AC 23-13A
FATIGUE, FAIL-SAFE, AND DAMAGE
TOLERANCE EVALUATION OF METALLIC
STRUCTURE FOR NORMAL, UTILITY,
ACROBATIC, AND COMMUTER CATEGORY
AIRPLANES September 29, 2005 For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20401 Initiated by: ACE-100
FOREWORD This Advisory Circular (AC) sets forth an acceptable means of showing compliance with Title 14 Code of Federal Regulations (14 CFR), part 23. This guidance is applicable to fatigue, fail-safe, and damage tolerance evaluations of metallic structure in normal, utility, acrobatic, and commuter category airplanes. This AC pr ovides information on approval of continued operational flight with known cracks in the structure of small airplanes, regardless of certification basis. This AC also clarifies the use of AC 20-128A in the evaluation of rotorburst structural hazards in small airplanes. Finally, this AC consolidates existing policy documents and certain technical reports into one document.
s/ David R. Showers Acting Manager, Small Airplane Directorate Aircraft Certification Service i (and ii)
CONTENTS
Section PAGE
1. What is the purpose of this AC?1
2. What is incorporated in this AC and, thereby, superseded?1
3. What is the background for this AC and what time period does this AC cover?1
4. How should the information contained in this AC be applied?1
5. What regulations are discussed in this AC?1
6. What other publications are related to this AC?2
7. What are the definitions for the common terms used in this AC?2
CHAPTER 1. SMALL AIRPLANE FATIGUE REGULATIONS4
1-1. What is the goal of the sma ll airplane fatigue requirements?4
1-2. How have the fatigue requi rements changed over time?4
1-3. Can you summarize the current fatigue regulations for small airplanes?4
1-4. Which metallic components should I include in the fatigue evaluation?6
1-5. Are the small airplane fatigue regula tions different in other countries?6
1-6. How do I show compliance to the fatigue requirements for alterations, modifications, or
changes to the design?6
CHAPTER 2. SAFE-LIFE FATIGUE EVALUATION7
2-1. What is a safe-life fatigue evaluation?7
2-2. What is the definition of failure in a safe-life structure?7
2-3. What are the steps in evaluati ng the safe-life of a structure?7
2-4. What loading spectra should I use?8
2-5. How were the load spectra in Appendix 1 developed?9
2-6. Can I use other loading spectra in place of the Appendix 1 spectra?9
2-7. What should I consider in developing a mission profile?9
2-8. Does the FAA require a flight test to valid ate the loads I use in my fatigue evaluation?10
2-9. Does the FAA require validation of the stresses I use in my fatigue evaluation?10
2-10. How do I substantiate a fatigue saf e-life with full-scale fatigue tests?11
2-11. What should I include in the test plan for my full-scale fatigue tests?11
2-12. Do I need to complete the test before the FAA will issue the type certificate (TC)?13
2-13. How do I calculate the fatigue safe-lif e using full-scale fatigue test results?13
2-14. How do I calculate the mean test life?14
2-15. What are the scatter factor s for full-scale fatigue tests?14
2-16. What about statistical confidence inte rvals on full-scale fatigue test results?16
2-17. What counts as one test specimen, and what counts as two test specimens?16
2-18. Can I use a full-scale test scatter factor lower than that obtained from Equation 4?17
2-19. What about the '3-sigma' method of computing the safe-life?17
2-20. How do you substantiate a fatigue safe-life with component tests?17
2-21. What loads do I apply in a component test?18
2-22. For component testing, how many specimens do I need to test?18
2-23. For component testing, what scatter factor should I use?18
2-24. How do you substantiate a fatigue safe-life with analysis supported by test evidence?19
2-25. What are some concerns with s ubstantiation by fatigue analysis?19
2-26. What scatter factors do I use in a fatigue analysis?20
2-27. How can I extend the safe-life, or retirement life, of my airplane?21
CHAPTER 3. FAIL-SAFE DESIGN23
3-1. What is a fail-safe design?23
iii
3-2. What is the history of fail-safe design?23
3-3. How can I assure that my design complies with the fail-safe design requirements?24
3-4. What are some design considerations in achieving a fail-safe design?24
3-5. What is a Principal Structural Element?24
3-6. How much fatigue damage should I consider in my fail-safe evaluation?25
3-7. How do I address inaccessible areas?26
3-8. How do I account for dynamic effects?26
3-9. How do I test principa l structural elements?26
3-10. Can I use analysis to s ubstantiate a fail-safe design?27
3-11. How do I select the critical areas?27
CHAPTER 4. DAMAGE TOLERANCE EVALUATION28
4-1. Are there any differences between the part 23 and part 25 damage tolerance regulations?28
CHAPTER 5. ROTORBURST REQUIREMENTS29
5-1. What are the requirements for turbine or tu rbopropeller installations in small airplanes?29
5-2. I thought there was not a discrete s ource damage requirement in part 23?29
CHAPTER 6. FLIGHT WITH KNOWN CRACKS30
6-1. What is the purpose of this policy?30
6-2. What is the policy for operational flight with known cracks?30
6-3. Are there any exceptions to this policy?30
6-4 Which airplanes are covered by this policy?30
6-5 What will the FAA consider before allowi ng operational flight with known cracks?30
REFERENCES34
APPENDIX 1. FLIGHT AND GROUND LOAD SPECTRAA1-1
APPENDIX 2. S/N CURVES FOR ALUMINUM COMPONENTSA2-1
1. What is the purpose of this AC?
This advisory circular (AC) provides information and guidance concerning acceptable means, but not the only means, of compliance with Title 14 of the Code of Federal Regulations (14 CFR) part 23. This guidance is applicable to fatigue, fail-safe, and damage tolerance evaluations of metallic structure in normal, utility, acrobatic, and commuter category airplanes. This AC provides information on approval of continued operational flight with known cracks in the structure of small airplanes, regardless of their certification basis. This AC also clarifies the use of AC 20-128A in the evaluation of rotorburst structural hazards in small airplanes. This AC consolidates existing policy documents, and certain technical reports, into a single document.
This AC incorporates, and supersedes, FAA Report No. AFS-120-73-2, Fatigue Evaluation of Wing and Associated Structure on Small Airplanes, dated May 1973. Material in this AC is neither mandatory nor regulatory in nature and does not constitute a regulation.
2. What is incorporated in this AC and, thereby, superseded?
The following AC has been incorporated into this revision in whole or in part and is hereby superseded:
AC 23-13, Fatigue and Fail-Safe Evaluation of Flight Structure and Pressurized Cabin for Part 23 Airplanes, dated April 15, 1993.
3. What is the background for this AC and what time period does this AC cover?
The AC is current through Amendment 23-55, effective March 1, 2002. This material spans approximately 38 years of Federal Aviation Administration (FAA) aviation history.
4. How should the information contained in this AC be applied?
a. This material does not have any legal status and should be treated accordingly. However, to ensure standardization in the certification process, this guidance should be considered during all small airplane Type Certification (TC) and Supplemental Type Certification (STC) activities.
The user of this AC should check the latest amendments to the applicable 14 CFR part to verify that the regulations referenced in this AC have not been superseded or that additional regulations have not been added.
b. New approaches to fatigue evaluation, design features, methods of fabrication, and new airplane configurations may require variations and deviations from the guidance described in this AC. Engineering judgment, guided by the extens ive literature on the subject, should be exercised for each particular application. An applicant should evaluate the primary structure of the pressurized cabin, wing, empennage, and associated structures for any original design and for any design changes that affect the loading spectra, internal stresses, or stress concentrations or that change the construction methods or materials. Changes to the design that may be minor from a static strength standpoint can have a major effect on fatigue characteristics.
5. What regulations are discussed in this AC?
a. The requirements for fatigue evaluation included in 14 CFR, part 23, §§ 23.571, 23.572,
23.573, 23.574, and 23.627.
b. The requirements for inspections and Instructions for Continued Airworthiness included in 14 CFR, part 23, §§ 23.575, 23.611, and 23.1529.
c. The requirements for minimization of rotorburst hazards included in 14 CFR, part 23, § 23.903.
6. What other publications are related to this AC?
a. AC 23-19, Airframe Guide for Certification of Part 23 Airplanes, dated January 27, 2003.
b. AC 25.571-1C, Damage Tolerance and Fatigue Evaluation of Structure, dated April 29, 1998.
c. AC 25.613-1, Material Strength Properties and Material Design Values, dated August 6, 2003.
d. AC 20-128A, Design Considerations for Minimizing Hazards caused by Uncontained Turbine Engine and Auxiliary Power Unit Rotor Failure, dated March 25, 1997.
e. AC 20-107A, Composite Aircraft Structure, dated April 25, 1984.
f. AC 91-56A, Continuing Structural Integrity Program for Large Transport Category Airplanes, dated April 29, 1998.
g. AC 91-60, The Continued Airworthiness of Older Airplanes, dated June 13, 1983.
h. Aging Airplane Inspection and Maintenance Baseline Checklist for Airplanes Without a Type Specific Checklist (Best Practices Guide for Maintaining Aging General Aviation Airplanes). Electronic version available at:
http://www.faa.gov/aircraft/air_cert/design_approvals/small_airplanes/cos/aging_aircraft/media/ aging_aircraft_best_practices.pdf
7. What are the definitions for the common terms used in this AC?
This Advisory Circular uses the definitions listed below. Some of these terms are used throughout and will only be defined here. Sometimes, the term definitions may be repeated in the relevant document section as an aid to the reader.
a. Principal Structural Element (PSE). A PSE is an element that contributes significantly to the carrying of flight, ground, or pressurization loads, and whose integrity is essential in maintaining the overall structural integrity of the airplane.
b. Primary Structure. Primary structure is that structure which carries flight, ground, or pressurization loads, and whose failure would reduce the structural integrity of the airplane.
c. Fatigue. The process of progressive localized permanent structural change occurring in a material subjected to conditions that produce fluctuating stresses and strains at some point or points, which may result in cracks or complete fracture after a sufficient number of fluctuations.
d. Safe-Life. The safe-life of a structure is that number of events, such as flights, landings, or flight hours, during which there is a low probability that the strength will degrade below its design ultimate value due to fatigue cracking.
e. S-N or ε–N. Stress–Life (S-N) or Strain–Life (ε -N) curves depict the magnitude of applied stress (S) or strain (ε) necessary to develop a fatigue crack in a specimen at a given life (N), where N is expressed in the number of cyclic applications of stress or strain.
f. Scatter Factor. The scatter factor, or life reduction factor, is a statistically derived divisor applied to fatigue test results to account for the variation in fatigue performance of built-up or monolithic structures. A scatter factor can also be used in a fatigue analysis to address the uncertainties inherent in a fatigue analysis.
g. Fail-Safe. Fail-safe is the attribute of the structure that permits it to retain its required residual strength for a period of unrepaired use after the failure or partial failure of a principal structural element.
h. Damage Tolerance. Damage tolerance is the attribute of the structure that permits it to retain its required residual strength for a period of use after the structure has sustained a given level of fatigue, corrosion, accidental, or discrete source damage.
i. Discrete Source Damage. Damage to the structure caused by a discrete source, such as fragments from uncontained engine failures.
j. Residual Strength. The strength capability of a structure after the structure has been damaged due to fatigue, corrosion, or discrete source damage. The residual strength capability includes consideration of static strength, fracture, and stiffness.
CHAPTER 1. SMALL AIRPLANE FATIGUE REGULATIONS
1-1. What is the goal of the sma ll airplane fatigue requirements?
The goal of the fatigue requirements is to maintain the design strength capability, both limit and ultimate load, throughout the operational life of the airplane.
1-2. How have the fatigue requirements changed over time?
a. Before 1957 . The only fatigue requirements for sm all airplanes appeared in the Civil Air Regulations (CAR), § 3.307. The CAR 3 paragr aph required the airplane designer to choose design details that avoided stress concentrations. This airworthiness requirement applied to all structural design details on the whole airplane. When the CAR requirements were recodified to the 14 CFR parts, this requirement was carried over as § 23.627.
b. 1957. Amendment 3-2 added a specific requirement to CAR 3 for the fatigue assessment of pressurized fuselage structure. The requirement gave applicants the option of a safe-life evaluation or incorporating fail-safe design features. CAR § 3.270 was recodified as 14 CFR, part 23, § 23.571.
c. 1969. Amendment 23-7, effective September 14, 1969, added fatigue requirements for the wing, wing carry-through, and attaching structure in § 23.572.
d. 1979. Special Federal Aviation Regulation No. 41 (SFAR 41), effective September 17, 1979, required a fatigue substantiation of “those parts of the wing, wing carrythrough, vertical fin, horizontal stabilizer, and attaching structure whose failure would be catastrophic.”
e. 1989. Amendment 23-38, effective October 26, 1989, added the empennage to the § 23.572 fatigue requirements.
f. 1993. Amendment 23-45 introduced § 23.573 requiring damage tolerance and fatigue evaluation for composite structures. This amendment also added damage tolerance as an option for metallic pressurized cabin in § 23.571 and for wing, empennage, and associated structure in § 23.572. Amendment 23-45 also expande d the fatigue assessment of wing structure to include canards, tandem wings, and winglets/tip fins.
g. 1996.
(1) Amendment 23-48 introduced § 23.574, which requires damage tolerance for metallic structure on commuter category airplanes. This amendment became effective on March 11, 1996.
(2) Amendment 23-48 also introduced § 23.575, which clarifies that inspections and any other procedures necessary to prevent catastrophic failure must be included in the Limitations Section of the Instructions for Continued Airworthiness (ICA).
1-3. Can you summarize the current fatigu e regulations for small airplanes?
a. For metallic pressurized cabin, wing, empennage, and associated structure in normal, utility, and acrobatic category airplanes, you have the option of the following fatigue evaluation methods:
(1) A fatigue strength evaluation, more commonly known as a safe-life evaluation. See § 23.571(a) for pressurized cabin and § 23.572(a)(1) for wing, empennage, and associated structure.
(2) A fail-safe strength evaluation. See § 23.571(b) for pressurized cabin and § 23.572(a)(2) for wing, empennage, and associated structure.
(3) A damage tolerance evaluation. See § 23.573(b) and § 23.571(c) for pressurized cabin and § 23.572(a)(3) for wing, empennage, and associated structure.
b. For wings, empennage, and associated structure, you may be able to show compliance to the fatigue requirements by comparing your design to an existing design. This method of substantiation includes showing that the structure, operating stress level, materials, stress concentrations, and expected uses are equivalent, from a fatigue standpoint, to a similar design that has had extensive satisfactory service experience. See § 23.572(a).
c. For metallic commuter category airplanes, you must use a damage tolerance evaluation. If you can show that damage tolerance is impractical for a particular structure, you may use a fatigue strength, or safe-life evaluation. See § 23.574.
d. For all categories of small airplanes constructed with composite materials, you must use a damage tolerance evaluation. AC 20-107A provides guidance for composite structures.
e. The following flowchart may help clarify which method of fatigue evaluation is required for your application.
FIGURE 1. WHICH FATIGUE REGULATIONS APPLY TO MY NEW TYPE DESIGN?
(Amendment 23-48, effective February 9, 1996.)
Is your structure constructed of composite materials?
NO Required: Damage Tolerance Evaluation YES 14 CFR §§ 23.573(a), 23.575 See AC 20-107A Required: 1.) Damage Tolerance Evaluation
14 CFR §§ 23.574(a), 23.575
Are you seeking certification YES in the commuter category?
or,
2.) If applicant can substantiate that the damage tolerance approach is impractical for the application, they may use a safe-life approach. 14 CFR §§ 23.574(b), 23.575 Required: For metallic structure in normal, utility, and acrobatic categories, an applicant may choose any of the following:
NO
1.) Safe-Life Evaluation For pressurized cabin, 14 CFR §§ 23.571(a), 23.575 For wing, empennage, and associated structure, 14 CFR §§ 23.572(a)(1), 23.575
2.) Fail-Safe Evaluation For pressurized cabin, 14 CFR §§ 23.571(b), 23.575 For wing, empennage, and associated structure, 14 CFR §§ 23.572(a)(2), 23.575
3.) Damage Tolerance Evaluation For pressurized cabin, 14 CFR §§ 23.571(c),
23.573(b), and 23.575 For wing, empennage, and associated structure, 14 CFR §§ 23.572(a)(3), 23.573(b), and 23.575 5
f. The most recent amendment to part 23 that revised the fatigue requirements was Amendment 23-48, effective February 9, 1996. Always check the latest amendment to part 23 to verify that the regulations referenced in this AC have not been superseded or that additional regulations have not been added.
1-4. Which metallic components should I include in the fatigue evaluation?
Although the fatigue regulations discuss “those parts of the airframe structure whose failure would be catastrophic,” a 1998 policy letter clarifies that you are required to include only certain sections of metallic airplanes in your fatigue evaluation. For airplanes certificated in any small airplane category, these sections include the pressurized cabin, wing (includes tandem wings, canards, winglets and tip fins), empennage, and associated structures. You do not need to include other portions of the airplane, including the landing gear and engine mounts.
1-5. Are the small airplane fatigue re gulations different in other countries?
a. The FAA fatigue regulations for small airplanes are the same as those of many foreign countries. The FAA and the CAA (Civil Aviation Authority) of these countries have worked together to harmonize the part 23 regulatory wording.
b. Even though the regulations may be identical, some differences in policy, guidance, and acceptable means of compliance may exist between the FAA and the CAA of foreign countries. These differences include:
(1) The CAA of some countries may require fatigue evaluation of the engine mounts and landing gear.
(2) The CAA of some countries may require different scatter factors in a safe-life evaluation.
1-6. How do I show compliance to the fatigue requirements for alterations, modifications, or changes to the design?
a. The fatigue performance of the structure should be evaluated for any alteration, modification, or change to the type design. P hysical changes to the design that may affect fatigue performance include changes that increase the external or internal loads, alter the distribution of internal loads, change the stress concentrations, or change the materials or method of construction. You should also ev aluate the effect of any change to the airplane’s operational characteristics that invalidate the assumptions used in the original fatigue substantiation.
Changes to the operational characteristics that may be important for fatigue include higher design airspeeds or higher average speed. They also include changes to the maximum allowable weight and center of gravity envelope, changes to the average weight and center of gravity location, and engine or propeller changes.
b. Modifiers and applicants for supplemental type certificates (STCs) often do not have access to the Type Certificate holder’s original fatigue substantiation data. In these cases, applicants often complete a “comparative fatigue analysis” in which the fatigue performance of the modified and unmodified airplane are compared. Applicants use the results of the comparative analysis to determine the appropriate changes to the safe-life or inspection program of the modified airplane.