What happened
On August 11, 2001, at approximately 17:08 local time, a Fokker DR1 replica took off from the Michelstadt/Odenwald airfield to perform a demonstration flight during an aviation event. Shortly after takeoff, the aircraft entered a formation flight alongside a Nieuport biplane at an altitude of approximately 400 feet. As the formation separated and the Fokker DR1 entered a shallow right-hand turn, the vertical stabilizer detached from the aircraft above the upper attachment fitting. The detached component fell onto the elevator, causing the aircraft to enter an uncontrolled flight path. At 17:11, the aircraft struck the ground with high impact force northwest of the runway. The pilot sustained fatal injuries in the crash, and the aircraft was destroyed.
The investigation
The BFU investigation utilized video footage captured by spectators to reconstruct the sequence of events. Examination of the wreckage revealed that the fuselage had broken near the pilot's seat and the left wing had been lost. The investigation focused on the structural integrity of the vertical stabilizer, which was a pendulum-type design covered in fabric. An expert examination of the stabilizer identified that the leading tube, constructed of steel, had been weakened by fatigue cracks. Specifically, the area above the welded attachment fitting had lost up to 40% of its cross-sectional strength due to fatigue. These cracks were not visible from the exterior because they were partially obscured by the fabric covering. The investigation also noted that while a helicopter hovering near parked aircraft earlier that day had caused control surfaces to move violently, this event was not the primary cause of the cracks, though it may have contributed to the final failure.
Findings
- The primary cause of the accident was the structural failure of the vertical stabilizer due to fatigue cracking.
- The fatigue cracks originated from vibrations inherent to the tail surfaces of a single-engine aircraft operating within the propeller slipstream.
- The loss of the vertical stabilizer resulted in a total loss of directional stability, and the presence of the detached component on the elevator likely compromised longitudinal stability as well.
- The aircraft became unrecoverable immediately following the structural separation.