Composite Aircraft Structure

9/8/09 AC 20-107B Appendix 1. Applicable Regulations and Relevant Guidance 1. Applicable Regulations . A list of applicable regulations is provided for subjects covered in this AC. In most cases, these regulations apply regardless of the type of materials used in aircraft structures.

AC Paragraphs Part 23 Part 25 Part 27 Part 29 ---------------------- ------------------------- 1. Purpose of this AC Not Applicable ---------------------- ------------------------- 2. To Whom this AC Applies Not Applicable ---------------------- ------------------------- 3. Cancellation Not Applicable ---------------------- ------------------------- 4. Related Regulations and Guidance Not Applicable ---------------------- ------------------------- 5. General Not Applicable 6. Material and Fabrication Development 603 603 603 603 605 605 605 605 609 609 609 609 613 613 613 613 619 619 619 619 7. Proof of Structure – Static 305 305 305 305 307 307 307 307 8. Proof of Structure – Fatigue and Damage Tolerance 573 571 571 571 9. Proof of Structure – Flutter 629 629 629 629 10 . Continued Airworthiness 1529 1529 1529 1529 App. G App. H App. A App. A 11. Additional Considerations a. Crashworthiness 561 561 561 561 562 562 562 562 601 601 601 601 631 631 721 721 783 783 783 783 785 785 785 785 787 787 787 787 807 789 801 801 965 801 807 803 967 809 965 809 963 963 967 967 981 1413 Page A1-1 9/8/09 AC 20-107B b. Fire Protection, Flammability, 609 609 609 609 and Thermal Issues 787 863 861 861 863 865 863 863 865 867 1183 903 867 903 1185 967 954 967 1191 1013 1121 1121 1193 1121 1182 1181 1194 1183 1183 1182 1185 1189 1183 1189 1191 1185 1191 1193 1189 1193 1359 1191 1194 1365 1193 c. Lightning Protection 867 581 610 610 954 954 954 954 1309 1316 1309 1309 --------------------------------------- Notes: (1) This list may not be all inclusive and there may be differences between regulatory authorities.

(2) Special conditions may be issued for novel and unusual design features (e.g., new composite materials systems).

2. Guidance. FAA issues guidance providing supportive information of showing compliance with regulatory requirements. Guidance may include the AC and policy statements (PS). In general, an AC presents information concerning acceptable means, but not the only means, of complying with regulations. The guidance listed below is deemed supportive to the purposes of this AC. These FAA documents can be located via website: http://www.faa.gov/regulations_policies /.

a. ACs.

(1) AC 20-53, Protection of Airplane Fuel Systems Against Fuel Vapor Ignition Due to Lightning [6/06] (2) AC 20-135, Powerplant Installation and Propulsion System Component Fire Protection Test Methods, Standards, and Criteria [2/90] (3) AC 20-136, Protection of Aircraft Electrical/Electronic Systems Against the Indirect Effects of Lightning [12/06] (4) AC 20-155, SAE Documents to Support Aircraft Lightning Protection Certification [4/06] (5) AC 21-26, Quality Control for the Manufacture of Composite Structures [6/89] (6) AC 21-31, Quality Control for the Manufacture of Non-Metallic Compartment Interior Components [11/91] Page A1-2 9/8/09 AC 20-107B (7) AC 23-15, Small Airplane Certification Compliance Program [12/03] (8) AC 23-20, Acceptance Guidance on Material Procurement and Process Specifications for Polymer Matrix Composite Systems [9/03] (9) AC 25.571-1, Damage Tolerance and Fatigue Evaluation of Structure [4/98] (10) AC 29 MG 8, Substantiation of Composite Rotorcraft Structure [4/06] (11) AC 35.37-1, Guidance Material for Fatigue Limit Tests and Composite Blade Fatigue Substantiation [9/01] (12) AC 145-6, Repair Stations for Composite and Bonded Aircraft Structure [11/96] b. Policy Statements (1) Static Strength Substantiation of Composite Airplane Structure [PS-ACE100-2001-006, December 2001] (2) Final Policy for Flammability Testing per 14 CFR Part 23, Sections 23.853 , 23.855 and 23.1359 [PS-ACE100-2001-002, January 2002] (3) Material Qualification and Equivalency for Polymer Matrix Composite Material Systems [PS-ACE100-2002-006, September 2003] (4) Bonded Joints and Structures - Technical Issues and Certification Considerations [PS-ACE100-2005-10038, September 2005] (5) Policy Statement on Acceptance of SAE International Aerospace Recommended Practice 5577 as an Acceptable Method of Compliance to the Lightning Direct Effects requirements of § 25.581 [ANM-111-05-004, April 2006] Page A1-3

9/8/09 AC 20-107B Appendix 2. Definitions 1. Allowables: Material values that are determined from test data at the laminate or lamina level on a probability basis (e.g., A or B basis values, with 99% probability and 95% confidence, or 90% probability and 95% confidence, respectively). The amount of data required to derive these values is governed by the statistical significance (or basis) needed.

2. Anisotropic: Not isotropic; having mechanical and/or physical properties which vary with direction relative to natural reference axes inherent in the material.

3. Arrested Growth Approach: A method that requires demonstration that the structure, with defined flaws present, is able to withstand appropriate repeated loads with flaw growth which is either mechanically arrested or terminated before becoming critical (residual static strength reduced to limit load). This is to be associated with appropriate inspection intervals and damage detectability.

4. Category of Damage: Five categories of damage have been defined based on residual strength capability, required load level, detectability, inspection interval, damage threat and whether (or not) the event creating damage is self evident.

5. Component: A major section of the airframe structure (e.g., wing, body, fin, horizontal stabilizer) which can be tested as a complete unit to qualify the structure.

6. Coupon: A small test specimen (e.g., usually a flat laminate) for evaluation of basic lamina or laminate properties or properties of generic structural features (e.g., bonded or mechanically fastened joints).

7. Critical Structure: A load bearing structure/element whose integrity is essential in maintaining the overall flight safety of the aircraft. This definition was adopted for this AC because there are differences in the definitions of primary structure, secondary structure, and principle structural elements (PSE) when considering the different categories of aircraft. For example, PSE are critical structures for Transport Category Aircraft.

8. Damage: A structural anomaly caused by manufacturing (processing, fabrication, assembly or handling) or service usage.

9. Debond: Same as Disbond.

10. Degradation: The alteration of material properties (e.g., strength, modulus, coefficient of expansion) which may result from deviations in manufacturing or from repeated loading and/or environmental exposure.

11. Delamination: The separation of the layers of material in a laminate. This may be local or may cover a large area of the laminate. It may occur at any time in the cure or subsequent life of the laminate and may arise from a wide variety of causes.

Page A2-1 8/24/10 AC 20-107B 12. Design Values: Material, structural elements, and structural detail properties that have been determined from test data and chosen to assure a high degree of confidence in the integrity of the completed structure. These values are most often based on allowables adjusted to account for actual structural conditions, and used in analysis to compute margins-of-safety.

13. Detail: A non-generic structural element of a more complex structural member (e.g., specific design configured joints, splices, stringers, stringer runouts, or major access holes).

14. Disbond: An area within a bonded interface between two adherends in which an adhesion failure or separation has occurred. It may occur at any time during the life of the substructure and may arise from a wide variety of causes. Also, colloquially, an area of separation between two laminae in the finished laminate (in this case, the term “delamination” is normally preferred).

15. Discrepancy: A manufacturing anomaly allowed and detected by the planned inspection procedure. They can be created by processing, fabrication or assembly procedures.

16. Element: A generic element of a more complex structural member (e.g., skin, stringers, shear panels, sandwich panels, joints, or splices).

17. Environment: External, non-accidental conditions (excluding mechanical loading), separately or in combination, that can be expected in service and which may affect the structure (e.g., temperature, moisture, UV radiation, and fuel).

18. Factor(s): a. Life (or Load) Enhancement Factor: An additional load factor and/or test duration applied to structural repeated load tests, relative to the intended design load and life values, used to account for material variability. It is used to develop the required level of confidence in data.

b. Life Scatter Factor: Same as Life/Load Enhancement Factor.

c. Overload Factor: A load factor applied to a specific structure test which is used to address parameters (e.g., environment, a short test pyramid, etc.) not directly addressed in that test. This factor is usually developed from lower pyramid testing addressing such parameters.

19. Heterogeneous: Descriptive term for a material consisting of dissimilar constituents separately identifiable; a medium consisting of regions of unlike properties separated by internal boundaries.

20. Impact Damage: A structural anomaly created by foreign object impact.

21. Intrinsic Flaw: Defect inherent in the composite material or resulting from the production process.

Page A2-2 8/24/10 AC 20-107B 22. Manufacturing Defect: An anomaly or flaw occurring during manufacturing that can cause varying levels of degradation in structural strength, stiffness and dimensional stability. Those manufacturing defects (or permissible manufacturing variability) allowed by the quality control, manufacturing acceptance criteria are expected to meet appropriate structural requirements for the life of the aircraft part. Other manufacturing defects that escape detection in manufacturing quality control should be included in a damage threat assessment and must meet damage tolerance requirements until detected and repaired.

23. No-Growth Approach: A method that requires demonstration that the structure, with defined flaws present, is able to withstand appropriate repeated loads without detrimental flaw growth for the life of the structure.

24. Primary Structure: The structure which carries flight, ground, or pressurization loads, and whose failure would reduce the structural integrity of the airplane.

25. Point Design: An element or detail of a specific design which is not considered generically applicable to other structure for the purpose of substantiation, e.g., lugs and major joints. Such a design element or detail can be qualified by test or by a combination of test and analysis.

26. Slow Growth Approach: A method that requires demonstration that the structure, with defined flaws present, is able to withstand appropriate repeated loads with slow, stable, and predictable flaw growth for the life of the structure, or beyond appropriate inspection intervals associated with appropriate damage detectability.

27. Structural Bonding: A structural joint created by the process of adhesive bonding, comprising of one or more previously-cured composite or metal parts (referred to as adherends).

28. Subcomponent: A major three-dimensional structure which can provide completed structural representation of a section of the full structure (e.g., stub-box, section of a spar, wing panel, body panel with frames).

29. Weak Bond: A bond line with mechanical properties lower than expected, but without any possibility to detect that by normal NDI procedures. Such situation is mainly due to a poor chemical bonding.

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9/8/09 AC 20-107B Appendix 3. Change of Composite Material and/or Process 1. It is necessary to re-certify composite structures, which during production, incorporate substitutions of, or changes to, the materials and/or processes from those originally substantiated at the time of initial certification. For example, the original material supplier may either change its product, or cease production. Manufacturers may also find it necessary to modify their production processes to improve efficiency or correct product deficiencies. In either case, care must be taken to ensure that modifications and/or changes are adequately investigated to ensure the continued adequacy of already certified composite structure. This appendix covers such material and/or process changes, but does not address other changes to design (e.g., geometry, loading). The definition of the materials and processes used is required in the specifications by 14 CFR 21.31. Changes to the material and process specifications are often major changes in type design and must be addressed as such under 14 CFR part 21, subpart D.

2. The qualification and structural substantiation of new or modified materials and/or processes used to produce parts of a previously certified aircraft product requires: a. The identification of the key material and/or process parameters governing performances; b. The definition of the appropriate tests able to measure these parameters; and c. The definition of pass/fail criteria for these tests.

3. “ Qualification” procedures developed by every manufacturer include specifications covering: a. Physical and chemical properties, b. Mechanical properties (coupon level), and c. Reproducibility (by testing several batches).

4. Specifications and manufacturing quality procedures are designed to control specific materials and processes to achieve stable and repeatable structure for that combination of materials and processes. However, the interchangeability of alternate materials and processes for a structural application cannot be assumed if one only considers the properties outlined in those specifications (as it could be for materials that are much less process dependent, e.g., some metallic material forms). A structure fabricated using new or modified materials and/or processes, which meets the “qualification” tests required for the original material and process specifications, does not necessarily produce components that meet all the original engineering requirements for the previously certified structure.

5. Until improvements in identifying the complex relations between key material parameters that govern composite processing occurs, there will be a need for extensive and diverse testing that directly interrogates material performance using a range of representative specimens of increasing complexity in building block tests. Furthermore, failure modes may vary from one material and/or process to another, and analytical models are sometimes insufficiently precise to Page A3-1 9/8/09 AC 20-107B reliably predict failure without sufficient empirical data. Therefore, a step-by-step test verification with more complex specimens may be required.

6. Classification of Material or Process Change.

a. Any of the following situations requires further investigation of possible changes to a given composite structure: (1) Case A: A change in one or both of the basic constituents, resin, or fiber (including sizing or surface treatment alone) would yield an alternate material. Other changes that result in an alternate material include changes in fabric weave style, fiber aerial weight, and resin content.

(2) Case B: Same basic constituents, but any change of the resin impregnation method.

Such changes include: (i) prepregging process (e.g., solvent bath to hot melt coating), (ii) tow size (3k, 6k, 12k) for tape material forms with the same fiber areal weight, (iii) prepregging machine at the same suppliers, (iv) supplier change for a same material (licensed supplier).

(3) Case C: Same material, but modification of the processing route (if the modification to the processing route governs eventual composite mechanical properties). Example process changes of significance include: (i) curing cycle, (ii) bond surface preparation, (iii) changes in the resin transfer molding process used in fabricating parts from dry fiber forms, (iv) tooling, (v) lay-up method, (vi) environmental parameters of the material lay-up room, and (vii) major assembly procedures.

b. For each of the above cases, a distinction should be made between those changes intended to be a replica of the former material/process combination (Case B and some of Case C) and those which are “truly new material” (Case A and some of Case C). So, two classes are proposed: (1) “Identical materials/processes” in cases intended to create a replica structure.

(2) “Alternative materials/processes” in cases intended to create truly new structure.

c. Within the “identical materials/processes” class, a subclassification can be made between a change of the prepregging machine alone at the supplier and licensed production elsewhere.

For the time being, a change to a new fiber produced under a licensed process and reputed to be a replica of the former one, will be dealt with as an “alternative material/process.” d. Some minor changes within the class representing identical materials/processes may not interact with structural performances (e.g., prepreg release papers, some bagging materials, etc.)

and should not be submitted as part of the recertification. However, the manufacturers (or the supplier) should develop a proper system for screening those changes, with adequate proficiency at all relevant decision levels. Other minor material changes that fall under Case B may warrant sampling tests to show equivalency only at lower levels of building block substantiation.

e. Case C changes that may yield major changes in material and structural performance Page A3-2 9/8/09 AC 20-107B need to be evaluated at all appropriate levels of the building block tests to determine whether the manufacturing process change yields identical or alternate materials. Engineering judgment will be needed in determining the extent of testing based on the proposed manufacturing change.

f. Case A (alternative material) should always be considered as an important change, which requires structural substantiation. It is not recommended to try a sub-classification according to the basic constituents being changed, as material behavior (e.g., sensitivity to stress concentrations) may be governed by interfacial properties, which may be affected either by a fiber or a resin change.

7. Substantiation Method. Only the technical aspects of substantiation are addressed below.

a. Compliance Philosophy . Substantiation should be based on a comparability study between the structural performances of the material accepted for type certification, and the second material. Whatever the modification proposed for a certificated item, the revised margins of safety should remain adequate. Any reduction in the previously demonstrated margin should be investigated in detail.

(1) Alternative Material/Process : New design values for all relevant properties should be determined for any alternate material/process combination. Analytical models initially used to certify structure, including failure prediction models, should be reviewed and, if necessary, substantiated by tests. The procurement specification should be modified (or a new specification suited to the selected material should be defined) to ensure key quality variations are adequately controlled and new acceptance criteria defined. For example, changing from first to second generation of carbon fibers may improve tensile strength properties by more than 20% and a new acceptability threshold will be needed in the specification of the alternate material to ensure the detection of quality variations.

(2) Identical Material : Data should be provided that demonstrates that the original design values (whatever the level of investigation, material or design) remain valid. Statistical methods need to be employed for data to ensure that key design properties come from the same populations as the original material/process combination. Calculation models including failure prediction should remain the same. The technical content of the procurement specification (Case B) should not need to be changed to properly control quality.

b. Testing.

(1) The extent of testing needed to substantiate a material change should address the inherent structural behavior of the composite and will be a function of the airworthiness significance of the part and the material change definition. For example, the investigation level might be restricted to the generic specimens at the test pyramid base (refer to figures in paragraph 7) for an identical material, but non-generic test articles from higher up the pyramid should be included for an alternative material. Care needs to be taken to ensure that the test methods used yield data compatible with data used to determine properties of the original structure.

Page A3-3 9/8/09 AC 20-107B (2) The testing that may be required for a range of possible material and/or process changes should consider all levels of structural substantiation that may be affected. In some instances (e.g., a minor cure cycle change), possible consequences can be assessed by tests on generic specimens only. For other changes, like those involving tooling (e.g., from a full bag process to thermo-expansive cores), the assessment should include an evaluation of the component itself (sometimes called the “tool proof test”). In this case, an expanded NDI procedure should be required for the first items to be produced. This should be supplemented – if deemed necessary – by “cut up” specimens from a representative component, for physical or mechanical investigations.

c. Number of Batches.

(1) The purpose for testing a number of batches is the demonstration of an acceptable reproducibility of material characteristics. The number of batches required should take into account: material classification (identical or alternative), the investigation level (non-generic or generic specimen) the source of supply, and the property under investigation. Care should be taken to investigate the variation of both basic material and the manufacturing process.

(2) Existing references (e.g., The Composite Materials Handbook (CMH-17) Volumes 1 and 3, FAA Technical Report DOT/ FAA/AR-03/19), addressing composite qualification and equivalence and the building block approach, provide more detailed guidance regarding batch and test numbers and the appropriate statistical analysis up to laminate level. Changes at higher pyramid levels, or those associated with other material forms, e.g., braided VARTM (Vacuum- Assisted Resin Transfer Molding) structure, may require use of other statistical procedures or engineering methods.

d. Pass/Fail Criteria. Target pass/fail criteria should be established as part of the test program. For strength considerations for instance, a statistical analysis of test data should demonstrate that new design values derived for the second material provide an adequate margin of safety. Therefore, provision should be made for a sufficient number of test specimens to allow for such analysis. At the non-generic level, when only one test article is used to assess a structural feature, the pass criteria should be a result acceptable with respect to design ultimate loads. In the cases where test results show lower margins of safety, certification documentation will need to be revised.

e. Other Considerations. For characteristics other than static strength (all those listed in AC 20-107B, paragraphs 8, 9, 10, and 11), the substantiation should also ensure an equivalent level of safety.

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