BELL 206 near Sevierville, TN — 2015-02-15

Probable cause

An uncontained engine failure, which resulted from insufficient lubrication of the No. 4 bearing due to carbon buildup on the filter screen of the bearing's oil supply nozzle.

NTSB narrative

The commercial pilot was departing in the helicopter for a local aerial sightseeing flight with five passengers. The engine was operating during the boarding process. After the passengers were seated and had fastened their seat belts, the pilot increased the engine power to 100%, raised the collective, and felt the helicopter shudder. He lowered the collective and looked to make sure all passengers were seated. He raised the collective a second time, again felt the shudder, lowered the collective, and then heard "a loud bang." He subsequently turned off the fuel to shut down the engine. A fire began in the engine compartment, and ground personnel helped evacuate the passengers and extinguish the fire. Multiple fragments of engine turbine section components were found resting in the engine bay, on the ground around the helicopter, and embedded in the bottom surfaces of the main rotor blades. Postaccident examination of the engine revealed that the power turbine shaft had decoupled due to a No. 4 bearing failure. Carbon buildup was found on the filter screen of the No. 4 bearing's oil supply nozzle that partially obstructed oil flow to the bearing and likely resulted in insufficient lubrication of the bearing. The No. 4 bearing inner race spun on the bearing journal instead of remaining stationary as designed. The turbine rotates at extremely high speeds, and the resistance from the failed bearing caused the power turbine pinion splines to decouple, leading to an instantaneous power turbine overspeed. The overspeed led to a third stage turbine disk burst and radial uncontainment of fragmented power turbine components through the exhaust support and airframe. All turbine component fractures were due to overload failure, and there was no evidence of fatigue. Therefore, it is likely that the uncontained engine failure was the result of insufficient lubrication of the No. 4 bearing due to carbon buildup on the oil supply nozzle's filter screen.

Analysis

Primary failure mode

Mechanical failure

First missed decision gate

Pilot could have aborted takeoff after first shudder instead of increasing power again.

NTSB coding

• Uncontained engine failure · Takeoff
• Maintenance

Evidence available

• Photos
• 11 docket documents

Docket documents11

• Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1form
• Release of Aircraft Wreckage, NTSB Form 6120.15form
• Statement of Party Representatives to NTSB Investigationinterview
• Airframe Maintenance Records [Excerpts]other
• Engine Maintenance Records [Excerpts]other
• Engine Serial Number CAE-860606 Oil Sample Analysis Resultsother
• M250 SIII/SIV No. 4 Bearing Historyother
• Memorandum for Record - Gatlinburg Weatherother
• Photo Arrayphotos
• Powerplant Factual Reportreport
• Rolls Royce Metallurgical Investigation Report- 15FA8-047report