skids. The aircraft then slid approximately 100 feet before coming to rest. At the time the aircraft stopped, the engine was operating at a low idle speed and the main rotor continued to rotate at a reduced rpm.
The investigation
The investigation determined that the partial loss of power was caused by an inconsistency of greater than 3 percent between the two N2 (free turbine speed) sensors on the engine. This discrepancy caused the fuel control to enter a fixed mode, which froze the fuel flow at the level present when the inconsistency was detected and triggered the red 'GOV' light in the cockpit.
Findings
Because the power required for forward flight is less than that required for a hover, the frozen fuel flow provided insufficient power to maintain flight or land the aircraft safely at speeds below 35 knots. This led to low rotor rpm and a high rate of descent prior to impact.
Additionally, at the altitude and airspeed recorded, the pilot was operating within the avoidance zone of the documented height-velocity diagram. Consequently, there may have been insufficient time to respond to the power loss by selecting an emergency landing site, disengaging the twist grip flight notch, and implementing manual engine control.