AVIAT A-1C-200 — Swan Valley, ID

What happened

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.

An editorial "what led to it / how to avoid it" analysis for this accident is generated separately and will appear here.