Incident Overview
During a cruise at 10,500 feet, the pilot heard a bang followed by a loss of engine power. The pilot declared an emergency and attempted to glide to a nearby airport but was unable to reach the runway. The airplane subsequently collided with power lines and fell to the ground approximately 4 miles west of the airport.
Engine Examination
Post-accident inspection revealed that the N1 and N2 shafting systems were bound up and would not move. The upper magnetic chip detector contained no metal but did show coke deposits, with slight varnishing of the anode and cathode. The lower magnetic chip detector contained a substantial amount of metallic particles, later identified as material from the No. 5 bearing; these particles bridged the gap between anode and cathode.
A detailed examination of the engine determined that three sections of the first stage turbine wheel had separated. The stub shaft was also fractured and separated, with a portion of the fractured tie bolt remaining in the wheel.
Metallurgical Findings
Metallurgical examination of the first and second stage turbine wheels revealed evidence of gamma prime solutioning and localized melting of the second stage turbine wheel airfoil tips, indicating temperature exposures above 2,100 degrees Fahrenheit—a range outside the normal operating envelope. This elevated temperature exposure resulted in rim separation due to thermal fatigue initiating at the rim, followed by accelerated interdendritic creep crack growth.
No evidence of an oil fire was associated with the first stage wheel separation. The composition, hardness, and microstructure of the first stage turbine wheel met engineering drawing requirements. The No. 5 bearing exhibited heat distress, but the cause of this could not be conclusively determined.
According to Rolls-Royce Allison, the internal evidence of excessive thermal damage to the first and second stage turbine wheels was consistent with prior examples of known hot starts.