Ground Test Experience With Large Composite Structures For Commercial Transports

Composite technology applicable to transport empennage structures has been developed through contracts sponsored by the NASA Aircraft Energy Efficiency (ACEE) Program Office. The empennage components, the horizontal stabilizer for the Boeing 737, the vertical fin for the Lockheed L-1011, and the vertical stabilizer for the Douglas DC-10, have been designed to replace the existing metal structures without modification of other parts of the aircraft. A principal element in the program for each component is an extensive ground test series of a full size structure. The programs for two of the components, tho 737 Horizontal Stabilizer and the DC-10 Vertical Stabilizer, include, as objectives, FAA certification and airline flight service. The ground test series for these components are, consequently, essential steps in verifying compliance with FAA certification requirements. The initial ground test of each component resulted in structural failure at less than ultimate design loads. While such failures represent major program delays, the investigation and analysis of each failure revealed significant lessons for effective utilization of composites in primary structure. Foremost among these are secondary loads that produce through-the-thickness forces which may lead to serious weaknesses in an otherwise sound structural design. The sources, magnitude, and effects of secondary loads need to be thoroughly understood and accounted for by the designers of composite primary aircraft structures.