Aviation Administration ( FAA ) airworthiness certification for ceramic matrix composite components in civil aircraft systems

Ceramic matrix composites (CMCs) are being designed and developed for engine and exhaust components in commercial aviation, because they offer higher temperature capabilities, weight savings, and improved durability compared to metals. The United States Federal Aviation Administration (FAA) issues and enforces regulations and minimum standards covering the safe manufacture, operation, and maintenance of civil aircraft. As new materials, these ceramic composite components will have to meet the certification regulations of the FAA for “airworthiness”. The FAA certification process is defined in the Federal Aviation Regulations (Title 14 of the Code of Federal Regulations), FAA policy statements, orders, advisory circulars, technical standard orders, and FAA airworthiness directives. These regulations and documents provide the fundamental requirements and guidelines for design, testing, manufacture, quality assurance, registration, operation, inspection, maintenance, and repair of aircraft systems and parts. For metallic parts in aircraft, the FAA certification and compliance process is wellestablished for type and airworthiness certification, using ASTM and SAE standards, the MMPDS data handbook, and FAA advisory circulars. In a similar manner for polymer matrix composites (PMC), the PMC industry and the FAA have jointly developed and are refining parallel guidelines for polymer matrix composites (PMCs), using guidance in FAA circulars and the CMH-17 PMC handbook. These documents discuss design methods and codes, material testing, property data development, life/durability assessment, production processes, QA procedures, inspection methods, operational limits, and repairs for PMCs. For ceramic composites, the FAA and the CMC and aerospace community are working together (primarily through the CMH-17 CMC handbook) to define and codify key design, production, and regulatory issues that have to be addressed in the certification of CMC components in civil aircraft.


Introduction
Ceramic matrix composites (CMCs) consist of a ceramic matrix reinforced with continuous ceramic fibers, yarns, or woven fabrics.These CMCs exhibit improved strain-to-failure (0.5% ∼ 1%) and improved damage tolerance (from a cumulative damage process), unlike monolithic ceramics that a e-mail: gatewaymt@aol.com;stevegmt@gmail.comThis is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.fracture in a brittle catastrophic manner at low strain.(See Figs. 1 and 2.) CMC systems are commonly classified as oxide systems (oxide fibers in alumino-silicate matrices) and non-oxide systems (silicon carbide fibers in silicon-carbide/nitride matrices).

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These CMCs are being designed and developed for "hot" engine and exhaust components in military and commercial aviation, because they offer higher temperature capability, weight savings, and improved durability compared to current nickel superalloys (Fig. 3).
Turbine components of direct interest for CMCs are -exhaust components, shrouds/seals, combustors, stators/vanes, and rotors/blades.General Electric and Rolls Royce are currently testing prototype exhaust and shroud components for the GE-CFM LEAP engine, the GE Passport 20 Engine, and the Rolls Royce Trent 1000 engine (Figs. 4 and 5).
As new materials, these ceramic composite components will have to meet the certification requirements of the United States Federal Aviation Administration (FAA) for "airworthiness".The term  "airworthiness" is defined as "that condition of an aircraft, engine, propeller, appliance, or component part where it conforms to its approved type design and is in a condition for safe operation (14 CFR s.3.5)".

Federal Aviation Agency responsibilities
In the United States the Federal Aviation Agency (FAA) has authority to regulate and oversee all aspects of American civil aviation.The mission of the FAA is to provide the safest, most efficient aerospace system in the world.The primary responsibilities of the FAA are: • Regulating civil aviation to promote safety -the safe manufacture, operation, and maintenance of civil aircraft.• Developing and operating a system of air traffic control and navigation for both civil and military aircraft.

Federal Aviation regulations
The FAA regulations that govern today's aircraft are found in Title 14 of the Code of Federal Regulations (14 CFR).For all aerospace materials engineers and designers the "foundation" regulations in 14 CFR are - (1) Where applied loads are eventually distributed through a single member within an assembly, the failure of which would result in loss of structural integrity of the component, 99 percent probability with 95 percent confidence.

CFR
(2) For redundant structure, in which the failure of individual elements would result in applied loads being safely distributed to other load carrying members, 90 percent probability with 95 percent confidence.
(c) The effects of temperature on allowable stresses used for design in an essential component or structure must be considered where thermal effects are significant under normal operating conditions.(d) The strength, detail design, and fabrication of the structure must minimize the probability of disastrous fatigue failure, particularly at points of stress concentration.(e) Greater design values may be used if a "premium selection" of the material is made in which a specimen of each individual item is tested before use to determine that the actual strength properties of that particular item will equal or exceed those used in design.

FAA aircraft certificates and approvals
FAA aircraft certificates and approvals are issued in six formats -• Airworthiness Certificate (AC) -approval to operate a specific aircraft.
• Type Certification (TC) -design certification for new aircraft/airframes, engines, and propellers.
• Production Certificate (PC) -approval to manufacture parts, components and systems.
• Supplemental Type Certificate (STC) -certification of a modification of aircraft, engine, or propeller (design, manufacture and install).• Parts Manufacturer Approval -certify replacement parts (third party).
• Technical Standard Order (TSO) -written specification published by FAA on materials, parts, and appliances.
Note that these certificates are for defined systems, products, components, and parts, not for material systems, alloys, or composite types.

The FAA certification process
The FAA certification process is defined and detailed through the following FAA documents. (http://www.faa.gov/regulations_policies/) • Part 21 of the 14 CFR -Certification Procedures for Products and Parts.
• FAA Policy Statements and Memos -policy directives and direction.
• FAA Orders -procedural rules and regulations.
• FAA Advisory Circulars (AC) -guidance documents for all aspects of design, production, operation, inspection, maintenance, and repair.These circulars are the primary detailed guidance on certification of components made of specific materials.• FAA Technical Standard Orders (TSO) -minimum performance standard issued by the Administrator for specified materials, parts, processes, and appliances used on civil aircraft.• FAA Airworthiness Directives -legally enforceable regulations issued by the FAA in accordance with 14 CFR part 39 to correct an unsafe condition in a product.• FAA Technical Reports.
• FAA and Industry Guide to Product Certification -A guide to the current certification process.
The FAA certification approval process has three stages -design approval (Type and Supplemental Type Certificates, Technical Standard Order, production approval (Production Certificate and Parts Manufacture Approval, Technical Standard Order) and operation and maintenance approval (Regulations, STCs, FAA Orders, and Advisory Circulars, Aircraft Handbooks and Manuals, Airworthiness Directives).
Design approval certifies that a given design meets operational, safety, and durability requirements.The design approval covers: Functional Design Definition, Validation and Documentation, Structural Substantiation (Static, Operational Temps), Manufacturing Process Definition and Documentation, Bonding/Assembly/Integration Design and Validation, Environmental Exposure, Damage Tolerance, Degradation Definition and Assessment, Impact Events, Fatigue and Creep, Fire Protection and Flammability, Lightening Strikes, Crashworthiness, and Inspection and Repair.
Production approval ensures that a defined manufacturing process produces parts that consistently meet design specifications and tolerances; it includes Process Definition and Verification, Critical Manufacturing Steps and Control, Quality Control System With Records, Defect Detection and Disposition.Operation and Field Maintenance approval covers: Damage Detection, Inspection and Assessment; Repair Criteria and Validated Repair Methods; and Life Prediction and Assessment.The certification process (for supplemental type certificate) is outlined and described in the "FAA and Industry Guide to Product Certification" (Sep. 2004 -117 pages).The process has five phases -conceptual design, requirements definition, compliance planning, implementation, and post certification.The certification process is a joint effort between the applicant team and the FAA team, based on communication, teamwork, and accountability from the beginning of the product development effort (requirements and design) through production, fielding, operation, and maintenance/ support.

The FAA certification of metal and polymer matrix composites components
The FAA certification process for metal parts and products (aluminum, steel, titanium, magnesium, etc.) is well established and detailed.The process uses FAA advisory circulars; industrial design practices; ASTM test standards, AMS and ASTM specifications for composition, processing, properties; certified -is integrated and installed in the system (joining and assembly), -can be maintained and inspected in service, -can be repaired or replaced in service.
Based on the similarities to PMCs, the CMC community can use the PMC certification guidance and tools as a template and baseline for CMC certification; but that PMC policy, guidance, and tools will have to be modified and expanded to address the differences in CMCs.
The near-term challenge for the CMC community is how the OEM's and producers will team with the FAA to define, formalize, and document the details of the certification process for CMCs.
The CMH-17 CMC handbook (Vol.5) is a major part of this CMC effort, with a focus on design, production, testing and evaluation, data development, and supportability.But the CMH-17 CMC handbook effort is an source for information developed by the CMC community.The CMC producers and users have responsibility to develop, validate and document the many "technical tools" for CMCs: • Design criteria, approaches, and methods.
• Test standards for mechanical, thermal, and physical properties.
• Analytical methods -stress, thermal, life prediction.• Degradation, damage, and failure mechanisms and models at different temperatures and environments.• Property data bases.
• Process control techniques and quality assurance protocols.
• Inspection methods (production line and field).
• Maintenance and repair techniques.
Materials simulation and numerical models will be critical tools for CMC design codes, material assessment, life prediction, and production control.

Summary and conclusions
Ceramic matrix composites (CMCs) are being developed, designed and will be produced for engine and exhaust components in commercial aviation.Those CMC components will have to be certified by the FAA for commercial use.
The FAA and industry are now working together (primarily through the CMH-17 CMC handbook) to define and codify key design, production, operation, and regulatory issues that have to be addressed in the certification of CMC components in civil aircraft.

Figure 1 .
Figure 1.Tensile Stress-Strain of High Density, High Modulus MI SiC Matrix with Nicalon SiC Fiber (w/Fiber Interface Coating).

Figure 3 .
Figure 3. Comparative Temperature Capability of Nickel Superalloys and CMCs.

Figure 5 .
Figure 5. Oxide-Oxide CMC Mixer, Center Body, and Engine Core Cowl on the GE Passport 20 Engine.
Sec. 25.601 -General.-The airplane may not have design features or details that experience has shown to be hazardous or unreliable.The suitability of each questionable design detail and part must be established by tests.14 CFR Sec.25.603 -Materials.-The suitability and durability of materials used for parts, the failure of which could adversely affect safety, must -(a) Be established on the basis of experience or tests; (b) Conform to approved specifications (such as industry or military specifications, or Technical Standard Orders) that ensure their having the strength and other properties assumed in the design data; and (c) Take into account the effects of environmental conditions, such as temperature and humidity, expected in service.4 CFR 25.605 -Fabrication methods (a) The methods of fabrication used must produce a consistently sound structure.If a fabrication process (such as gluing, spot welding, or heat treating) requires close control to reach this objective, the process must be performed under an approved process specification.(b) Each new aircraft fabrication method must be substantiated by a test program.14 CFR Sec.25.609.-Protection of structure.Each part of the structure must be suitably protected against deterioration or loss of strength in service due to any cause, including -(1) Weathering; (2) Corrosion; (3) Abrasion; and (4) Have provisions for ventilation and drainage where necessary for protection.14 CFR Sec.25.629.-Aeroelastic stability requirements.(a) General.The aeroelastic stability evaluations required under this section include flutter, divergence, control reversal and any undue loss of stability and control as a result of structural deformation.The aeroelastic evaluation must include whirl modes associated with any propeller or rotating device that contributes significant dynamic forces.Compliance with this section must be shown by analyses, wind tunnel tests, ground vibration tests, flight tests, or other means found necessary by the Administrator.(b) Each new aircraft fabrication method must be substantiated by a test program.14 CFR Sec.26.613.-Material strength properties and design values.(a) Material strength properties must be based on enough tests of material meeting approved specifications to establish design values on a statistical basis.00002-p.4Testing and Modeling Ceramic & Carbon Matrix Composites (b) Design values must be chosen to minimize the probability of structural failures due to material variability.Except as provided in paragraph (e) of this section, compliance with this paragraph must be shown by selecting design values which assure material strength with the following probability:

Figure 6 .
Figure 6.Candidate Components for CMCs in Aircraft Turbine Engines.