Occurrence
The flight was scheduled as an aerobatic flight, but no witnesses observed the airplane maneuvering. Hunters reported hearing the engine 'revving up' and saw pieces of the airplane falling to the ground.
Wreckage
Wreckage was scattered over an area approximately 2,400 feet long by 400 feet wide. Separated pieces of the airplane included an 8-foot section of the left wing, pieces of the left and right elevators, and other debris from the empennage and canopy. The top portion of the rudder assembly was found about 2,400 feet from the main wreckage.
Aileron Examination
The left aileron was found intact and weighed 7.15 pounds. Balance checks of the left aileron showed that its residual hinge moment was between +0.138 and +0.200 foot-pounds (trailing edge heavy), exceeding the Grob specification which ranges from -0.22 (leading edge heavy) to +0.074 foot-pounds (trailing edge heavy). Manufacturing records indicated that when the left aileron was originally installed, it weighed 6.40 pounds and had a residual hinge moment of +0.071 pounds.
Maintenance History
A review of the aircraft maintenance logs revealed the airplane had been repainted, but the flight control surfaces had not been rebalanced.
Other Control Surfaces
Estimates of the residual hinge moments of the other flight control surfaces, including the rudder, disclosed that they also exceeded Grob's trailing-edge-heavy service limits. Grob's flutter analysis, based on control connection stiffness, indicated potential rudder flutter under these conditions.
Additional Findings
Oxide deposits found on several of the elevator hinge contact surfaces evidenced disbonding/delamination.