Pilatus PC-12/45 accident near Stagecoach, Nevada, February 25, 2023

The NTSB's narrative final · quoted from the NTSB record

The pilot, two medical crew members, and two passengers departed on the medical transport flight, which was operating on an instrument flight rules (IFR) flight plan in night instrument meteorological conditions (IMC). Onboard data and ADS-B flight track information showed that, between 1 and 3 minutes after takeoff, the autopilot disengaged and then reengaged; however, the airplane continued to fly a course consistent with the published departure procedure. About 11 minutes after takeoff, the airplane turned about 90° right, away from the next waypoint along the departure procedure, and remained on that heading for about 47 seconds. Around this time, the airplane’s autopilot was disengaged again and was not reengaged for the remainder of the flight. Also, about this time, the airplane’s previously consistent climb rate stopped, and the airplane maintained an altitude of about 18,300 ft mean sea level (msl) for about 20 seconds, even though the pilot had been cleared to climb to 25,000 ft msl. The airplane subsequently turned left to a northeasterly heading and climbed to about 19,400 ft msl before entering a descending right turn. Shortly after entering the right turn, the airplane’s rate of descent increased from about 1,800 ft per minute (fpm) to about 13,000 fpm, and the rate of turn increased before ADS-B tracking information was lost at an altitude of about 11,100 ft msl, in the vicinity of the accident site. The distribution of the wreckage at the accident site was consistent with a low-altitude in-flight breakup. Examination of the airframe and engine revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation, and data obtained from onboard recording devices showed that the engine was developing power at the time of impact. Postaccident examination of the autopilot, trim servos, and trim actuators did not reveal any evidence of mechanical malfunction or failures that would have precluded normal operation. Recorded weather conditions at the departure airport about the time of departure included 1 ¾ statute miles visibility and a cloud ceiling 1,700 ft above ground level (agl). The departure airport and surrounding areas had been impacted by significant winter weather throughout the day of the accident, and the pilot who was on call for the accident operator earlier that day turned down a flight request due to the weather conditions. Another air medical operator, who operated the same make and model airplane as the accident airplane, also turned down a request for a flight in the area due to the low visibility, turbulence, and icing conditions. The accident airplane was equipped for flight into known icing conditions. Review of weather information indicated that the airplane was likely operating in IMC above 6,000 ft msl, and likely did not accumulate much, if any, structural icing. Icing conditions would have been present between 5,000 and 10,000 ft msl. Although turbulence was likely present in the area, there was no evidence to support that the accident airplane encountered hazardous turbulence during the flight. The airplane was equipped with several sources of recorded data, including a central advisory and warning system (CAWS) computer. The CAWS computer, which captured autopilot status, among other parameters, was significantly impact damaged and missing one of the memory chips that stored time information; therefore, the two autopilot disengagements could only be identified as occurring in two-minute windows after elapsed takeoff time, with the first about 1 to 3 minutes after takeoff, and the second between about 2 and 4 minutes before the accident. There are several ways in which the autopilot could have been automatically or manually disengaged during the accident flight; however, based on the available CAWS data and examination of the airplane and system components, the reason for the two autopilot disengagements during the accident flight could not be determined. Following the second autopilot disengagement, the pilot would have been required to manually maintain control of the airplane while operating in IMC, which increased his susceptibility to spatial disorientation. The airplane’s subsequent flight path was consistent with a phenomenon known as a “graveyard spiral,” a sensory illusion in which a pilot believes they are flying in a wings-level descent; however, the airplane is actually in a descending turn. Attempts to arrest the descent by pulling back on the control yoke have the effect of tightening the turn and losing altitude at an increasing rate until the airplane’s structural limits are exceeded, resulting in an in-flight breakup, or until the airplane impacts the ground. Graveyard spirals are most common at night or in poor weather conditions where no horizon exists to provide a visual reference to correct misleading inner-ear cues. Autopsy of the pilot revealed a 3 cm fibroblastic meningioma (tumor) in the right parietal section of the brain. The parietal lobe is one of the four major components of the cerebral cortex and has a primary role in the integration of sensory information, including spatial and navigational information. The parietal lobe is also primarily responsible for the integration of visual and vestibular information. The presence and location of the tumor may have impacted the pilot’s ability to synthesize and respond to sensory interpretation from the conditions under which he was flying; however, it is also possible that the tumor may have been an incidental finding without any significant symptoms, and the pilot’s next of kin indicated no changes in his behavior or health before the accident. Based on the available information, whether the effects of the pilot’s undiagnosed brain tumor contributed to the accident could not be determined. The accident pilot was not permanently assigned to the base from which the accident flight departed; rather, he was classified as a “float” pilot, who rotated across the operator’s bases throughout the country. The operator did not have any formal training or mentoring procedures in place to ensure that local area knowledge was passed along to pilots new to a specific operating area; the investigation was unable to establish the pilot’s experience operating in night IMC over mountainous terrain. All three crewmembers of the accident flight were relatively new in their respective roles. The pilot was hired by the operator about five months before the accident, while both clinicians had been assigned to the airplane for about six months. The company’s website highlighted the “Three to say go, one to say no” protocol as a best practice among air ambulance providers that empowers any member of the flight team, for any reason, to raise a safety concern. For rotorcraft flights, the operator required that clinicians with less than one year of experience be paired with clinicians with more than one year of experience, a practice that leveraged the flight team’s collective experience to benefit flight safety. However, fixed-wing operations were not subject to this requirement. The operator’s procedures also required company dispatchers to inform flight crews if a flight had been turned down by another operator. Although the weather can change throughout the course of a given day, the fact that other pilots and operators turned down flights due to weather in the area on the day of the accident should have been relayed to both the pilot and medical crew as part of their decision-making process. However, review of communication logs did not indicate that the company’s dispatchers made the accident crew aware of the turndowns earlier on the day of the accident. The operator also required that a flight risk assessment be completed before each flight; however, no such assessment was located for the accident flight. Even if a risk assessment had been conducted, the crew’s relative inexperience, and lack of information about the earlier turndowns, increased the likelihood of a knowledge-based error during the risk assessment and decision-making process. That an inexperienced flight crew was permitted to accept the accident flight given the weather conditions and the previous flight turndowns with no additional approval demonstrated an insufficient risk assessment process and lack of organizational oversight. Another fatal accident involving the accident operator occurred 71 days before this accident; the investigation into that accident also revealed the lack of a preflight risk assessment.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.