WPR24FA257

The pilot and six passengers departed on a cross-country flight on an instrument flight rules (IFR) flight plan with a planned refueling stop in Montana. The airplane flew on a northwest course consistent with its planned destination. The airplane leveled off at a maximum cruise altitude of about 26,000 ft mean sea level (msl). Satellite imagery indicated that, at the accident site about the time of the accident, cumulus cloud bases were about 10,000 ft msl with rain showers; and the cloud tops were about 28,500 ft msl. The sounding model indicated trace to light icing with a low probability of supercooled large droplets (SLD) along about the last 20 minutes of the airplane’s flight path. The accident airplane was equipped for flight into known icing conditions and likely would not accumulate much, if any, structural icing from trace to light icing. Additionally, SLDs were likely not a factor given their low probability of occurrence and the flight profile of the accident airplane during the last few minutes of flight. During the last portion of the flight, about 15 seconds before the airplane entered an area of precipitation as depicted by weather reflectivity charts, the pilot reported a loss of the autopilot and declared an emergency to the air route traffic control center controller. The controller asked the pilot where he would like to land and the pilot responded that he was trying to get control of the airplane; the controller told the pilot to advise if he required additional assistance. The controller then relayed the minimum IFR altitude and current altimeter setting with no further response from the pilot. Data indicate that shortly after entering the area of precipitation, the airplane maneuvered to the right of course, climbed to about 26,525 ft msl, and entered a 270° right turn. In the first 180° of the turn, the airplane descended to about 25,250 ft msl; however, it climbed to about 27,025 ft msl in the last 90° of the turn. Shortly after, the data showed the airplane made a slight left turn before it began a descending 180° right turn. The last data point showed the airplane on a southerly heading, at an altitude of about 21,900 ft msl, and with a groundspeed of 256 knots. Sound modeling data indicate that rain shower or thunderstorm activity had the potential for an updraft speed of up to 3,346 ft/min. ADS-B data indicated that the autopilot disconnected a few seconds after the airplane encountered the precipitation, likely when the airplane encountered an updraft because the autopilot was unable to counteract an updraft of the strength indicated by the modeling data. The airplane was likely operating in instrument meteorological conditions (IMC) throughout the remainder of the flight. Following the autopilot disengagement, the pilot had to manually maintain control of the airplane while operating in IMC, increasing his susceptibility to spatial disorientation. The airplane’s erratic flight track in the final 2 minutes offlight, culminating in the final rapidly descending right turn, were consistent with the known effects of spatial disorientation. Furthermore, the pilot’s focus to re-engage the autopilot likely created an operational distraction that contributed to a breakdown in his instrument scan. The airplane likely partially broke up at low altitude during its rapid descent to the ground, consistent with an exceedance of the airplane’s maximum operating speed. Pieces of the left wing were found up to 0.82 miles from the main wreckage and exhibited signatures of overload. Impact signatures at the accident site, witness observations, and review of the flight data were all consistent with the pilot losing control of the airplane. Furthermore, postaccident examination of the airplane wreckage revealed no evidence of preimpact failures or malfunctions that would have precluded normal operation. Specifically, while the airplane sustained significant impact damage, the portions of the flight control system that could be examined showed no evidence of disconnections or preimpact failures. In addition, the airplane’s setting for the pitch trim was near neutral and for the roll trim was slightly right wing down.

The FAA's Pilot's Handbook of Aeronautical Knowledge contained the following guidance: Under normal flight conditions, when there is a visual reference to the horizon and ground, the sensory system in the inner ear helps to identify the pitch, roll, and yaw movements of the airplane. When visual contact with the horizon is lost, the vestibular system becomes unreliable. Without visual references outside the airplane, there are many situations where combinations of normal motions and forces can create convincing illusions that are difficult to overcome. The FAA’s Airplane Flying Handbook (FAA-H-8083-3) described hazards associated with flying when visual references, such as the ground or horizon, are obscured: The vestibular sense (motion sensing by the inner ear) in particular tends to confuse the pilot. Because of inertia, the sensory areas of the inner ear cannot detect slight changes in the attitude of the airplane, nor can they accurately sense attitude changes that occur at a uniform rate over a period of time. On the other hand, false sensations are often generated; leading the pilot to believe the attitude of the airplane has changed when in fact, it has not. These false sensations result in the pilot experiencing spatial disorientation. The FAA publication “Medical Facts for Pilots” (AM-400-03/1) described several vestibular illusions associated with the operation of aircraft in low-visibility conditions. The somatogravic illusion, which involves the semicircular canals of the vestibular system, was generally placed into the "graveyard spiral" category. According to the publication text, the graveyard spiral: …is associated with a return to level flight following an intentional or unintentional prolonged bank turn. For example, a pilot who enters a banking turn to the left will initially have a sensation of a turn in the same direction. If the left turn continues (~20 seconds or more), the pilot will experience the sensation that the airplane is no longer turning to the left. At this point, if the pilot attempts to level the wings this action will produce a sensation that the airplane is turning and banking in the opposite direction (to the right). If the pilot believes the illusion of a right turn (which can be very compelling), he/she will reenter the original left turn in an attempt to counteract the sensation of a right turn. Unfortunately, while this is happening, the airplane is still turning to the left and losing altitude. According to the FAA Safety Briefing Magazine article, It’s a Confusing World Up There: When you make a sudden return to straight and level flight after a climb, it can feel like you are tumbling backward. This is known as “inversion illusion.” The disorientation you feel from this might lead you to push your aircraft abruptly into a nose-low attitude, which can intensify the illusion. Like the “inversion illusion,” the “elevator illusion” is also caused by an abrupt change. A sudden upward verticalacceleration, as can occur in an updraft, can stimulate your otolith organs and create the illusion of being in a climb. This could make you want to push the aircraft into a nose-low attitude. An abrupt downward vertical acceleration, usually in a downdraft, has the opposite effect making you want to pull the aircraft into a nose-up attitude. The FAA Office of Aerospace Medicine published a research report titled “Spatial Disorientation in Fatal General Aviation Accidents (2003-2021)” in 2025. The study’s results showed that the majority of spatial disorientation accidents occurred during the en route portion of the flight the pilot age group with the most accidents was between 60-69 years old, and identified that the type of airplane involved in these accidents is also an important consideration. The analysis identified that most accidents involved single-engine aircraft, but that a growing number of accidents involving turboprop or turbojet aircraft occurred over the 19-year span, beginning largely around 2013. These complex aircraft are highly automated to accommodate single-pilot operations, which may lead to more reliance on autopilot. FAA Advisory Circular (AC) 91-74B, Pilot Guide: Flight in Icing Conditions defines trace and light icing: Trace icing. Ice becomes noticeable. The rate of accumulation is slightly greater than the rate of sublimation. A representative accretion rate for reference purposes is less than ¼ inch (6 mm) per hour on the outer wing. Deicing/anti-icing equipment is not utilized unless encountered for an extended period of time (over 1 hour). Light Icing. Light Icing. The rate of ice accumulation may create a problem if flight is prolonged in this environment (over 1 hour). Requires occasional cycling of manual deicing systems to minimize ice accretions on the airframe. The Pilatus PC-12/47E airplane has a low wing, T-tail, and a retractable landing gear. The flight control system uses push-pull rods and steel cables, in addition to an electric trim system, for movement of the ailerons, rudder, and elevator. An aileron-rudder interconnect system provides turn coordination and lateral stability. PC-12/47E Pilot’s Operating Handbook and Airplane Flight Manual Section 2, Limitations: Airspeed Limitations: The maximum operating speed (VMO) was 240 knots indicated airspeed (KIAS) up to an altitude of about 15,200 ft. According to the Maximum Operating Speed chart, at 26,000 ft, the maximum operating speed was about 194 KIAS. Section 4, Normal Procedures, Severe Icing Conditions: The following weather conditions may be conducive to severe in-flight icing: • visible rain at temperatures below 0 degrees Celsius ambient air temperature. • droplets that splash or splatter on impact at temperatures below 0 degrees Celsius ambient air temperature. Section 7, Airplane and Systems Description, Airfoil Deice System: The airplane is equipped with inflatable pneumatic deicing boots fixed to the leading edges of the wings (two bootsper wing - inboard and outboard) and to the horizontal stabilizer. Their purpose is to inflate and dispense any ice which may accrete on their surface during flight in atmospheric icing conditions. The PC-12/47E is equipped with a Honeywell Primus APEX integrated avionics system. The Primus APEX is composed of multiple subsystems including, but not limited to, the electronic display system, automatic flight control system, crew alerting system, flight management system, traffic alert and collision avoidance system, terrain awareness and warning system, and weather radar system. The automatic flight control system has a dual-channel autopilot, yaw damper, flight director, and automatic pitch and yaw trim. The Automatic Flight Control System (AFCS) Autopilot: Auto flight control is accomplished with aileron, elevator and rudder servo actuator motors. Autopilot disengagement is defined as either normal or abnormal. A normal disengagement is initiated manually by pressing the AP DISC push-button on the control wheel or by the AP push button on the FC or by activating the manual trim system. A normal disconnect will cause the AP indication on the PFD to flash red/white and the aural “Cavalry Charge” warning tone to be activated. After 2.5 seconds the AP indicator and audio are removed. Any disengagement due to a monitor trip or failure is considered abnormal. An abnormal disconnect will cause the AP indication on the PFD to flash red/white and the aural warning tone to be activated until acknowledged via the AP DISC push-button. For some failures an autopilot disengagement will be accompanied by a CAS [crew alerting system] advisory indicating the reason for the disengagement. Section 3, Emergency Procedures: B. ABNORMAL DISCONNECT Indication: Flashing red AP on PFD and continuous “Cavalry Charge” aural warning. 1. Airplane Control Wheel GRASP FIRMLY and regain aircraft control 2. Autopilot Disengage Switch PRESS to cancel aural warning (pilot or co-pilot wheel) 3. Aircraft RETRIM manually as necessary 4. Aircraft If no AFCS associated CAS messages attempt to re-engage autopilot once A lightweight data recorder (LDR) was not found at the main wreckage site or in the recovered wreckage. Pilatus installed the LDR on new production PC-12 airplanes beginning with airframe S/N 1271 (the accident airplane was S/N 1203). A retrofit for certain airframe S/Ns preceding S/N 1271, including the accident airplane, was available via Pilatus service bulletin. The airworthiness file for the accident airplane did not show a record of a LDR installation prior to the accident. The LDR was not required to be installed. On July 26, 2024, about 1304 MDT, a Pilatus Aircraft LTD, PC-12/47E airplane, N357HE, was destroyed when it was involved in an accident near Recluse, Wyoming. The pilot and six passengers were fatally injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. A review of ADS-B and OpsVue data provided by the FAAshowed that the airplane departed Nebraska City Municipal Airport (AFK), Nebraska City, Nebraska, about 1200 local time, and flew on a northwest track with an intended destination of Billings Logan International Airport (BIL), Billings, Montana. The data showed that the airplane initially leveled off at about 24,000 ft msl for about 430 miles and then climbed to a maximum cruise altitude of about 26,000 ft msl for the last 140 miles. About 1302 MDT, the airplane maneuvered to the right of course and climbed to about 26,550 ft msl and entered a 270° right turn. In the first 180° of the turn, the airplane descended to about 25,250 ft msl; however, it then climbed to about 27,075 ft msl in in the last 90° of the turn. Shortly after, the data showed the airplane made a slight left turn before it began a descending 180° right turn. The last data point showed the airplane on a southerly heading, at an altitude of about 21,900 ft msl, and with a groundspeed of 256 knots. Figure 1. A graphic created with OpsVue data showing 10 points of a profile view of the accident flight. According to ADS-B data, the autopilot was disengaged at 1302:10. Subsequently, the airplane’s vertical rate indicated a climb for about the next 30 seconds. The maximum rate of climb indicated was 2,496 fpm. The airplane’s vertical rate indicated no change before the autopilot disconnected. Between 1303:21 and 1303:23, the autopilot indicated it was re-engaged. During the last portion of the flight, just before the airplane veered off course, the pilot reported to the Salt Lake City Air Route Traffic Control Center controller the loss of the autopilot and declared an emergency. An aural alert was heard in the background of the pilot’s radio transmissions that was consistent with the “cavalry charge” warning tone, which annunciates during either a normal or abnormal autopilot disconnect. The controller asked the pilot where he would like to land and initially received no response. The pilot then responded that he was trying to get control of the airplane; the controller told the pilot to advise if he required additional assistance. The controller then relayed the minimum IFR altitude and current altimeter setting with no further response from the pilot. Following a loss of radio and radar contact, the FAA issued an Alert Notice (ALNOT) for the airplane. Afterward, the airplane was located by local law enforcement in remote terrain about 12 miles northeast of Recluse, Wyoming. One witness near the accident site heard a “loud whining noise” that diminished and then increased, followed shortly thereafter by smoke emanating from the area of the accident site. Another witness observed the airplane overhead in a “barrel roll” type maneuver and heard the airplane’s engine “roaring loud” until they heard the airplane impact terrain. The witness stated that they saw smoke in the area shortly after. A third witness, who started a conversation with the accident airplane occupants at AFK,departed a few minutes before the accident airplane and planned to fly a similar route to Montana. The pilot commented that he intentionally flew a more southerly route to avoid the icing at 20,000 ft msl and above. His route was mostly clear of clouds until he descended but he observed the weather the accident airplane encountered at 26,000 ft msl and was surprised that they did not go around or fly above the weather. In a postaccident interview, he commented that the accident flight’s routing would have been a rough ride from the clouds building up and further said he was sure they were in the clouds and icing. An autopsy and toxicology testing of the pilot were unable to be conducted due to the circumstances of the accident. The NWS Surface Analysis Chart centered over the northcentral United States about the time of departure (1200) depicted a low-pressure center over central Idaho at 1010-hectopascals (hPa) along a frontal boundary that stretched across southern Montana, Wyoming, and into South Dakota with the accident site located north of the front on the cold side of the boundary. Fronts can act as lifting mechanisms to help produce clouds and precipitation if sufficient moisture is present. The NWS Storm Prediction Center Constant Pressure Charts for 1800 MDT indicated a mid- and upper-level trough at 700- and 300-hPa above the accident site. The SPC Day 1 Convective Outlook issued at 1026 MDT noted the accident site was located in an area where a chance of general thunderstorms was predicted with a marginal risk of severe storms further east over eastern Wyoming and the Dakotas. Northeast Wyoming Regional Airport (GCC) had the closest official weather station to the accident site. GCC had an Automated Surface Observing System (ASOS), and its longline reports were not augmented. The GCC ASOS was located 32 miles south of the accident site at an elevation of 4,365 ft. Automated GCC weather at 1253 MDT indicated wind from 030° at 10 knots, visibility 10 miles or greater, clear skies below 12,000 ft above ground level (agl), temperature of 25°C, dew point temperature of 6°C, and an altimeter setting of 30.05 inches of mercury (inHg). Remarks noted that the station had a precipitation discriminator, sea level pressure was 1011.8 hPa, the temperature was 25.0°C, and the dew point temperature was 5.6°C. The observations from the GCC reported VFR conditions prevailing at the time of the accident with a cloud ceiling at 11,000 ft agl. The 1300 MDT High-Resolution Rapid Refresh (HRRR) model sounding identified a mainly conditionally unstable environment from the surface through 20,000 ft, then a stable layer above 20,000 ft. The Rawinsonde Observation Program (RAOB), an interactive sounding analysis program, indicated an overcast cloud layer between 17,000 and 26,000 ft. Moderate clear and rime icing was indicated by RAOB between 15,000 and 24,000 ft. A most-unstable layer was calculated using between 10,000 and 16,000 ft as a cloud base and,given that range of cloud bases, the maximum vertical velocity (MVV) of potential updrafts was calculated to be 17 m/s or 3,346 ft/min. At the accident airplane’s altitude before descent at approximately 26,000 ft, the wind was from 222° at 38 knots, the temperature was around -20.3°C, and the dew point temperature was around -26.3°C. An overcast layer of cloud cover was indicated above the accident site with the cloud cover moving from southwest to northeast. The brightness temperature above the accident site was 248 Kelvin, which would have been near 28,500 ft based on the vertical temperature profile provided by the 1300 MDT HRRR sounding. A weather satellite cross-section along the accident flight path was recreated and provided to the investigation by the Cooperative Institute for Research in the Atmosphere (CIRA). The accident flight was located in an area of supercooled cloud cover based on the GOES-16 imagery. Figure 2: Reflectivity initiated at 1302:20 MDT. (Note: Magenta dotted line is the flight path). The KUDX WSR-88D base reflectivity images for the 0.44° and 0.84° elevation scans initiated at 1256:25, 1257:08, 1301:37, 1302:20, 1306:50, and 1307:32 MDT depicted reflectivity values between 10 and 20 dBZ in between the accident flight’s location between 1301 and 1303 MDT, with the depicted reflectivity growing in size near and east of the accident site between 1301 and 1317 MDT. The reflectivity targets were moving from southwest to northeast at the time of the accident. There were no lightning flashes reported within 25 miles of the accident site within 10 minutes prior to and following the accident time. The Graphical Forecasts for Aviation (GFA) products issued by the NWS Aviation Weather Center around 0700 MDT and valid at 1200 and 1500 MDT indicated VFR conditions with isolated (between 10 and 20 percent chance) of thunderstorms and a north to east wind of 10 knots. The GFA cloud forecast applicable to the accident site for 1200 MDT indicated cirrus clouds above the accident site. The GFA cloud forecast applicable for 1500 MDT indicated overcast cloud cover at 16,000 ft with clouds tops to FL390 at the accident site. There were no Graphical AIRMETs (G-AIRMET) valid for the accident site for below FL280 (G-AIRMET Tango was valid above FL280). The FIP icing probabilities and icing severity products for the 1-hour forecast valid at 1300 MDT at 23,000 through 26,000 ft indicated a 10% to 30% probability of trace to light icing above the accident site with higher probabilities and intensities below 23,000 ft. The CIP products indicated a 20 to 30% probability of trace to light icing from 23,000 ft through 26,000 ft at 1300 MDT above the accident site. Higher CIP probabilities and intensities were below 23,000 ft. The CIP indicated a 0 to 20% probability of SLDs above the accident site at 23,000 through 26,000 ft at 1300 MDT. A search of archived information indicated that the accident pilot did not request weather information fromLeidos Flight Service. The accident pilot did have an account with ForeFlight but did not file his flight plan through ForeFlight nor did he review or receive a weather briefing. It is unknown what, if any, weather information the accident pilot viewed before or during the accident flight. The private pilot held ratings for airplane single-engine land, multi-engine land, and an instrument rating. The pilot was issued an FAA third-class airman medical certificate on September 15, 2022, with the following limitation: must have available glasses for near vision. The pilot reported on his most recent application that he had accumulated 2,200 total hours of flight experience, with 70 hours in the 6 months before the examination. The pilot successfully completed recurrent Pilatus PC-12NG training at the Recurrent Pilot Training Center on February 6, 2024. Included in the training was unusual attitude recovery. Examination of the accident site revealed that the airplane impacted terrain on a magnetic heading of about 099°. The main wreckage was consistent with the airplane impacting the ground nose first, and nearly vertically with a slightly inverted attitude. The elevation of the main wreckage site was about 4,030 ft msl and located on a slope of about 20°. The initial impact point was a ground disturbance about 10 ft long, 41 ft wide, and 6 ft deep; the majority of the airplane wreckage was embedded in the ground. Surrounding the ground disturbance was a large area of burnt vegetation. Small fragments of the airplane were scattered throughout an approximate 300 ft radius from the main wreckage. A post-impact fire ensued that encompassed an area of about 40 acres, primarily to the west of the main wreckage. Two sections of the left wing and various fragments of the airplane were located along a debris path that extended between about 0.64 to 0.82 miles northwest from the main wreckage site on a heading of about 298° to 309° magnetic. Pieces of the left wing were found up to 0.82 miles from the main wreckage and exhibited signatures consistent with overload. All major components of the airplane were contained within the accident site and debris path. Examination of the autopilot pitch, trim, and yaw servos revealed that based on the extension measurements taken from the pitch and roll trim actuators, the trim settings for the airplane’s pitch and roll axes were estimated to be slightly nose up, because the horizontal stabilizer was tilted about 0.97° leading-edge-down from the neutral position, which trims the airplane slightly nose up, and roll trim was slightly right-wing-down, about 3.7°. The yaw trim setting could not be determined because the yaw trim actuator was not recovered. To the extent possible because of the significant damage sustained, postaccident examination of the airframe and engine revealed no evidence of preimpact mechanical failures or malfunctions that would have precluded normal operation. The Primus APEX Modular Avionics Unit(MAU) was found in the wreckage site and was significantly damaged. No flight data was able to be recovered due to deformation damage to the compact flash card that stores flight data. Additionally, the nonvolatile memory chip, which normally stores the CAS messages, was not found.