CEN18FA149 · Eurocopter AS 350 B2

The helicopter pilot and two emergency medical service crewmembers were on a repositioning flight in night visual meteorological conditions after two helicopter air ambulance flights and a refueling stop. The total flight time for the first three flights was 94 minutes and occurred over a period of about 2 ½ hours.

The helicopter departed on the accident flight about 2107. About 1 minute later, the pilot asked the crewmembers whether they were "alright." One crewmember responded "yup" and then asked the pilot, "question is are you alright up there?" The pilot responded, "uhhh think so. Good enough to get us home at least." There was no further discussion related to fatigue. During the flight, the pilot adjusted his seat position and flexed his legs, which were actions consistent with signs of fatigue. Also, although the pilot participated in the medical crewmembers' conversations in the middle of the flight, he did not participate in their conversations near the end of the flight. During the last portion of flight, the helicopter entered a progressively steepening right bank, and the pilot did not respond as the medical crewmembers shouted his name. The helicopter descended and became inverted, and the pilot continued not to respond as the crewmembers' shouted his name.

After the helicopter began to roll to the right, the pilot slumped to the left, appearing incapacitated. There was no evidence indicating that the pilot was suffering from a medical condition that could have caused his incapacitation. There was also no evidence of poor quality or quantity of sleep; the pilot's wife reported no issues with the pilot falling asleep or staying asleep, and cellular telephone records and his wife's interview indicated adequate sleep opportunity in the days preceding the accident. On the day of the accident, cellular telephone activity revealed two possible opportunities for the pilot to sleep before going on duty but it is not known if the pilot rested during those times. Thus, the pilot could have been awake for about 15.5 hours at the time of the accident (based on telephone records showing activity at 0725 the morning of the accident) if he did not take advantage of the sleep opportunities. Although this time since awakening would not be considered excessive, this accident shift was the pilot's first after returning from a week-long vacation during which his circadian rhythm would have had him sleeping. Further, the environment created by the helicopter vibration, darkness of night, and few operational demands during the cruise phase of flight would have increased the pilot's fatigue and the body's biological desire to sleep. These factors, along with the pilot's movements observed on the image recorder, indicated that the pilot became fatigued during the flight. Thus, he pilot likely fell asleep during the flight as a result of the time of day, his time since awakening, and the fatigue-inducing effect of the 1 ½ hour flight.

No light or audio warnings that would indicate a mechanical malfunction were observed during the accident flight. Postaccident examination and testing revealed no preimpact anomalies that would have precluded normal operation of the helicopter.

Verbatim NTSB analysis and probable cause from the NTSB dataset