Mcdonnell Douglas Helicopter 369E N801SB incident near Allentown, Pennsylvania

Probable cause

The fatigue failure of a blade in the second-stage turbine wheel, which resulted in a total loss of engine power.

NTSB analysis

The helicopter experienced a total loss of engine power in flight and the pilot performed an off-airport landing in a hayfield. The pilot was uninjured and the helicopter was not damaged. Examination of the exhaust stacks revealed minor internal impact damage, consistent with high velocity objects exiting the engine. The fourth-stage turbine wheel outer ring was missing material along an approximately 30-degree arc and could not be rotated by hand. The engine had been overhauled at a facility in Canada and installed on the incident helicopter by the operator approximately 6 flight hours prior to the event. Several parts of the engine were removed and sent to the Safety Board's Material Laboratory for examination. Examination of the second-stage turbine wheel revealed a fatigue fracture through a blade. The fatigue progressed through approximately half of the airfoil, and the origin was located in the transition radius between the wheel rim outer diameter and the blade near the junction of a fore-to-aft mold line that was created during original manufacture. The blade failed due to fatigue, and the oxidation of the blade fracture surface suggested that the fatigue was due to thermal loads. No metallurgical or mechanical discontinuities were noted at the surface. Examination of the other 23 blades on the same wheel revealed small cracks in the same general location as the fatigue origin on the fracture blade. The fuel nozzle spray pattern was within specification, and there was no indication of flame streaking throughout the turbine components. Although a crack at the base of the blades on the second-stage power turbine wheel was a relatively new location, and similar failures had the same performance modifications performed, the investigation was unable to identify a connection between the performance modifications and the fatigue at the base of the blade.

Text from the NTSB. Open the NTSB docket for ERA09IA499 for the full record.