What happened
On April 26, 2018, a Eurocopter AS350 B2 helicopter, registration N127LN, was involved in a fatal accident near Hazelhurst, Wisconsin. The aircraft was operating a repositioning flight for Air Methods Corporation under night visual meteorological conditions.
Prior to the accident, the crew had completed two medical transport flights and a refueling stop. The helicopter departed Dane County Regional Airport (MSN) at approximately 21:07. On-board recordings from the Appareo Vision 1000 recorder captured the pilot conducting a preflight with the engine running, noting no mechanical anomalies. During the flight, the pilot and medical crew engaged in various conversations, and the pilot was observed performing movements such as flexing his legs and adjusting his seating position.
At approximately 22:43, while flying at 2,280 feet MSL with an airspeed of 126 knots, the helicopter entered a right bank. The roll rate increased rapidly, eventually exceeding 90 degrees. The aircraft became inverted, and the torque gauge indicated a value beyond the red line. The helicopter descended rapidly, losing altitude at a rate of approximately 5,000 feet per minute, before impacting the ground. The wreckage was located in a wooded area, and the cockpit and cabin area were destroyed.
The investigation
Investigators examined the wreckage and the aircraft's flight data. The engine was found separated from the fuselage, and the compressor blades showed damage consistent with foreign object ingestion. Examination of the module 5 reduction gearbox revealed it had migrated rearward from its installed position.
Mechanical testing of the hydraulic pump and main rotor actuators showed no evidence of pre-impact failure or defects. The Appareo unit recorded no light or audio warnings indicating a mechanical malfunction, nor was there evidence of a bird strike.
Analysis of the pilot's activity via cellular records showed he had been active since 07:25 on the day of the accident. While the pilot had a history of adequate sleep in the preceding week, the investigation noted the flight occurred during a period when the pilot's circadian rhythm would have been adjusted for sleep following a week-long vacation. The environment, characterized by night flight, helicopter vibrations, and low operational demands during the cruise phase, was noted as a factor in the pilot's fatigue.
Findings
- The pilot's loss of control was a result of fatigue during the cruise phase of flight.
- The pilot's movements, including leg flexing and seat adjustments, were consistent with signs of fatigue.
- The flight environment, including darkness, vibration, and monotonous flight conditions, increased the biological desire to sleep.
- There were no mechanical failures or malfunctions that contributed to the accident.