Beechcraft B36TC N75RM

The flight departed from an airport near Fort Worth, Texas, and proceeded on a northeast course. The airplane was enroute to Missouri for an annual inspection. About 30 minutes after takeoff, the airplane leveled off near 15,500 ft mean sea level (msl). About 24 minutes after leveling off, as the airplane approached a line of thunderstorms, the airplane started to climb, and the flight path began to deviate consistent with an attempt by the pilot to maneuver around those thunderstorms. Isolated thunderstorm tops in the area were 20,000 ft to 24,000 ft. About 28 minutes after the climb started, the airplane leveled off about 24,500 ft msl and returned to a northeast course. Attempts by air traffic control to contact the pilot after the airplane climbed through 18,000 ft msl were not successful. After about 2 hours and 37 minutes, the airplane entered a descent from about 24,500 ft msl. About 2 minutes later, the airplane course became slightly erratic as the descent continued; however, the airplane ultimately returned to the northeast course. At that time, the airplane was in a stabilized descent of about 1,000 ft per minute (fpm), which continued until the data ended. The airplane impacted a cornfield about 480 ft northeast of the final recorded data point. The impact path was aligned with the final portion of the flightpath. The airplane impacted a corn field and slid about 150 ft before coming to rest. The landing gear and wing flaps were retracted. Flight control continuity was confirmed, and an engine examination did not reveal any anomalies consistent with an inability to produce rated power. Both fuel tanks appeared to be intact and about 35 gallons of fuel remained in the left tank; however, no fuel remained in the right fuel tank. The cockpit fuel selector was set to the right tank at the time of the on-scene examination. These findings were consistent with a loss of engine power due to fuel starvation. Just under 2.5 hours after the airplane reached 24,500 feet msl, the airplane was intercepted and the pilot was observed to be incapacitated. The time when the pilot most likely became incapacitated could not be determined. Nevertheless, flightpath deviations around weather and the pilot’s lack of communication with air traffic control during the climb indicated performance deficiencies that were inconsistent with the pilot’s skill and experience. Thus, the pilot likely became impaired when the airplane was below 18,000 ft but might not have been incapacitated when the airplane climbed into class A airspace. In other words, the pilot likely became impaired at an altitude below 18,000 feet, and later became incapacitated. The airplane was not pressurized, but it was equipped with an onboard oxygen system. Examination of the onboard oxygen system revealed a cracked fitting behind the left (pilot) sidewall, which caused a leak when the system was pressurized. Metallurgical examination of the fitting determined that it had a localized casting defect and that a portion of the fracture contained thread sealant embedded onto the surface, indicating that a crack was present at the time the fitting was installed. The investigation was unable to determine the oxygen supply onboard before the accident flight and was, therefore, unable to make any determination of whether the pilot was being adequately supplied with oxygen during the flight. Given the altitudes at which the airplane was operating and the accident circumstances, the possibility of altitude-related hypoxia must be considered. If the airplane’s onboard oxygen system was properly functioning and properly used, the system would have generally been expected to prevent significant hypoxia in a pilot operating below 18,000 feet. Even with the nasal cannula instead of a mask at 24,500 feet, a pilot would generally not likely experience significant hypoxia if the oxygen system was functioning properly. However, an impaired or incapacitated pilot might have a diminished ability to use the nasal cannula effectively. In addition, if the supplemental oxygen supply became exhausted while the airplane was at altitude, due to the crack in the fitting, hypoxia would have resulted. An average, healthy pilot’s performance is mostly unaffected by cabin altitudes below 10,000 ft but may seriously deteriorate within 15?minutes at a cabin altitude of 15,000 feet. The time of useful consciousness is the maximum time available for an average, healthy pilot to take protective action against hypoxia at a given cabin altitude. This time decreases rapidly with increasing cabin altitude; at 18,000 feet, it is 20 to 30 minutes; at 25,000 feet it is 3 to 5 minutes. Although the pilot’s toxicology results indicated that he had used a cannabis product, the THC level in his blood was very low and there were no detectable THC metabolites in his blood or urine, making it unlikely that cannabis effects contributed to the accident. The accident pilot’s impairment and/or subsequent incapacitation could plausibly have resulted from known effects of hypoxia. Hypoxia might have resulted from supplemental oxygen depletion, diminished altitude tolerance due to disease, or a combination of those factors. If the supplemental oxygen supply was exhausted while the aircraft was at altitude, hypoxia would have resulted, as the pilot did not take action to descend. The aircraft was at 24,500 ft for almost 2.5 hours before the pilot was seen to be incapacitated. At that altitude, the time of useful consciousness without supplemental oxygen for an average, healthy pilot is about 3 to 5 minutes. The pilot’s autopsy identified cardiopulmonary disease that conveyed increased susceptibility to hypoxia as well as some increased risk of experiencing an impairing or incapacitating cardiovascular event such as arrhythmia or ischemic stroke. Without knowing the pilot’s usual altitude tolerance or when the supplemental oxygen supply was exhausted, there is no way to determine the likelihood that the pilot’s cardiopulmonary disease contributed to the accident. Given the airplane’s flight path after the pilot deviated around weather, it is likely that the autopilot was engaged during most of the final portion of the flight. The pilot likely became impaired at some point during the flight below 18,00 ft and subsequently became incapacitated. The exact timing and underlying cause of those events could not be determined. Based on the flight path after the pilot’s deviation around weather, it is plausible the airplane completed most of the final portion of the flight with the autopilot engaged. Once the fuel supply in the selected tank was exhausted, the engine lost power and the airplane entered an extended glide that continued until the airplane impacted the cornfield.

Quoted verbatim from the NTSB record.