American AA5 N5880L

The flight instructor reported that, during a local instructional flight, while descending in smooth air at an airspeed below the yellow arc, the airplane began shaking and buffeting violently and loudly. The control yoke was also shaking violently (left, right, fore, and aft), and the airplane was pitching up and down. The instructor took control of the airplane from the student pilot, declared an emergency, then returned for landing after being informed by the pilot of a chase airplane that their left elevator was “flapping in the wind.” With reduced elevator authority due to the displaced position of the left elevator, the airplane landed hard, and the nose landing gear collapsed. Postaccident examination of the airplane revealed that the left elevator remained attached to the bellcrank and supported at the inboard support bearing assembly, but the outboard support bearing assembly was separated from the outer rib of the left horizontal stabilizer, leaving the elevator displaced down from its normal position. The outboard support bearing assembly and a separated aft section of the outboard rib of the left stabilizer were not located or recovered. Relatively coarse striations intermixed with dimple features, consistent with cyclic overstress loading, were noted on the fractured outer rib of the left horizontal stabilizer. Additionally, the aft spar for the left horizontal stabilizer was buckled on the upper and lower flanges, consistent with upward and downward overstress loading during a flutter event. The cracks and fractures on the right outboard rib had coarse striations intermixed with dimple features, consistent with cyclic overstress loading, providing further evidence of loading associated with elevator flutter. The elevator flutter likely occurred due to separations at the bondlines of the left horizontal stabilizer that reduced the overall stiffness of the structure. Bondline separations in the horizontal stabilizer, such as those observed on an exemplar horizontal stabilizer, may have been present at the outboard rib-to-skin bondlines, which could have weakened the area around the outboard bearing support bracket and made the elevator more susceptible to flutter. Since a separation of the left outboard elevator bearing support bracket would tend to relieve loads on the aft spar, this indicated the failure of the left horizontal stabilizer outboard rib likely occurred after the aft spar buckled due to overstress loading. Although both elevator trim tabs were disbonded along most of the length of the trailing edge, which would have made them more susceptible to flutter due to their reduced structural stiffness, a representative from the current type certificate holder reported that the overall damage pattern was not consistent with a trim tab flutter event. While the airplane maintenance manual (AMM) contained a specific instruction to inspect the bondlines, and a 100-hour inspection was performed in accordance with the AMM about 51 flight hours before the accident, it is likely that internal corrosion on the interior of the upper skin of the left horizontal stabilizer and bondline separation at the outboard rib of the left horizontal stabilizer existed at the time of the inspection; therefore, the failure of maintenance personnel to detect the disbonding at the outer rib of the left horizontal stabilizer likely contributed to the accident. Following this accident, a service bulletin and Federal Aviation Administration airworthiness directive were issued regarding bondline inspections.

Quoted verbatim from the NTSB record.