Volume I: Composites Applications and Design 777 EMPENNAGE CERTIFICATION APPROACH A. Fawcett1, J. Trostle2, S. Ward3 1777 Program, Structures Engineering, Principal Engineer and DER 2777 Program, Structures Engineering, Manager 3Composite Methods and Allowables, Principal Engineer Boeing Commercial Airplane Group, PO. Box 3707, Seattle, Washington, USA SUMMARY: This paper presents the Boeing approach to certification of the 777 composite empennage structure. The design team used carbon-fiber-reinforced plastic (CFRP) materials for the horizontal and vertical stabilizers, the elevators, and the rudder of the new 777 twinjet. Boeing based its approach to certification on analysis supported by coupon and component test evidence in compliance with guidelines issued by the FAA and JAA. The test program validated analysis methods, material design values, and manufacturing processes. The new toughenedresin material used on the 777 provides improved damage resistance over conventional thermoset materials. The 777 empennage represents a major commitment to composites in commercial aircraft service. KEYWORDS: commercial transport aircraft, certification, structural testing, carbon-fiber reinforced plasticError! Bookmark not defined., composite structure, strength, damage tolerance, and fatigue. INTRODUCTION Many components on the 777 aircraft contain composite materials (figure 1). Examples include fairings, floorbeams, engine nacelles, movable and fixed wing trailing edge surfaces, gear doors, and the empennage-including the horizontal and vertical stabilizers, elevators, and rudder. Composite materials are used primarily to reduce weight and improve aircraft efficiency. For some components, composite materials are appropriate, based on other requirements such as fatigue resistance, surface complexity, corrosion resistance, or manufacturing preference. The use of CFRP in 777 empennage structure follows developmental work and commercial service from the early 1980s. The NASA/Boeing 737 horizontal stabilizer was the first major component of composite structure certified for commercial use [1]. The company introduced five shipsets into service in 1984. The approach taken by Boeing to obtain certification and acceptance by the FAA was a key milestone in the development of composite structure for commercial aircraft applications. This certification approach complies with FAA and JAA regulations and maintains the Boeing philosophy that aircraft structure certification is shown by analysis with supporting test evidence. Following the 737 horizontal stabilizer program, Boeing designed a composite empennage for the 7J7 airplane. Although the company did not commit this model to production, Fuji Heavy Industries (FHI) fabricated a full-scale horizontal stabiliser test article (figure 2). Japan I - 178 Proceedings of ICCM–11, Gold Coast, Australia, 14th-18th July 1997 Aircraft Development Corporation (JADC) performed static, fatigue, and damage tolerance testing of the stabilizer test article [2]. The 7J7 empennage represented the first significant use of a toughened-resin CFRP material. The test program complied with applicable regulations and addressed items outlined in the certification agency advisory circulars for composite aircraft structures [3]. The full-scale test, along with a comprehensive ancillary test program, met all of the advisory circular recommendations. It provided full-scale validation of the design and analysis methodology, fabrication processes, and damage tolerance capability of toughened-resin materials. Boeing has expanded the use of composite materials to other applications. The Boeing 737, 747, 757, and 767 aircraft use composite materials in wing fixed leading and trailing edge structure and control surfaces. The rudder, elevator, aileron, and spoilers on these models use CFRP sandwich construction. The use of composite materials on the 777 horizontal and vertical stabilizers originated as part of a company-funded program. Boeing built a prototype or preproduction composite horizontal stabiliser, based on the 767-200 planform (figure 3). Designers selected the 767-200 as a base configuration with known weight, loads. cost, and structural characteristics. The 777 preproduction horizontal stabilizer design evolved with long-term production in mind; a major objective was to validate manufacturing costs. Other objectives for the program included: · Use composites where weight reduction can be achieved at a reasonable cost. · Use toughened-resin materials or additional gage to increase resistance to service threats. · Provide for a wide range of repair options, including mechanically fastened repairs. · Provide access for maintenance, inspections, and repair. I - 179 Volume I: Composites Applications and Design · Design for visual in-service inspections only. · Design to maximize automated fabrication processes. Because of the similarity in structure, geometry, and materials, the FAA and JAA accepted the preproduction stabilizer and supporting test program as applicable experience and part of the certification basis for the 777 empennage. Fu]l-scale testing of production aircraft structure, including the empennage components, is complete. This, together with other test data, forms the basis by which joint FAA/JAA certification has been granted. The 777-200 is in production and operating in service with the first eight customers. Fig. 2: 7J7 CFRP Horizontal Stabilizer Test Article I - 180 Proceedings of ICCM–11, Gold Coast, Australia, 14th-18th July 1997 DESCRIPTION OF STRUCTURE The 777 empennage consists of the horizontal and vertical stabilisers, elevators, and rudder (figure 4). The design team configured each stabilizer as a two-cell box, consisting of a main structural box and an auxiliary or forward torque box, leading edges, tip, and fixed trailing edges. The main torque boxes are made from CFRP composite material: solid-laminate front and rear spars, honeycomb sandwich ribs, and integrally stiffened laminate skin panels. The main box panels and spars feature a toughened-matrix CFRP material from Toray. The preimpregnated fiber/resin system is T800/3900-2. It provides improved resistance to impact damage over previous brittle materials. The auxiliary torque box and fixed trailing edges are glass or glass/CFRP sandwich panels with aluminum ribs. The leading edge, tip, and auxiliary spar are aluminum construction. The Boeing Company manufactures both stabilisers at the Composite Manufacturing Center near Tacoma, Washington. The elevator and rudder are also constructed from CFRP sandwich panels, ribs, and spars and are hinged from the stabiliser or fin fixed trailing edge. The rudder incorporates a lower tab of CFRP sandwich construction. ASTA fabricates the rudder in Australia; Hawker de Havilland fabricates the elevators. Reference 4 contains a more detailed description of the 777 empennage components. I - 181 Volume I: Composites Applications and Design CERTIFICATION APPROACH This section describes the certification approach, together with the regulations and means of compliance. Topics include internal loads, environment, static strength, damage tolerance, and fatigue. FAR Part 25 and JAR Part 25 [5, 6] define the regulatory requirements applicable to the 777 aircraft. Table I summarizes the principal requirements for structural strength, design, and construction. In addition to the regulations, the FAA and JAA have identified an acceptable means of compliance for certification of composite structure [3]. The advisory circular includes requirements in the following areas: (1) effects of environment (including design allowables and impact damage); (2) static strength (including repeated loads, test environment, process control, material variability, and impact damage); (3) fatigue and damage tolerance evaluation; and (4, other items such as flutter, flammability, lightning protection, maintenance, and repair. The 777 empennage certification approach is primarily analytical, supported by test evidence at the coupon, element, subcomponent, and component levels and full-scale limit load test at ambient environment. The environmental effects on the composite structure are characterised at the coupon, element, and subcomponent levels and are accounted for in the structural analysis. Supporting evidence includes testing through a 'building-block" approach that obtains material characterisation, allowables and analysis methods development, design concept verification, and final proof of structure (figure 5). Experience with similar structure was important in developing the 777 certification program. The 7J7 horizontal stabilizer [2, 7] and the 777 preproduction horizontal stabilizer programs validated analytical methods, design allowab]es, and fabrication and assembly processes applied to the 777 empennage structure. Boeing has accumulated significant additional I - 182 Proceedings of ICCM–11, Gold Coast, Australia, 14th-18th July 1997 knowledge and experience in characterising the behavior of composite aircraft structure. Boeing has augmented this experience database with the 737 composite stabilizer fleet experience and numerous other production applications in control surfaces, fixed secondary structure, fairings, and doors. The FAA and JAA participated in the certification approach for the 777. They participated directly through discussion and approval of the certification plan and indirectly through Boeing designated engineering representatives (DER). FAA representatives witnessed nearly all significant subcomponent