Idaho

The pilot and passenger departed on a cross-country flight. Automatic dependent surveillance – broadcast (ADS-B) data indicated that the airplane flew at a cruise altitude between 11,000 ft and 13,000 ft mean sea level (msl) on a north to northeast heading consistent with its planned destination for most of the flight. About 30 minutes before the end of the flight, the airplane began a descent and then turned east. Shortly thereafter, the pilot of the airplane declared minimum fuel with air traffic control (ATC). A few minutes later the pilot declared critical fuel due to a fuel leak. The pilot stated in his last communication that he would attempt to make a nearby airport. Subsequently, the airplane impacted hilly, desert terrain at an elevation of about 5,780 ft and on a heading of about 034°. An acquaintance of the pilot who was a flight instructor stated that, on the two previous flights he had flown with the pilot, the left wing of the airplane felt heavy. The accident pilot thought it was because of a fuel imbalance. The postaccident examination revealed that the left tank fuel valve was positioned ON and the right tank valve was positioned OFF, consistent with the pilot balancing the fuel by feeding from the left-wing fuel tank. It is possible that when the pilot noticed the minimum fuel status, he failed to recall that he had previously selected the rightside fuel tank OFF, and thus did not have this fuel available. Given that the cruise altitudes on the accident flight were similar to what the previous owner used to make his fuel range and duration estimates, even with about a 20% reduction in fuel due to the pilot allowing 2 inches from the top of the fuel tanks during refueling, the airplane should have had adequate fuel to make its destination. A strong smell of fuel and fuel staining were also observed at the accident site. Page 2 of 11 WPR22FA173 A review of radar imagery from Boise, Idaho, revealed that the airplane flew through several areas of light to moderate intensity echoes as it proceeded northward, and then after turning eastward, the airplane’s fight track was through an area of moderate to heavy intensity echoes. The accident site was located on the southeast edge of the echo. Light-to-moderate icing conditions in the clouds with clear to mixed type icing below 12,000 ft msl were expected. Thus, it is likely that the airplane, which was not certified for flight in icing, encountered icing in the final portion of the flight. The pilot was flying with insulin-dependent diabetes, having type 1 diabetes mellitus. Given the urine glucose level of 29mg/dL, no detectable glucose in vitreous fluid, and ongoing verbal communication, it is unlikely that the pilot was experiencing significant metabolic disturbance from high blood glucose. Whether he was experiencing less severe effects of high blood sugar could not be determined. Whether he had symptoms of low blood glucose, such as diminished concentration or increased nervousness, is unknown. The pilot’s use of diphenhydramine (Benadryl), which can cause sleepiness, was likely not a factor due to fact that it was detected only in the urine and not in the blood. Thus, it is unlikely that effects of the pilot’s diphenhydramine use contributed to the accident. Accident site signatures and a review of the weather were consistent with a loss of control of the airplane. In addition, an examination of the airframe and engine revealed no evidence of any preimpact mechanical failures or malfunctions that would have precluded normal operation. It is likely that, while maneuvering to an alternate airport due to a critical fuel situation, in icing conditions, the pilot failed to maintain the proper airspeed, which resulted in the exceedance of the airplane’s critical angle of attack and the airplane experiencing an aerodynamic stall.

The pilot flew two RNAV (GPS) runway 20 instrument approaches at the Burley Municipal Airport, Burley, Idaho in instrument meteorological conditions (IMC). The accident occurred during the second approach. For the first instrument approach, the pilot configured the airplane with flaps up and flew the final approach segment at speeds above the operator’s training standard of 120 knots indicated airspeed (KIAS).The pilot flew a low pass over the runway, most likely to assess the landing conditions in accordance with company policy, determined the conditions were acceptable, initiated the missed approach and requested to return flying the same approach. The pilot elected to not use flaps during the second approach but slowed the approach speed during the final approach leg. Reported weather had improved and visibility had increased to about 2.5 miles. During this approach, the airplane intercepted and remained on the glide path to the stepdown fix. The last automatic dependent surveillance - broadcast (ADS-B) equipment plot recorded the airplane about a mile past this fix, or about 0.6 nautical miles (nm) from the displaced threshold, on the glide path, and at an estimated 85 knots calibrated airspeed (KCAS), which was slower than the airplane’s 95-knot minimum speed for flaps up in icing conditions. Shortly afterward, the airplane descended about 130 ft below the glide path, striking an agglomerate stack atop a potato processing plant, fatally injuring the pilot and substantially damaging the airplane. A witness reported seeing the airplane come out of the clouds and immediately enter a steam cloud coming from six other stacks before striking the accident stack. A security camera at the processing plant captured the last moments of the airplane’s flight as it came into view in a wings-level, flaps-up, nose-high descent and just before it impacted the stack. While snow and visible moisture were present, the agglomerate stack was always in clear view during the Page 2 of 24 WPR22FA151 video, with only partial sections obscured. The witness’s account of hearing the engine noise increase and then the nose lift-up may have been the pilot’s attempt to avoid the obstacle. The Federal Aviation Administration’s (FAA) Aeronautical Information Manual advises pilots to avoid overflight of exhaust stacks; however, the accident stack was directly underneath the instrument approach course and overflight would be expected. Postaccident examination of the airplane, conducted hours after the accident, revealed no structural icing on the wings and empennage. Examination of the airframe and powerplant revealed no mechanical malfunctions or failures that would have precluded normal operation. The flaps were up, and a review of the manifest revealed the airplane was loaded within the specifications of the manifest and within the center of gravity limits. Between 2016 and 2017, the FAA conducted two aeronautical studies regarding the stack structures. In the first study, the FAA determined that many of the stack structures were a hazard to air navigation that required mitigation by the processing plant. As an interim measure, the FAA placed the runway 20 visual approach slope indicator (VASI) out of service because the stacks penetrated the obstruction clearance surface and were deemed hazardous to aviation. After determining that they needed to increase the height of the stacks, the plant then modified their proposal; the proposed height increase necessitated a second study. The second study determined the agglomerate stack and the row-of-six stacks exceeded the Code of Federal Regulations (CFR) section 77 standards and provided mitigating actions that included painting the stacks with high visibility white and aviation orange paint and equipping the stacks with red flashing warning lights. The control measures also included the permanent removal of the VASI. On the day of the accident, the agglomerate stack and row-of-six stacks had not been painted to the standard required by the FAA. The warning lights had been installed, and five of the row-of-six stacks were equipped with flashing red lights. The agglomerate stack warning light was stolen following the accident, so an accurate determination of its operating status could not be made. The existing paint scheme and the visible moisture emitted by the stacks provided a low contrast to the environmental background. This low contrast and the lack of a visual glide slope indicator may have caused difficulty for the pilot in maintaining a safe altitude during the visual portion of the approach to the runway. A white and aviation orange paint scheme, as identified in the regulations, may have offered a higher contrast and thus an adequate warning once the pilot transitioned to visual conditions.

The float-equipped De Havilland DHC-2 was on a tour flight, and the Cessna 206 was on a personal flight. The airplanes collided in midair over a lake during day visual meteorological conditions. No radar or automatic dependent surveillance-broadcast data were available for either airplane. Witnesses reported that the airplanes were flying directly toward each other before they collided about 700 to 800 ft above the water. Other witnesses reported that the Cessna was at a lower altitude and had initiated a climb before the collision. Review of 2 seconds of video captured as part of a witness’ “live” photo showed that both airplanes appeared to be in level flight before the collision. No evidence of any preexisting mechanical malfunction was observed with either airplane. Recovered wreckage and impact signatures were consistent with the upper fuselage of the Cessna colliding with the floats and the lower fuselage of the De Havilland. The impact angle could not be determined due to the lack of available evidence, including unrecovered wreckage. The available evidence was consistent with both pilots’ failure to see and avoid the other airplane.