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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.
TO THE FEDERAL AVIATION ADMINISTRATION: Work with Title 14 Code of Federal Regulations Part 135 certificate holders that operate under visual flight rules in the aircraft’s required terrain awareness and warning system (TAWS) class to (1) ensure that management and pilots are aware of the risks associated with distraction (from continuous nuisance alerts) and complacency (brought about by routine use of the terrain inhibit feature); (2) develop plans for mitigating those risks and minimizing nuisance alerts; and (3) develop procedures that specifically address when pilots should test, inhibit, and uninhibit the TAWS alerts, considering the operator’s typical operations and the TAWS manufacturer’s guidance.
Original recommendation transmittal letter:
Open - Acceptable Response
Togiak, AK, United States
Collision with Terrain Hageland Aviation Services, Inc. dba Ravn Connect Flight 3153 Cessna 208B, N208SD
Addressee(s) and Addressee Status:
FAA (Open - Acceptable Response)
Safety Recommendation History
We note that you are developing a safety alert for operators (SAFO) that will discuss the risks associated with nuisance alerts; encourage operators to ensure all pilots are aware of the risks associated with distraction and complacency brought about by routinely using the terrain inhibit feature; recommend that operators incorporate policies and procedures into their training programs to mitigate terrain inhibit features; include plans to minimize nuisance alerts; and minimize the risk associated with distraction and complacency brought about by routinely using the terrain inhibit feature. We believe that a SAFO is an appropriate way to alert management and pilots to the risks associated with continuous nuisance alerts and routine use of the terrain inhibit feature. However, we remain concerned that operators’ procedures (which you have approved) allow normal flight operations at altitudes where equipment (which you require) warns pilots of an unsafe condition. To address this concern and satisfy the second and third parts of this recommendation, we believe that you should work with operators to develop procedures that mitigate these risks, then ensure that those procedures are incorporated into operators’ approved flight procedures. Pending our review of the SAFO and of your plan to address the remaining parts of this recommendation, Safety Recommendation A-18-14 is classified OPEN--ACCEPTABLE RESPONSE.
-From Daniel K. Elwell, Acting Administrator: The FAA has initiated development of a Safety Alert for Operators that will – • Address risks associated with nuisance alerts and encourage operators to ensure all pilots are aware of the risks associated with distraction and complacency brought about by routine use of the terrain inhibit feature: and • Recommend operators incorporate policies and procedures into their training programs that will reduce terrain inhibit features, include plans to minimize nuisance alerts, and minimize risk associated with dis traction and complacency brought about by routine use of the terrain inhibit feature.
On April 17, 2018, the NTSB adopted its report, Collision with Terrain, Hageland Aviation Services, Inc., dba Ravn Connect Flight 3153, Cessna 208B, N208SD, Togiak, Alaska, October 2, 2016, NTSB/AAR-18/02. The details of this accident investigation and the resulting safety recommendations may be found in the attached report, which can also be accessed at http://www.ntsb.gov. Among the Safety Recommendations, five are issued to Federal Aviation Administration (FAA) and can be found on page 71 of the report; eight are reiterated to the FAA and can be found on pages 72-73 of the report. The NTSB is vitally interested in these recommendations because they are designed to prevent accidents and save lives. We would appreciate a response within 90 days, detailing the actions you have taken or intend to take to implement these recommendations. When replying, please refer to the safety recommendations by number. We encourage you to submit your response.
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