ERA24FA058 · Beech 35-C33

After departure on a 430-nautical mile (nm) cross-country flight, initial controller-pilot communications and headings were unremarkable. However, as the airplane climbed through 6,500 ft mean sea level (msl) and became established enroute, a performance study of ADS-B data showed that the airplane’s altitude and airspeed began to vary as the airplane repeatedly descended and climbed with fluctuating airspeeds. These oscillations became increasingly more severe over about 25 minutes, with altitude deviations up to 1,500 ft and significant fluctuations in airspeed. An air traffic controller advised the pilot that the airplane was left of course; although the pilot acknowledged the controller, the deviations continued to worsen and the pilot stopped responding to radio calls. Faint emergency transmissions were later received from the pilot and passenger as the airplane continued to descend. During the final portion of the flight, the airplane exceeded a descent rate of 10,000 ft per minute with a groundspeed of about 228 knots before terrain impact. The increasing altitude and heading deviations, loss of communication, and rapid uncontrolled descent were consistent with a loss of airplane control. Flight instructors who had flown with the pilot before the accident indicated that she routinely demonstrated poor manual aircraft handling and struggled with cockpit automation, including autopilot and trim use. The pilot’s training history showed that she had demonstrated difficulty maintaining situational awareness, suggesting the high-performance airplane she had purchased about 1.5 years before the accident was beyond her level of pilot proficiency, something explicitly noted by one of the instructors who had flown with her. Postaccident examination of the airplane wreckage did not reveal evidence of any anomalies or mechanical irregularities that would have precluded normal operation, though the degree of fragmentation and fire damage precluded a complete evaluation of the airplane’s flight controls and autopilot system. Given the airplane’s heading was stable during the initial portion of the flight, it is likely that the pilot had been using the autopilot. Based on the pilot’s reported misunderstanding of that system, including, but not limited to the autopilot’s pitch inputs and the requirement for manual pitch trim application, it is possible that the pilot inadvertently mis-trimmed the airplane, which resulted in oscillations that were exacerbated by the pilot’s application of force on the control yoke. These kinds of pilot-induced oscillations can occur when manual yoke or pitch trim inputs are in conflict with the autopilot's pitch control inputs, leading to delayed, or out-of-sync, corrections, rather than a smooth, constant altitude hold. The airplane’s vertical profile as evidenced by the ADS-B data, was consistent with these kinds of pilot-induced oscillations. As the oscillations became stronger, the pilot may have ultimately been unable to control the airplane’s pitch, even with the autopilot disengaged, resulting in the loss of airplane control. Toxicology results indicated that the pilot had used a combination of medications that may be used to treat conditions such as anxiety and depression, which may have performance-impairing effects. However, no details about any associated underlying conditions were available from reviewed records. Additionally, the medications trazodone, alprazolam, and buspirone may cause psychomotor and cognitive impairment such as slowed reaction time, diminished situational awareness, and sedation. However, due to the extent of the pilot’s injuries, measurement of drug levels was limited to tissue specimens only, preventing reliable comparison to antemortem reference ranges in blood. Overall, the pilot may have been experiencing impairing effects of medication use or an associated underlying condition at the time of the accident, and such effects may have diminished her ability to render effective control inputs. However, available medical and circumstantial evidence was insufficient to establish whether such effects contributed to the accident, particularly given the pilot’s demonstrated baseline proficiency in the high-performance airplane.

Verbatim NTSB analysis and probable cause from the NTSB dataset