Amateur-built TORNADO S N318WH accident near Haskell, Oklahoma

Probable cause

The student pilot/builder’s addition of winglets to the stabilator, which resulted in flutter and overstress failure of the stabilator control horn.

NTSB analysis

The student pilot departed on a flight in a kit-built airplane to test the winglets he had recently installed on the airplane’s stabilator, which were intended to eliminate the airplane’s uncommanded yaw. The flight was the airplane's first winglet test flight away from the airport traffic pattern and at a higher altitude. The student stated that during the flight, he decreased the pitch attitude to level off at 2,000 ft above ground level and the airplane suddenly vibrated aggressively and it "felt like the tail was thumping." He decreased the engine power, but the thumping and vibrating continued, so the pilot pitched the airplane down for an off-field emergency landing. Before he was able to land, the airplane rolled inverted and descended into the trees, resulting in the separation of the right wing.

Postaccident examination revealed that the stabilator control horn had fractured due to overstress and separated from the push-pull tube. According to an airplane kit manufacturer engineer, winglets added to the stabilator produce flutter. Based on the evidence, it is likely that the winglets led the stabilator to flutter, which overstressed the stabilator control horn and resulted in the control horn’s failure.

Text from the NTSB. Open the NTSB docket for CEN20LA173 for the full record.