What happened
On September 8, 2017, an Airbus MBB BK117 C-2 helicopter, registration N146DU, was conducting an air ambulance flight near Hertford, North Carolina. The flight was part of a mission to transport a patient to a medical center.
About eight minutes after departing from the Sentara Albemarle Medical Center Heliport, the helicopter began a left turn toward the south. During this period, the No. 2 engine experienced a rear bearing seizure. This mechanical failure likely produced several cockpit warnings, including an engine chip detection and an engine parameter discrepancy.
Evidence suggests the pilot likely misdiagnosed these indications and erroneously shut down the No. 1 engine. The helicopter continued to fly briefly using only the No. 2 engine. Shortly thereafter, the No. 2 engine lost all power. The aircraft entered a near-vertical descent and impacted a drainage pathway. The crash resulted in 4 fatal injuries, including the pilot, two flight nurses, and the patient.
The investigation
Investigators examined the wreckage and found that the No. 2 engine's gas generator shaft rear bearing was severely damaged, with all roller elements found seized, flattened, and overheated. Metallic debris was discovered in the engine's oil return strainer and chip detector.
Analysis of the cockpit controls revealed that the No. 1 engine throttle was in the OFF position, while the No. 2 engine throttle remained in the FLIGHT position. This confirmed the pilot had manually shut down the functioning engine.
Regarding the engine instrumentation, a simulation performed by the manufacturer indicated that the bearing failure would cause the First Limit Indicator (FLI) to switch from displaying torque to displaying turbine outlet temperature. This change would create a large, unexpected split between the two engine needles on the display, which could be confusing to a pilot.
Maintenance records showed that while the concentration of iron in the No. 2 engine oil was below the manufacturer's alert threshold, the levels were significantly higher than those in the No. 1 engine and had fluctuated notably in the months prior to the accident.