Page:CAB Accident Report, Mooney M-18 Crash on 7 September 1959.pdf/3

- 3 - The vertical stabilizer failed to the left and with the rudder attached fall 150 feet from the wreckage. When the rudder separated from the aircraft the lower hinge failed in the weld. The rib trailing edge glued butt joint adjacent to the middle hinge was loose. It was being held in place by one brad and the joint showed very little wood failure.

All other damage to the aircraft was the result of impact forces. There was some evidence of deterioration present in several of the glued joints which had separated with very little wood failure. In other joints, which had failed in a similiar manner, there was no evidence of any deterioration. (It was noted that none of the glued joints were reinforced with gusset plates but depended entirely upon the strength of the glue to hold them.)

In view of the findings during this investigation and the fact that the model 18 production eased in 1956, the Board carried its investigation into the Mooney Aircraft Company present production methods for the model 20A. The present company, which took over control of the Mooney Aircraft Company in 1956, maintains a strict inspection system to check all stock as at is received into the plant to ensure conformity to specifications. Quality control of all fabricated parts in maintained by visual inspection of each operation and comparison with blueprints. In addition, a conformity report on each lot of parts has to be filled out stating the number of units inspected, the workmanship, and conformance to the drawings. Further, each step in the assembly and subassembly must be inspected and signed off before the next operation can be started.

Analysis

The evidence indicates that the initial inflight failure occurred when the glue joint of the No. 2 rib separated allowing the trailing edge member of the right horizontal stabilizer to pull out. Airload bowed the elevator down in the middle, its inboard and outboard hinges remaining intact. As the elevator was bowed downward and the lateral distance between the inboard and outboard hinges decreased, an abnormally high downward loading was consequently imposed on the stabilizer, failing it downward as its attach point. It is probable that as the stabilizer separated from the aircraft the outboard elevator hinge failed and the elevator remained attached to the aircraft momentarily until the inboard hinge also failed. This is indicated by the location of the stabilizer farther from the wreckage than the elevator.

The immediate result of the loss of the stabilizer was a violent nosedown pitching of the aircraft, which failed the right wing spar downward in two places. Following this the aircraft rolled rapidly to the right and the vertical fin separated. The aircraft then fell nearly vertically to the ground.

The generally poor condition of glued wood joints throughout the aircraft appears to as a result of poor production techniques. Although deterioration from weathering was noted in some which had failed, it is not believed to have been of such a degree as to cause separation with little or no wood failure. The design practice of using glued butt joints without gussets or the equivalent is considered poor. In the later models 20 and 20A this problem is overcome because the stabilizer and wings are completely covered with plywood and this surface acts as gusset for the structure.

Similarly, poor production techniques were noted in numerous welds throughout the aircraft. Because of poor penetration several had failed in the weld deposit or at the juncture of the weld deposit and the parent metal. The weld should, if