The airline transport pilot and a passenger were conducting a formation pleasure/sightseeing flight in an Aviat A-1C-200 with another airplane in mountainous terrain near Swan Valley, Idaho.
The pilot of the second airplane, who was flying behind the Aviat, 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 hit terrain. The pilot of the lead airplane died in the crash.
The density altitude about the time of the accident was over 10,990 feet. Data recovered from the airplane’s avionics system indicated that the engine was producing full power throughout the 24-minute flight. Before impact, the 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, who was seriously injured in the crash, 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.
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 Identification: WPR16FA013
This October 2015 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.