The wreckage of a Grob G115D, a German-built, all-composite, aerobatic two-seat trainer, was spread out over an area nearly half a mile long and 400 feet wide-this despite the fact the airplane was sighted shortly before the accident by a witness on the ground who estimated its height, in level flight, as only 500 feet. The weather was good and was not considered a factor in the mishap.
Parts widely scattered along a line indicate an inflight breakup. The first items to separate from the airframe are at the beginning of the wreckage path; the fuselage and engine are usually found at the end. In this case, the first item in line was the top of the rudder. Various other pieces of empennage debris, including the stabilizer and both elevators, were found 900 feet from the main wreckage, as was a portion of the left wing and flap. Fragments of the acrylic canopy were scattered along the wreckage path, but although both pilots were wearing parachutes, there was no evidence of their having jettisoned the canopy. The lower portion of the rudder was never found.
The only part of the airframe left intact was the left aileron, and it was carefully examined by accident investigators. They knew from the maintenance logs that the airplane had been repainted 96 flight hours before the accident, but the flight control surfaces had not been rebalanced. They found that the aileron, at 7.15 pounds, was both three-quarters of a pound heavier than when originally installed (according to the manufacturer's records) and considerably more tail heavy.
The nose- or tail-heaviness of a control surface is expressed as a "residual hinge moment." "Residual" simply means what unbalance remains after the surface balance weights have been installed. "Hinge moment" is what you get when you support the surface on its hinges, put a weighing scale under the trailing edge and multiply the scale reading by the distance from the hinge line to the point where the surface rests on the scale. For example, a scale reading of four ounces with the scale contact 12 inches behind the hinge line would be described as a residual hinge moment of 0.25 foot-pounds (one-quarter pound times one foot).
Grob specifications for aileron balance permitted a hinge moment range of -0.22 foot-pounds (a negative hinge moment means the surface is leading-edge heavy) to +0.074 foot-pounds. The measured hinge moment of the recovered aileron was "between +0.138 and +0.200 foot-pounds"-considerably out of factory limits in a tail-heavy direction.
It's hard to understand how a coat of paint can add three-quarters of a pound to a surface area of only around 15 square feet, but the National Transportation Safety Board's (NTSB) report on the accident passes over this oddity without comment. Apparently, however, investigators hypothesized that the other control surfaces, including the mostly vanished rudder, might have been similarly overweight and underbalanced, and Grob's analysts affirmed that rudder flutter was possible under these conditions. The probable cause of the accident, the NTSB concluded, was "failure of maintenance personnel to rebalance the flight controls after the airplane had been repainted, which resulted in rudder flutter and inflight breakup of the airplane."
