Pilot Debrief

PILATUS PC-12/45 near Christoval, TX — 2023-12-14

Final reportCEN24FA064
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Date
2023-12-14
Location
Christoval, TX, USA
Aircraft
PILATUS PC-12/45
Registration
N188PC
Category
Airplane
Highest injury
Fatal
Fatalities
2

Probable cause

The pilot’s loss of control due to spatial disorientation while operating in instrument meteorological conditions. Contributing to the accident were the controller’s failure to issue an appropriate intercept angle for the final approach course, failure to advise the pilot that vectors would take the airplane through the final approach course, and the pilot’s entry of an incorrect heading into the autopilot.

Contributing factors

Contributing to the accident were the controller’s failure to issue an appropriate intercept angle for the final approach course, failure to advise the pilot that vectors would take the airplane through the final approach course, and the pilot’s entry of an incorrect heading into the autopilot.

NTSB narrative

The instrument-rated pilot and a passenger departed on an instrument flight rules (IFR) flight. After about 4.5 hours, while approaching the first planned fuel stop, the airplane was in instrument meteorological conditions (IMC). According to air traffic control (ATC) data, the controller instructed the pilot to turn left to heading 210° for vectors to the final approach course for an RNAV (GPS) approach. About three minutes later, the controller issued a heading of 270°. Thirty-seven seconds later, the controller instructed the pilot to turn right to 030° and maintain 4,300 ft mean sea level (msl) until established on the final approach course. The final approach course heading was 002° magnetic (6.7° true). About 15 seconds later, the airplane crossed the final approach course from east to west on a track of 298° true, an angle of about 69° left of the final approach course. About three seconds later, the pilot reported to ATC “we overshot it,” to which the controller replied, “I know, that’s why I gave you a 030.” According to onboard GPS data, at about the same time, the airplane was in a 26.5°-roll right turn when the roll abruptly steepened. Over the next 10 seconds, the right roll increased from 26.5° to 92.4°. With the airplane rolled 92° to the right, the pilot reported “we got a problem.” This was the last transmission from the airplane. Between the pilot’s transmissions of “we overshot it” and “we got a problem,” the pitch angle decreased from about 1° to about -32°, the airspeed increased from 200 knots indicated airspeed (KIAS) to 240 KIAS, and the rate of descent increased from 0 feet per minute (fpm) to 13,000 fpm. Shortly before impact, the pitch angle reached -37° before starting to recover. At the end of the recorded GPS data, the pitch angle was -22.5°, the roll angle was 51.6° right, the airspeed was 283 KIAS, and the rate of descent was about 15,000 fpm. The airplane impacted terrain about 2.3 nm west and 14.7 nm south of the runway 36 threshold. Postaccident examination of the accident site revealed a long debris field consistent with a high-speed impact and a right-wing-low and a nose-low pitch attitude. The surrounding vegetation exhibited fuel blight. All major components of the airplane were located in the debris field. Examination of the airframe did not reveal evidence of any mechanical malfunctions or failures that would have precluded normal operation. However, the degree of fragmentation and fire damage precluded a complete evaluation of the airplane’s flight controls and autopilot system. Rotational contact signatures on the internal engine components were consistent with the engine producing power at impact. Three propeller blades exhibited pronounced bending opposite the direction of rotation. No evidence of preimpact mechanical malfunctions or failures of the engine or propeller was identified that would have precluded normal operation. Postaccident examination of the airplane’s central advisory and warning system (CAWS) annunciator lights revealed that the master caution, autopilot trim and autopilot disengage lights were illuminated at the time of impact. The airplane’s configuration was commensurate with flight in precipitation but not icing conditions. The illuminated annunciators were consistent with the airplane’s upset and autopilot disconnect sequence. A performance and simulation study supports several observations regarding the airplane’s performance during the descent from 4,300 ft msl, its increasingly steep descending right turn shortly after it flew across the final approach course, and its final dive and impact with terrain. Based on the sequence of events and the recorded autopilot parameters, the autopilot remained engaged while the roll angle increased past normal autopilot limits (typically 30°), until it disconnected after the roll angle increased past the autopilot’s engagement limit of 75°. The study further suggested a scenario consistent with the data where, while the pilot was receiving vectors for the approach, the autopilot’s lateral mode was likely “heading hold”, because the ADS-B “SelHdg” parameter matched the headings provided by the controller, except for the final 030° heading. After the controller gave the 030° heading, the pilot changed SelHdg to 300°, and not 030°. After the airplane crossed the final approach course, the ADS-B LNav discrete switched from 0 to 1, which indicated that lateral navigation was engaged. The active waypoint at that time was likely the final approach fix (FAF), with a bearing of 003°. Under these circumstances, it is improbable that the autopilot would have steered the airplane to the 030° heading assigned by the controller. Instead, with heading hold selected, the autopilot would have continued on the selected heading of 300°. To keep the airplane turning right toward the 030° assigned heading, the pilot could have pressed and held the control wheel steering (CWS) button on the yoke or manually overpowered the autopilot servos, which would have allowed him to manually roll the airplane further to the right without disengaging the autopilot. The apparent pitch angle—that is, the pitch attitude perceived by the pilot’s vestibular system—would have ranged between -1° and 1°, and so conditions consistent with the production of a somatogravic illusion of level flight were present. As the airplane rolled through a right roll angle of 75°, the autopilot would have disconnected automatically, consistent with the change in the ADS-B autopilot discrete. Shortly after, the pilot announced that “we got a problem.” The sudden increase in the recorded normal load factor immediately before impact was consistent with an abrupt recovery attempt. Because the airplane had descended beneath the cloud base, the pilot may have acquired outside visual references and attempted to recover, but there was insufficient altitude to return the airplane to level flight before impact. The circumstances of this accident are consistent with the pilot experiencing spatial disorientation, most likely the somatogravic illusion. The performance and simulation study determined the apparent pitch angle remained nearly level during the initial 20 seconds of the final descent. In the absence of reliable outside visual references, such an apparent pitch attitude is conducive to the somatogravic illusion. In addition, the controller did not provide an appropriate intercept angle to the final approach course and did not advise the pilot that vectors would take the airplane through the final approach course and state the reason for doing so, as required by FAA Order JO 7110.65. While operating in IMC, the pilot was required to respond to an abrupt and unusually large heading change to re-intercept the final approach course. These atypical vectors required the pilot to rapidly modify the approach while operating in IMC, increasing his workload and cognitive demand as he attempted to quickly re-intercept the final approach course, a procedure that he likely did not anticipate. By increasing the pilot’s workload during a critical phase of flight, the deficient ATC services likely increased the pilot’s workload and contributed to the pilot’s spatial disorientation.

NTSB coding

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