Beechcraft C35 N2025D

The noninstrument-rated private pilot was conducting a night cross-country flight in the piston-engine-powered, unpressurized airplane. According to the radar track data and ATC transcript, the pilot flew between about 14,000 ft and 18,000 ft for about the last 40 minutes of the flight. Review of weather information indicated that the airplane was likely operating in instrument meteorological conditions for most of the flight, with icing possible below 14,000 ft. About 10 minutes before the accident, the air traffic controller observed the airplane had climbed through 18,000 ft. The air traffic controller subsequently informed the pilot that he would require an instrument flight rules (IFR) clearance to fly at that altitude and asked the pilot if he wanted to file IFR.  The pilot responded, "that would be great, I just want to get over [this] so I can get back down." The controller also asked the pilot if the airplane was IFR-equipped and -capable, to which the pilot responded that he was IFR-equipped. During subsequent communications with air traffic control, the pilot's communications became inconsistent and the airplane began flying erratically with changes in direction and altitude during the final minutes of flight. The airplane entered a series of descending turns before experiencing an in-flight break-up. Examination of the wreckage revealed no preimpact mechanical malfunctions or anomalies that would have precluded normal operation. Although the airplane was equipped with a home-built system of medical-grade oxygen onboard, the valves on the bottles were found in the off positions; since the valves were inaccessible from the cockpit, was not possible for the pilot to have used oxygen during the flight. As altitude increases, the partial pressure of oxygen in the air decreases, leading to hypoxia. The FAA Aeronautical Information Manual (AIM) notes that hypoxia is a "state of oxygen deficiency in the body sufficient to impair functions of the brain," and "the effects of hypoxia are usually quite difficult to recognize, especially when they occur gradually." The AIM also states that between 12,000 and 15,000 ft "judgment, memory, alertness, coordination and ability to make calculations are impaired, and headache, drowsiness, dizziness and either a sense of well-being (euphoria) or belligerence occur." It also states "pilot performance can seriously deteriorate within 15 minutes at 15,000 ft. FAA regulations (14 CFR 91.211) require a pilot to use supplemental oxygen when operating above 12,500 ft in excess of 30 minutes and during the entire flight time when operating above FL140. The pilot's inconsistent communications and erratic flight track was characteristic of a state of hypoxia, and hypoxia likely resulted in the pilot's inability to maintain control of the airplane. Toxicological testing of the pilot revealed the presence of multiple central nervous depressants and a total of 6 psychoactive substances in his system (diazepam, nordiazepam, hydrocodone, dihydrocodeine, norcyclobenzaprine, and zolpidem). While there is no direct conversion between the liver and kidney levels and blood or serum levels of these drugs, the presences of even lesser amounts of all 6 substances in combination strongly suggests the pilot was influenced by the effects of the multiple medications he was using. It is likely that the combined effects of the 6 impairing substances he was using contributed to his unsafe decision making, which led to him to initiate the flight in the dark, over mountainous terrain, in instrument meteorological conditions, and without ensuring ready access to his oxygen system.

Quoted verbatim from the NTSB record.