Safety Recommendation A-18-014

On October 2, 2016, about 1157 Alaska daylight time, Ravn Connect flight 3153, a turbine-powered Cessna 208B Grand Caravan airplane, N208SD, collided with steep, mountainous terrain about 10 nautical miles northwest of Togiak Airport (PATG), Togiak, Alaska. The two commercial pilots and the passenger were killed, and the airplane was destroyed. The scheduled commuter flight was operated under visual flight rules (VFR) by Hageland Aviation Services, Inc., Anchorage, Alaska, under the provisions of Title 14 Code of Federal Regulations (CFR) Part 135.

The National Transportation Safety Board’s (NTSB) investigation determined that instrument meteorological conditions (IMC) were likely in the vicinity of the accident site at the time of the accident. The flight departed Quinhagak Airport, Quinhagak, Alaska, at 1133 and was en route to PATG. Data available for the accident flight showed that, after departure in visual meteorological conditions, the airplane proceeded along a generally direct route toward the destination at an altitude of about 1,000 ft mean sea level (msl), which resulted in terrain clearances between 500 and 700 ft above ground level (agl).

During the last 4 minutes of the flight, the airplane climbed as it approached the mountain ridge that it eventually struck at an elevation of about 2,300 ft msl after having likely entered IMC. The airplane was equipped with a Class B terrain awareness and warning system (TAWS) that had an en route required terrain clearance (RTC) of 700 ft agl; flight at altitudes below the RTC (and not within 15 miles of an airport, given certain criteria) would result in TAWS terrain alerts. Hageland flights operated under VFR were allowed to fly as low as 500 ft agl, as was seen with this flight, which was flown en route below the TAWS alerting threshold.

The system was equipped with a terrain inhibit switch that allowed the pilot to manually inhibit all TAWS aural and visual caution and warning alerts. A TAWS simulation that used an estimated flightpath for the accident airplane (assuming a level cruise altitude between known data points and a climb after the last data point to the accident elevation) showed that, if the alerts were not inhibited, the TAWS would have provided continuous alerts for most of the assumed flight. The investigation concluded that the TAWS alerts were likely inhibited for most, if not all, of the flight, eliminating a margin of safety.