Gulfstream Corporation 695A N965BC accident near San Bernardino, California

Probable cause

The pilot’s continued use of the airplane’s autopilot after flight into moderate to severe icing conditions, which resulted in an exceedance of the airplane’s critical angle of attack and an aerodynamic stall.

NTSB analysis

The accident pilot had been in a recurrency training program in the accident airplane the week before the accident flight, which was the return flight after completing the week with the instructor. According to the instructor, the pilot wanted to proceed home so they had planned for the accident pilot to depart on that leg of the flight no later than 1830 for a route of flight that remained west of specific terrain due to weather forecasts and that would arrive at the destination airport before dark. The pilot subsequently departed about 30 minutes past the planned departure time and flew a route east of the terrain that had been discussed, which placed the airplane in severe weather conditions after dark.

While descending from its cruising altitude, the airplane encountered turbulence and the autopilot disengaged. For the remainder of the flight, multiple transmissions were recorded between air traffic control (ATC) and the pilot regarding course corrections, altitudes, icing conditions, and issues with the airplane’s autopilot. ATC offered vectors from the arrival procedure; however, the accident pilot declined the vectors. After receiving another pilot’s report of airframe icing from ATC, the accident pilot confirmed to ATC that he had accumulated moderate rime ice and agreed when ATC offered a lower altitude. When ATC asked, the pilot confirmed that the airplane remained in icing conditions during the continued descent, and the airplane subsequently leveled at 7,200 ft above mean sea level (msl).

ATC instructed the pilot to climb the airplane to the last assigned altitude of 7,400 ft msl; however, the airplane remained at 7,200 ft msl for 29 seconds and began a right turn. During flight at 7,200 ft msl, the airplane’s groundspeed slowed from 124 kts to 84 kts. ATC asked the pilot if he was ready to turn back to the left and the pilot responded “turning.” The airplane subsequently began a rapid descent from which the pilot did not recover.

Recorded weather closest to the accident site reported wind from the southwest at 15 kts, with gusts to 23 kts, rain, and a cloud ceiling of 1,100 ft above ground level (agl). Multiple AIRMETS and SIGMETS were in effect throughout the planned route of flight, and areas of moderate to severe icing were forecast. The airplane flew its descent in instrument meteorological conditions and moderate to severe icing conditions, including supercooled liquid droplets (SLD).

Given the conditions, the airplane likely encountered serious icing during the descent. The pilot continued to use the airplane’s autopilot system while flying in sustained moderate to severe icing conditions. It is likely that either the autopilot masked the effects of the ice accretion on the airframe until it either reached its limits of use and disengaged at a low attitude or that it remained engaged until the airplane exceeded its critical angle of attack and entered an aerodynamic stall. The sudden onset of a rapid descent and subsequent near-vertical impact are consistent with a loss of control due to an aerodynamic stall.

Postaccident examination of the airframe and engines revealed no evidence of any preexisting mechanical failures or anomalies that would have precluded normal operation.

Medical review indicated that the pilot had used a cannabis product and it is possible that he could have been experiencing associated impairing effects at the time of the accident. However, the exact timing of his last cannabis use or level of impairment could not be determined from muscle tissue results alone.

Text from the NTSB. Open the NTSB docket for WPR24FA124 for the full record.