Piper PA-18AS N1905A accident near Tyonek, Alaska

Probable cause

The noninstrument-rated pilot's decision to initiate a visual flight rules flight into dark night, marginal visual flight rules to instrument flight rules conditions, which resulted in a loss of control due to spatial disorientation.

NTSB analysis

The noninstrument-rated private pilot had spent the previous 4 days and nights conducting a solo sheep hunting trip and was returning home when the accident occurred. Before departing the remote, mountain airstrip, the pilot contacted a Federal Aviation Administration (FAA) Flight Service Station (FSS) on his satellite phone for a weather briefing and asked whether visual flight rules (VFR) flight was recommended for his route that night.

The flight service specialist (FS-S) provided the pilot with the terminal area forecast for the destination airport; information from the current Area Forecast, which indicated cloud ceilings around 5,000 ft mean sea level (msl), marginal VFR conditions, and rain; and a pilot report from a nearby mountain pass, which advised that VFR was not recommended. The pilot asked several times about a specific mountain pass, and although the FS-S described the conditions at that pass based on FAA weather camera images and expressed pessimism about the prospect of the pilot attempting VFR flight, she did not provide the pilot with the information contained in the Area Forecast for the pass, which indicated instrument flight rules conditions, rain, and mist, as well as isolated moderate turbulence in the area below 6,000 ft msl. The FS-S also did not provide weather radar information, which showed the intended route of flight under an extensive area of precipitation, including areas of moderate or greater intensity. Despite the pessimism of the FS-S on the prospect of the pilot attempting a VFR flight to his intended destination that night, had the FS-S provided the current forecast information for the specific pass and the regional turbulence found in the Area Forecast, as well as a description of the current weather radar depiction for his intended route of flight, the pilot would have had a more accurate picture of the weather over the route of flight.

Data obtained from an onboard GPS unit for the last several minutes of flight showed the airplane conduct two descending, spiraling turns. The airplane continued to descend before the data terminated. The airspeed during the last several minutes of the flight ranged from 49 knots to 82 knots.

The pilot departed on the flight about 10 minutes before sunset. Twilight conditions during much of the flight would have provided the pilot with some illumination from which to see; however, cloud cover would have decreased the amount of illumination during this period, as would the end of civil twilight, which occurred about an hour after takeoff. Thus, dark night conditions would have existed for at least the last 14 minutes of the flight, as the flight proceeded over a remote area devoid of cultural lighting. The dark night conditions, lack of available ground lighting, and possible instrument meteorological conditions present at the time were conducive to the development of spatial disorientation, and the airplane's flight track is consistent with the known effects of spatial disorientation. Whether the pilot may have been experiencing fatigue before and/or during the flight given his hunting activities of the previous 4 days, it could not be determined based on the available information.

Based on the available radar weather and airplane track data, it is likely that the airplane flew into or came very close to an area of moderate or greater precipitation just before the accident; however, the extent to which the weather contributed to the pilot's spatial disorientation could not be determined.

The pilot chose to depart at a time that would have required him to operate in dark night conditions with a lack of cultural lighting, eliminating his reference to a visual horizon and requiring reliance on the airplane's flight instruments for attitude control. The dark night conditions would also have precluded the detection and avoidance of weather.

Text from the NTSB. Open the NTSB docket for ANC17FA049 for the full record.