WPR22FA173

The pilot’s failure to maintain proper airspeed and his exceedance of the airplane’s critical angle of attack, which resulted in an aerodynamic stall.

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 right-side 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. 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 Surface Analysis Chart depicted a low-pressure system over central Idaho, north of the accident site, with a trough of low pressure extending east and west from the low. A cold front extended across central Nevada. The accident site was located on the cold air side, or north of the cold front and south of the low-pressure system in central Idaho. The station models near the accident site depicted a counterclockwise circulation related to the low-pressure system, with northwesterly to southwesterly winds of 4 to 10 knots, light snow showers, overcast cloud cover, temperatures from 35° to 36° F, and the dew point temperatures from 33°to 35°F. The NWS Storm Prediction Center’s Convective Outlook Chart that was valid for the period and included the accident site revealed a general risk of thunderstorm activity over Idaho during the period. The NWS 12-hour Low-Level Significant Weather Prognostic Chart depicted general marginal visual flight rules (MVFR) conditions expected to prevail over northern Nevada, Idaho, and Oregon during the period, with the freezing level between 4,000 and 8,000 ft. No significant turbulence below 24,000 ft was expected over Idaho for the period. NOAA numerical model sounding indicated the density altitude of about 5,452 ft msl and indicated winds were west to northwest about 3 knots. The freezing level was depicted at 711 ft above ground level or about 6,000 ft msl. The atmosphere was characterized as conditionally unstable and supported broken to overcast cumulus clouds capable of producing light rain from about 3,000 ft msl to 20,000 ft msl. The analysis program expected light-to-moderate icing conditions in the clouds, with clear to mixed type icing below 12,000 ft msl. 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 turned eastward. At 1417, the airplane’s fight track was through an area of moderate-to-heavy intensity echoes, with the accident site on the southeast edge of the echo. Figure 1: Base reflectively image for 1417 MDT with flight track. The wind profile indicated little support for any turbulence below 16,000 ft over the area outside of cumulus clouds. The 3-hour Graphic Forecast for Aviation, valid from 1200 to 1500 mountain daylight time (MDT), depicted a small area of instrument flight rules and MVFR conditions over the accident site, with surface precipitation in the form of rain, with rain/snow east and west of the accident site. Broken to overcast clouds were expected with bases from 5,000 to 7,000 ft msl and tops to 22,000 ft. A search of PIREPs revealed that there were no reports in the immediate vicinity of the accident site. One PIREP near Twin Falls, Idaho, reported patchy clouds. Several PIREPs were reported near Boise, Idaho. 2 PIREPs reported moderate turbulence, one at 6,000 ft and one at 10,000 ft. Additionally, one PIREP reported light clear icing at 10,000 ft. Finally, a PIREP from a pilot out of Fresno, California, and about 85 miles southwest of the accident site reported moderate to severe turbulence at 8,000 ft. There were no active SIGMETs, Convective SIGMETS, or CWA over the accident site at the time of the accident. There was a current AIRMET for mountain obscuration conditions over Idaho, which warned of mountains obscured in clouds, precipitation, and mist. A search of the FAA contract Automated Flight Service Station provider Leidos indicated that they and no 3rd party vendors using the Lockheed Flight Service (LFS) system had any contact with the pilot of N801DT on May 8, 2022. A separate search of ForeFlight indicated that the accident pilot had an account that included N801DT; however, no flight plans were filed for the accident flight and no weather imagery, or charts were viewed using the ForeFlight app. The pilot of N801DT created a route string from BVU to ONO with a cruising altitude of 7,500 ft, and he did review airport pages for BVI at 1154 MDT, which includes METAR, TAFs, and NOTAMs. There were no indications the pilot reviewed the GFA enroute forecast or in-flight weather advisories before or inflight. The Owyhee County Coroner Office, Marsing, Idaho, conducted an autopsy on the pilot; the cause of death was “severe blunt force trauma.” Toxicology testing performed by the FAA’s Forensic Sciences Laboratory on the pilot’s specimens, detected atorvastatin, salicylic acid, and diphenhydramine. Glucose was measured in the urine at 29 mg/dL. Atorvastatin (Lipitor) is used to treat high cholesterol and is acceptable for FAA medical certification. Salicylic acid is a metabolite of aspirin and is also used in several preparations to treat conditions. Aspirin is used to treat minor pain and as an antiplatelet medication to prevent blood clots. Use of aspirin is acceptable for flying if the underlying condition being treated is also acceptable. Diphenhydramine (Benadryl) is a non-prescription antihistamine for treating seasonal allergies and can cause sedation. It is acceptable for pilots but should not be used within 60 hours of flying and regular use is unacceptable. Diphenhydramine (Benadryl) can cause sleepiness, and depending on when the drug was administered, could have posed a hazard to flight safety. According to the previous owner, the airplane’s fuel system consisted of wet wings that held 230 gallons. In addition, there was a fuel header tank located by the pilot’s feet. There was a fuel lever for each side that was selected ON or OFF. For normal operation, he would turn on both side fuel levers. He would check for fuel imbalances and then turn on the fuel lever for the high side and turn off the fuel lever for the low side, if required. He had no major fuel imbalances in his 14 years of flying the airplane. During refueling he would leave about 1 inch from the top. The highest altitude he flew the airplane was about 11,500 ft –12,500 ft msl. At that altitude, the airplane’s flight duration was about 4.0 to 4.5 hours and a range of about 800 miles. The accident flight’s duration was less than 3 hours. A previous acquaintance of the pilot, who was a flight instructor, flew in the airplane with the accident pilot on the two flights before the accident. He stated that they both believed that the left wing felt heavy and thought it was because of a fuel imbalance. The flight instructor further stated at times he used two hands to keep the wings level and the airplane felt heavy, especially when in bank. During the postaccident examination, 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. During the last airplane refueling, the refueler stated that the pilot requested that the main tanks be filled to about 2 inches from the top. The tanks were observed to be about 10 inches in height during the examination, which could make for about a 20% reduction in the amount of fuel that could be serviced into the tanks. The previous owner stated that he would leave about 1 inch from the top when refueling, to allow for expansion of the fuel. Furthermore, the previous owner stated that the airplane was instrument certified but had no anti-ice equipment other than the pitot heat. The commercial pilot held ratings for airplane single-engine land, multi-engine land, instrument airplane, and flight instructor. The pilot was issued a FAA third-class airman medical certificate on August 14, 2020, with the following limitations: must wear corrective lenses. He had reported Type I diabetes mellitus. He also reported using insulin lispro, often marketed as Humalog, and insulin largine, often marketed as Lantus. Both types of insulin are used to treat diabetes. The first authorization for Special Issuance for type I diabetes was June 6, 1998, with multiple subsequent renewals. The most recent