History of Flight
On August 27, 2023, at approximately 1000 Alaska daylight time, a Beechcraft B36TC airplane, registration N6709Q, was destroyed when it struck terrain near Cape Yakataga, Alaska. The pilot and passenger received fatal injuries. The flight was operated under Title 14 Code of Federal Regulations Part 91 as a personal trip.
The airplane departed Snowshoe Lake Airport (5AK4) in Glennallen, Alaska, at 0850, destined for Ketchikan, Alaska. Review of preliminary air traffic control (ATC) communications and ADS-B data revealed that radar contact and radio communications were intermittent along the route due to high terrain.
At about 0915, the pilot stated he was in visual flight rules (VFR) conditions at an altitude of 12,500 ft mean sea level (msl). The Anchorage Air Route Traffic Control Center (ARTCC) controller asked if he could climb higher, and the pilot responded that he would climb to 13,500 ft msl.
Around 0920, the controller confirmed radar contact and asked the pilot's requested final altitude. The pilot replied 13,000 ft and mentioned that he knew there was weather ahead. The controller noted that the pilot would encounter both weather and higher terrain and needed to climb higher. The pilot responded that they had oxygen onboard and the airplane was turbocharged, so a climb to a higher altitude would not be a problem. The controller then instructed the pilot to climb to 14,000 ft, and the pilot confirmed.
At 0926, the controller told the pilot he would lose him on the active frequency and provided multiple backup frequencies to maintain radio communication. At 0928, the pilot established communications on a new frequency. The controller advised that they would lose contact on all frequencies for about 10 minutes, issued the current altimeter setting, and provided another frequency to monitor.
At 0938, the controller instructed the pilot to turn 20° to the right to avoid higher terrain but received no response. At 0945, the controller asked another airplane to relay instructions to turn 20° right and climb to a higher altitude. The other aircraft's pilot relayed that N6709Q acknowledged the instructions.
At 0953, after reestablishing radio contact, the pilot of N6709Q requested a climb to 15,000 ft, which the controller approved. At 0958, the controller called N6709Q twice with no response, then asked another aircraft to broadcast on the frequency. The controller then stated, "N6709Q, Anchorage center, you up? N6709Q, I see you rapidly descending at 11,000."
No further communications were received from the pilot. According to archived ADS-B data, at 0957 the airplane reached its highest altitude of 14,950 ft, then began a left turn and rapid descent. At 0959, the final radar target was near the accident site at 8,875 ft and 138 knots groundspeed.
The next day, a U.S. Coast Guard HC-130 airplane crew located the wreckage site near Mt. Leeper in Wrangell-St. Elias National Park and Preserve, at an elevation of about 4,787 ft. However, terrain and continued poor weather conditions prevented reaching the site.
Personnel and Aircraft Information
The pilot's logbook was not obtained. His total flight time was recorded from his most recent FAA airman medical application dated June 12, 2023. He obtained an instrument rating in Texas about two months before the accident.
According to a mechanic who worked on the airplane, the pilot had been in Alaska and planned a trip to Washington state to visit family. He chose the route from Glennallen to Ketchikan to avoid flying through Canada.
Review of the airplane's Pilot Operating Handbook (Section 2) included a warning: "FLIGHT IN ICING CONDITIONS PROHIBITED."
Meteorological Information
The pilot obtained three preflight weather self-briefings from ForeFlight with a planned cruising altitude of 13,000 ft. Between the second and final briefing, the pilot requested the maximum icing severity image, icing potential for 11,000 ft and 12-hour icing forecast, and the 18,000 ft winds aloft forecast. Archived versions of the icing product were not available in the ForeFlight database or from the NWS. The NWS Current Icing Potential (CIP) and Forecast Icing Potential (FIP) products the pilot requested were only available for the contiguous United States and not for Alaska.
The only icing information available to the pilot in ForeFlight was on the vertical cross-section of winds aloft, which also included turbulence. The cross-section depicted a change of light to moderate turbulence with color-shaded boxes. The chart also included icing symbols ranging from trace to heavy icing between 14,000 and 16,000 ft. The freezing level was identified at 14,000 ft.
The NWS Alaska Aviation Weather Unit (AAWU) issued a Significant Weather Prognostic Chart depicting a large area of continuous precipitation over interior Alaska and the central and eastern gulf coast, with fog over southeast coastal waters.
NOAA satellite imagery showed cloud tops near 18,000 ft and -8°C over the accident site. Cloud tops warmer than -15°C are typically associated with liquid-phase clouds, indicative of structural icing. The nearest NWS weather surveillance radar indicated liquid-phase clouds, supporting the existence of supercooled large droplets (SLD).
The NWS Graphical Forecast for Aviation (GFA) ceiling and visibility and cloud forecast images depicted marginal VFR to IFR conditions and light rain over the route.
A search of pilot reports (PIREPs) within 120 miles of the accident site from 0900 to 1300 yielded no reports.
RAOB analysis identified the freezing level at 12,280 ft and depicted favorable conditions for light-to-moderate rime icing from the freezing level through about 24,000 ft. The accident airplane cruised between 13,950 and 15,000 ft until it began descending.
The GFA icing severity depiction for 15,000 ft at 1000 showed moderate to heavy icing conditions bordering the route and accident site. This product was derived from NWS computer model data with no forecaster modifications and was not available to the public but accessible to Alaska Flight Service weather briefers.
The NWS AAWU Icing Forecast expected an isolated risk (less than 10% probability) of moderate icing from 16,000 ft through FL220 over the route and near the accident site.
The National Center for Atmospheric Research (NCAR) developed internal CIP and FIP products for Alaska; at the time, they were only produced for the NWS AAWU and not available to the public.
NCAR recreated CIP imagery for the accident period. The CIP severity categories are: none, trace, light, moderate, and heavy, based on accretion rate. The output identified up to moderate-to-heavy icing severities, high icing probabilities of 80-85%, and some indications of potential SLD in the region. The CIP data indicated that the icing threat increased as the airplane climbed from 14,000 ft to 15,000 ft.
Research indicates that severe icing environments are found in liquid-phase clouds with temperatures between 0 to -20°C. Strong orographic lifting due to terrain enhances the likelihood and severity of icing in mountainous areas. Icing with mountains can be hazardous because a pilot may be unable to descend below the freezing level due to terrain elevation.
FAA Advisory Circular AC 91-74B defines icing intensities: trace (less than 1/4 inch per hour), light (1/4 to 1 inch per hour), moderate (1 to 3 inches per hour), and severe (more than 3 inches per hour). Deicing/anti-icing equipment is required accordingly.
Wreckage and Impact
On September 5, about nine days after the accident, weather conditions improved briefly, and a National Park Service (NPS) ranger flew over and photographed the accident site. The fragmented wreckage came to rest in a highly crevassed area on the Yahtse Glacier that continually accumulates snow. A sizable portion was found in a large snow crater on the snow- and ice-covered glacial terrain, with smaller portions adjacent.
On September 13, the Wrangell-St. Elias National Park and Preserve mountaineering team determined that due to the location, crevasse danger, and poor weather conditions, recovery of the airplane's two occupants or wreckage would not be attempted.
Additional Information
The pilot obtained three self-briefings before the flight and reviewed weather information from several sources, including the NWS AAWU website.
The NWS AWC produced graphical forecast products (CIP and FIP) depicting potential for significant icing, including SLD, for the contiguous United States, but a product depicting SLD was not available for Alaska.
On May 24, 2022, the NTSB issued Safety Recommendations A-22-21 (to the FAA) and A-22-22 (to the NWS) to develop a graphical forecast showing SLD icing conditions in Alaska and make it available to pilots.