What happened
On August 26, 2012, an experimental Fokker DR-1 triplane, registration N152RB, departed controlled flight and impacted terrain approximately two miles northeast of Parker, Colorado. The aircraft, a full-scale replica of a German World War I fighter, was being operated on a personal flight from Centennial Airport (APA) to Platt Valley Airpark.
After receiving takeoff clearance for an eastbound departure, the aircraft climbed to 6,700 feet MSL. Radar data indicated that approximately five minutes after departure, the airplane initiated a descending left turn. The aircraft disappeared from radar at 6,400 feet MSL, shortly before impacting the ground at an elevation of 6,170 feet MSL.
Witnesses near the accident site reported hearing the engine sounding normal as the aircraft approached. One witness observed the airplane traveling west to east at a low altitude, noting it appeared to be yawing to the left. Upon moving around a nearby building, the witness observed the aircraft in a tight left descending spiral with a nose-down pitch of approximately 45 degrees.
The pilot, the sole occupant of the aircraft, sustained one serious injury and was not killed in the accident.
The investigation
Investigators examined the wreckage, which included the engine, propeller, fuselage, and all three wings. The engine, a Lycoming IO-360-B4D, was found to have been functioning normally; testing confirmed that the fuel system, magnetos, and compression were all within operational parameters. Fuel samples taken from the refueling vehicle used prior to the flight were found to be clean and free of contaminants.
Mechanical examination of the flight controls confirmed that the ailerons and rudder remained functional. The investigation also noted that the aircraft's design, intended to mimic a highly maneuverable military fighter, featured inherent instability and high pitch sensitivity.
Findings
At the time of the accident, the density altitude in the area was approximately 8,250 feet. Due to the high density altitude, the aircraft's rate of climb was estimated to be reduced by approximately 67 percent. The investigation concluded that the airplane likely stalled due to the combination of reduced airspeed and diminished climb performance caused by these high density altitude conditions.