Aviat A-1C-200 N280TB accident near Swan Valley, Idaho

Probable cause

The pilot's failure to maintain airspeed while maneuvering in high density altitude conditions over mountainous terrain, which resulted in the airplane exceeding its critical angle of attack and experiencing an aerodynamic stall.

NTSB analysis

The airline transport pilot, and one passenger, were conducting a formation pleasure/sightseeing flight with another airplane in mountainous terrain. The pilot of the second airplane, who was flying in trail behind the accident airplane, stated that the two airplanes flew into a canyon area. As they approached a ridgeline, the second pilot performed a 360° climbing turn to gain altitude prior to crossing the ridgeline. After he completed his turn, he noticed a dust cloud on the ground ahead of him, and realized that the lead airplane had impacted terrain. Postaccident examination of the airplane revealed no evidence of mechanical malfunctions of failures that would have precluded normal operation. The density altitude about the time of the accident was over 10,990 ft.

Data recovered from the airplane's avionics system indicated that the engine was producing full power throughout the 24-minute flight. Before impact, the airplane's airspeed decreased to between 48 and 50 knots. The airplane's published stall speed was between 46 and 55 knots, depending on the airplane's configuration. Signatures at the accident site and the damage to the airplane indicated a near-vertical impact, consistent with an aerodynamic stall. The passenger reported that, before impact, the airplane was in a turn, and that she heard a beeping sound, consistent with activation of the aural stall warning.

Despite the fact that the airplane's engine was producing full power, the high density altitude conditions would have degraded the engine's performance and the airplane's ability to climb. It is likely that, as the pilot was maneuvering the airplane in an attempt to climb over the rising terrain, he allowed the airspeed to decay and the airplane exceeded its critical angle of attack and subsequently experienced an aerodynamic stall.

Text from the NTSB. Open the NTSB docket for WPR16FA013 for the full record.