Helicopter Tail Boom Severed During Forced Landing After Engine Failure (N1106Z)

Incident Overview

A helicopter pilot reported flying straight and level at approximately 700 feet above ground level when the engine-out light illuminated. The pilot initiated an autorotation towards a ridgeline. During the landing, the main rotor blades contacted the tail boom, severing it from the main fuselage.

Investigation Findings

An investigation revealed several issues with the engine. The engine burner drain valve exhibited an air leakage of 3000 cc/min, exceeding the maximum allowable limit of 800 cc/min (at 20 psi). The combustion liner showed excessive heat distortion adjacent to a large diameter hole on the right side of the skirt. The first-stage turbine nozzle had severe heat distress on 14 trailing edge vanes and 11 leading edge vanes. The fuel nozzle had heavy carbon deposits on the spray tip and air shroud. Additionally, a white residue was found in the engine, and a fuel sample contained 425 parts per million of phosphorus.