N6709Q

HISTORY OF FLIGHTOn August 27, 2023, about 1000 Alaska daylight time, a Beech B36TC airplane, N6709Q, was destroyed when it was involved in an accident near Cape Yakataga, Alaska. The pilot and passenger were fatally injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The airplane departed Snowshoe Lake Airport (5AK4), Glennallen, Alaska, at 0850, destined for Ketchikan, Alaska. A review of preliminary air traffic control (ATC) communications and ADS-B data from the FAA revealed that radar contact and radio communications were intermittent along the route of flight due to high terrain.

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 the pilot if he could climb higher, and the pilot responded that he would climb to 13,500 ft msl.

About 0920, the controller confirmed radar contact with the airplane. The controller asked what the pilot’s requested final altitude was; the pilot replied 13,000 ft, and that he knew he had some weather ahead of him. The controller responded that he would encounter both weather and higher terrain and that he would need to climb higher. The pilot responded that they had oxygen onboard, the airplane was turbocharged, and 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 subsequently confirmed that he would start the climb.

At 0926, the controller told the pilot he would lose him on the active frequency and issued multiple backup frequencies to maintain radio communication.

At 0928, the pilot established radio communications on a new frequency. The controller told the pilot that they would lose contact on all frequencies for about 10 minutes, then issued the current altimeter setting and provided the pilot another frequency to monitor.

At 0938, the controller instructed the pilot to turn 20° to the right to avoid higher terrain along the route of flight but received no response.

At 0945, the controller asked another airplane to relay instructions to the pilot of N6709Q, stating, in part, to make a 20° right turn and to climb to a higher altitude.

At 0945, the pilot of the other airplane relayed a message to the controller that the pilot of N6709Q had acknowledged the updated instructions.

At 0953, after reestablishing radio contact with the controller, the pilot of N6709Q requested a climb to 15,000 ft and the controller approved the request.

At 0958, the controller stated “N6709Q, Anchorage center,” then again, “N6709Q, Anchorage center,” with no response from the pilot. The controller then asked another airplane to broadcast on the frequency to ask the pilot of N6709Q about his intentions.

At 0958, the controller stated, “All other aircraft standby real quick, N6709Q, Anchorage center, you up? N6709Q, I see you rapidly descending at 11,000.”

There were no further communications from the pilot of N6709Q.

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, the crew of a U.S. Coast Guard HC-130 airplane located the wreckage site near Mt. Leeper, located in the Wrangell-St. Elias National Park and Preserve, at an elevation of about 4,787 ft, but terrain and continued poor weather conditions precluded reaching the site (see figure 1).

Figure 1. Aerial image of the accident site, with red box depicting area of accident site. (Source: Alaska Rescue Coordination Center) PERSONNEL INFORMATIONThe pilot’s logbook was not obtained during the investigation. The pilot’s total flight time was recorded from his most recent FAA airman medical application dated June 12, 2023. The pilot acquired an instrument rating in June, in Texas where he lived, about two months prior to 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 for a few days before returning to Alaska for the start of moose season. He planned the route from Glennallen to Ketchikan, then to Washington state because he preferred not to fly through Canada. AIRCRAFT INFORMATIONReview of the airplane’s Pilot Operating Handbook, Section 2, KINDS OF OPERATIONS, includes a warning that, "FLIGHT IN ICING CONDITIONS PROHIBITED.” METEOROLOGICAL INFORMATIONThe 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 viewed by the pilot were not available in the ForeFlight database or available 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 were not created for the Alaska region.

The only icing information available to the pilot in the Foreflight weather briefing was on the vertical cross-section of the winds aloft, which also includes the winds aloft and turbulence. The vertical cross-section depicted a change of light to moderate turbulence by color shaded boxes with the corresponding values. The chart also included icing symbols, which ranged 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 the Significant Weather Prognostic Chart which depicted a large area of continuous precipitation over the interior of Alaska, and over the central and eastern gulf coast, with fog over the southeast coastal waters of Alaska.

The National Oceanic and Atmospheric Administration satellite imagery depicted cloud tops near 18,000 ft and -8°C over the accident site. Cloud tops warmer than -15 C are typically associated with clouds in the liquid phase and are indicative of structural icing. The nearest NWS weather surveillance radar indicated the existence of liquid phase clouds, which further supported the existence of SLD.

The NWS GFA ceiling and visibility and cloud forecast images depicted marginal visual flight rules (MVFR) to IFR conditions and light rain over the route of flight.

A search of pilot weather reports (PIREPs) within 120 miles of the accident site surrounding the period from 0900 through 1300 resulted in no reports in the national database.

The freezing level was identified at 12,280 ft by the RAOB Analysis depicted favorable conditions for light-to-moderate rime type icing from the freezing level through about 24,000 ft. The accident airplane was cruising between 13,950 and 15,000 ft until it began a descent and impacted terrain.

The GFA is a graphical depiction of surface wind, thunderstorms, precipitation, cloud bases and tops, and color-coded general flight categories. The GFA is available on the NWS AWC website and other briefing sources. The GFA icing severity depiction for 15,000 ft for 1000 depicted moderate to heavy icing conditions bordering over the route of flight and the accident site. This icing severity product was derived from NWS computer model data with no forecaster modifications. This graphic is not available to the public or pilots on the NWS website but was accessible for Alaska Flight Service weather briefers.

The NWS AAWU Icing Forecast expected an isolated (less than 10% probability) risk of moderate icing from 16,000 ft through FL220 over the route of flight and in the immediate vicinity of the accident site.

The National Center for Atmospheric Research (NCAR) developed an AAWU internal CIP and FIP products for Alaska; at the time of the accident they were only produced for the NWS AAWU and were not available to the public.

Figure 2. On the left is the vertical cross-section chart that was available to the pilot. On the right is the icing graphic that was not available to the pilot.

NCAR recreated the CIP imagery for the time period surrounding the accident. The CIP icing severity has five categories: none, trace, light, moderate, and heavy. The severity estimates are roughly based on the accretion rate of ice on an airplane, and the levels are determined by the time it would take for an airfoil to accrete 1/4” of ice: trace = 1 hour; light = 15 minutes to 1 hour; moderate = 5 to 15 minutes; severe = less than 5 minutes. The CIP output identified up to moderate to heavy icing severities, high icing probabilities of 80-85%, and some indications of potential SLD in the accident region during the time of the accident.

The NCAR CIP data also indicated that the icing threat increased as the accident airplane climbed from 14,000 ft to 15,000 ft.

Research has shown that the most severe icing environments are found in liquid-phase clouds, with temperatures between 0 to -20 C. Strong orographic lifting due to terrain typically enhances the likelihood and severity of icing in mountainous areas. Icing with mountains can also be hazardous because a pilot may be unable to descend below the freezing level due to terrain elevation.

FAA Advisory Circular AC 91-74B “Pilots Guide: Flight in Icing Conditions” provides pilots with a general reference on the principal factors related to flight in icing conditions and the location of additional information in related publications. The advisory circular defines icing intensities as follows:

Trace Icing. Ice becomes noticeable. The rate of accumulation is slightly greater than the rate of sublimation. A representative accretion rate for reference purposes is less than 1/4 inch (6 mm) per hour on the outer wing. Deicing/anti-icing equipment is not utilized unless encountered for an extended period of time (ov...