SEA07LA090 · Burton Fly Baby

The pilot departed the airport for a local personal flight in the amateur built experimental-category airplane. About 25 minutes after takeoff, the airplane was observed flying in an easterly direction at an undetermined altitude. The witness reported hearing the engine slow down and then speed up. Seconds later, the witness heard "what sounded like an explosion" followed by the airplane falling to the ground in multiple pieces. The witness was located approximately 1 mile from where the wreckage came to rest. Post accident examination of the wreckage revealed that the right wing's forward and aft solid flying wires, turnbuckles and respective wire anchor attachment brackets remained intact, however the attachment brackets were separated from the forward and aft spar assemblies. The four through bolts (AN3 3/16-inch-diameter) associated with the forward brackets were fractured along the bolt shank near the bolt head. The bolts were necked down and were bent near the fracture points; the fracture surfaces were angular. Only two of the four through bolts associated with the aft bracket were recovered and similar findings were noted; the bolts were necked down and the fracture surfaces were angular. The builder's instruction manual for the airplane recommends AN3 (3/16-inch-diameter) anchor attach bracket bolts for standard operations and AN4 (1/4-inch-diameter) anchor attach bracket bolts if aerobatics are intended to be performed in the airplane. The specified anchor attachment bracket bolts (both AN3 and AN4) range in length from 5-1/2-inches to 6-3/4-inches. A personal friend of the pilot reported that about 2 weeks before the accident, the pilot completed a flight in the accident airplane. Upon returning from the flight, the pilot stated that he had completed a "slow roll." The pilot stated the airplane performed well during the flight.

Verbatim NTSB analysis and probable cause from the NTSB dataset