Hughes 369D N8356F accident near Greenville, Virginia

Probable cause

The inadequate design of the fourth-stage turbine wheel, resulting in the fatigue failure of one airfoil and a subsequent loss of engine power.

NTSB analysis

Following completion of aerial tree cutting near power lines, the pilot returned to the designated landing zone and, while maneuvering for landing after placing the saw on the ground, the engine experienced a loss of power. The pilot attempted to perform an autorotative landing but landed hard, causing substantial damage to the helicopter.

Postaccident inspection of the engine revealed one airfoil of the fourth-stage power turbine wheel fractured near the hub due to high-cycle fatigue that initiated on the pressure side of the airfoil at the trailing edge. No surface defects were noted along the root filet radius adjacent to the fracture initiation region. The enhanced fourth-stage power turbine wheel was manufactured in October 2006, installed into the turbine assembly, operated for 1,769 hours, then removed for a 1,750-hour inspection. The fourth-stage power turbine wheel remained installed in the turbine assembly at the completion of the inspection, and the turbine assembly was reinstalled onto the engine. At the time of failure, the wheel had accumulated approximately 2,690 hours since new and approximately 921 hours since the last 1,750-hour inspection was performed.

Since 2005, there have been four other fatigue failures of an airfoil of an enhanced fourth-stage turbine wheel. All other failures initiated at the trailing edge near the hub. In December 2006, Rolls-Royce developed an engine alert bulletin (CEB-A-1400) alerting owners and operators of the accident make and model engine to avoid steady state N2 speed between 75 to 88 percent above 85 shaft horsepower (shp). Additionally, Rolls-Royce reported that their research of failures of airfoils of the fourth stage power turbine revealed the possibility of a higher stress state in the airfoil at the trailing edge root than originally modeled. The stress, which occurs during engine start, results from thermal differentials in the airfoil geometry that produce a residual stress at the trailing edge root that can lead to a fatigue crack. Once a crack occurs, it can propagate in high-cycle fatigue followed by overload failure if the engine is operated in a steady state in the N2 speed avoidance range (75 to 88 percent N2) above 85 shp, as specified in CEB-A-1400.

Text from the NTSB. Open the NTSB docket for ERA09LA317 for the full record.