Loss of control in flight: Steen Skybolt, N202GM, fatal accident occurred on June 8, 2023, near Rio Vista Municipal Airport (O88), Rio Vista, California

• Location: Rio Vista, California 
• Accident Number: WPR23FA219 
• Date & Time: June 8, 2023, 11:24 Local 
• Registration: N202GM 
• Aircraft: Steen Skybolt 
• Aircraft Damage: Substantial 
• Defining Event: Loss of control in flight 
• Injuries: 2 Fatal 
• Flight Conducted Under: Part 91: General aviation - Personal

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On June 08, 2023, about 1124 Pacific daylight time, an experimental, amateur-built Steen Skybolt, N202GM, was substantially damaged when it was involved in an accident in Rio Vista, California. The two pilots were fatally injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The two pilots, both of whom were owners of the airplane, were returning to their home airport. Radar track information correlated to the accident airplane was consistent with the airplane departing and continuing southwest toward the destination airport before it turned south toward the accident airport. The airplane joined the left downwind leg of the traffic pattern (right traffic was specified for this runway), and after turning onto a base leg, made a sharp left turn and spiraled toward terrain, consistent with an aerodynamic stall. 

A witness reported hearing one of the pilots transmit "engine out" or "simulated engine-out"; however, the airport’s common traffic advisory frequency was not recorded, and the content of this transmission could not be confirmed. The witness also stated that the airplane crossed over the airport to join the traffic pattern and that the engine sounded normal.

Examination of the engine revealed continuity of the valve and drive train and compression in each of the cylinders. Removal of the cylinders revealed light scratches and corrosion of the combustion chambers and barrels. The intake and exhaust valves were intact and undamaged; the No. 3 exhaust valve was consistent in appearance with exposure to high temperatures. Severe spalling was noted on the faces of the camshaft intake lifters and several exhaust lifters; the camshaft lobes did not appear rounded. Although visual examination of the oil filter media did not reveal any metallic debris, microscopic inspection revealed metal particles, and there were several small pieces of metal in the oil suction screen. It is unlikely that any of these findings would result in a total loss of engine power. There was evidence of fuel in the fuel system.

A video recorded about five months before the accident was recovered from an iPad owned by one of the pilots. The video indicated that the airplane had recently exhibited problems with the trim system leading to the airplane oscillating and “throwing the elevator up and down.” A modification was made to the trim system sometime after the pilots purchased the airplane about seven months before the accident; however, there was no record of this modification in the available maintenance records. At the accident site, the right trim tab was sheared off at the piano hinge and came to rest under the elevator, yet remained connected to the bellcranks and was continuous to the fuselage bellcrank. When attempting to move to nose-down trim, the control cable could not move to that position because the cable would bind on the sleeve and not move over the attach fitting. Whether the modified trim system caused a problem inflight and what effect it may have had on the control surface and airplane’s controllability could not be determined.

The circumstances of the pilots’ decision to land at the accident airport could not be determined; however, based on the available information, it is likely that they exceeded the airplane’s critical angle of attack while maneuvering for landing, which resulted in an aerodynamic stall and loss of control at an altitude too low for recovery.

- Probable Cause: The pilots’ exceedance of the airplane's critical angle of attack while maneuvering to land, which resulted in an aerodynamic stall and subsequent loss of control.