ASTM F3651/F3651M-23

• Abstract:

  SIGNIFICANCE AND USE; 4.1 This practice provides one means for determining the scatter factors to establish either the safe-life, or inspection threshold, or recurring inspection intervals, or combinations thereof, as a result of aeroplane durability and damage tolerance assessments. This information can be used in conjunction with Specification F3115/F3115M. ; 4.1.1 This practice defines scatter factors or factors to be used on the unfactored test or analytical mean lives, or both, for determining factored lives (that is, safe-life, inspection threshold, or recurring inspection intervals, or combinations thereof). These factors may be related to but are different from other factors such as load enhancement factor, and life factor that are used to compensate for long test duration. For guidance on life and load enhancement factors, refer to DOT/FAA/AR-10/6 or from relevant CAAs. ; 4.1.2 The unfactored test or analytical mean life, or both, must be determined prior to the usage of this standard practice (see 4.5.1). ; 4.2 The material presented herein is derived from the references listed in Section 2. ; 4.3 Either the safe-life or inspection thresholds can be determined for the entire aeroplane or separately for components such as wing, empennage, landing gear, control surfaces, etc. Such determinations are based on test(s), similarity to previous test(s), or analysis supported by tests. Recurring inspection intervals are typically determined on the same basis but may also be supported by in-service data. ; 4.4 The scatter factors described in this practice are applicable to cyclic test data that meets the following criteria: ; 4.4.1 The cyclic test article must be representative of the production article. Careful consideration must be given for any modifications or alterations, or both, made to the test article prior to or during testing, or both, for metallic structures. ; 4.4.2 At the completion of full-scale or component fatigue/cyclic tests (excluding landing gear), the resid...; SCOPE; 1.1 This practice provides guidance to determine scatter factors to establish either the safe-life, or inspection threshold, and inspection intervals to be published in the Airworthiness Limitation section of the maintenance manual in order to maintain continued airworthiness. The guidance materials presented herein for a means of compliance based on cyclic testing, damage tolerance testing, fatigue analysis, or damage tolerance analysis, or combinations thereof. The material was developed through open consensus of international experts in general aviation. The information was created by focusing on Levels 1, 2, 3 and 4 Normal Category aeroplanes. The content may be more broadly applicable; it is the responsibility of the applicant to substantiate broader applicability as a specific means of compliance. ; 1.2 An applicant intending to propose this information as Means of Compliance for a design approval must seek guidance from their respective oversight authority (for example, published guidance from applicable civil aviation authorities, or CAAs) concerning the acceptable use and application thereof. For information on which oversight authorities have accepted this standard (whole or in part) as an acceptable Means of Compliance to their regulatory requirements (hereinafter “the Rules”), refer to the ASTM Committee F44 web page (www.astm.org/COMMITTEE/F44.htm). ; 1.3 Units—This document may present information in either SI units, English Engineering units, or both; the values stated in each system may not be exact equivalents. Each system shall be used independently of the other; combining values from the two systems may result in nonconformance with the standard. ; 1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applica...

• Status: Published
• Publication Date: 30-Nov-2023
• Technical Committee: F44 - General Aviation Aircraft
• Drafting Committee: F44.30 - Structures

Overview

ASTM F3651/F3651M-23 is an internationally recognized standard developed by ASTM International for the determination of safe-life, inspection threshold, and recurring inspection intervals in the context of general aviation aircraft. This document provides standardized practices to establish airworthiness limitations, ensuring continued safety and structural integrity of aircraft over their service life. The guidance is especially relevant for Levels 1, 2, 3, and 4 normal category aeroplanes but may be applied more broadly with substantiation.

By focusing on durability and damage tolerance assessments, the standard enables applicants to determine statistically robust intervals for mandatory maintenance activities. These determinations are based on a combination of cyclic testing, fatigue and damage tolerance analysis, and supporting data, and are designed to satisfy regulatory requirements for ongoing aircraft airworthiness.

Key Topics

• Safe-Life Determination
  Guidance on establishing the maximum allowable time, cycles, or landings before a structural component must be retired to minimize the probability of failure due to fatigue.

• Inspection Thresholds
  Methods to identify the point at which specific aircraft structures must initially be inspected to detect cracks or defects before they become critical.

• Recurring Inspection Intervals
  Recommendations for scheduling future inspections to monitor the ongoing condition of structures, ensuring continued airworthiness.

• Scatter Factors
  Statistically derived modifiers applied to test data or analytical mean lives to account for variability in material performance and loading, ensuring conservative and safe maintenance intervals.

• Means of Compliance
  Practices for demonstrating compliance based on full-scale or component fatigue testing, fatigue analysis, or damage tolerance evaluations. Applicants are advised to coordinate with aviation authorities to verify acceptable use.

• Referenced Documents and Data
  Integration with related standards, including ASTM F3115/F3115M and references to guidance from regulatory agencies like the FAA and EASA.

Applications

The practical applications of ASTM F3651/F3651M-23 include:

• Civil Aerospace Maintenance and Engineering
  Used by aircraft manufacturers, maintenance organizations, and regulatory authorities to determine maintenance schedules that uphold safety standards.

• Development of Airworthiness Limitation Sections
  Provides the foundation for specifying life-limits and mandatory inspections in maintenance manuals, ensuring compliance with ongoing airworthiness requirements.

• Support for Certification and Design Approval
  Offers a recognized means of compliance for aircraft designers seeking type certification or supplemental type approval.

  • Applicants are encouraged to consult oversight authorities regarding acceptance as a means of compliance with airworthiness rules.

• Assessment of New and Existing Aircraft Structures
  Useful for both new aircraft certification projects and for evaluating existing fleets and components for continued operational safety.

• Implementation Across Structural Elements
  Can be applied to the entire aircraft or discrete structural components, including wings, landing gear, control surfaces, and empennages.

Related Standards

Several related documents and regulatory references enhance the practical implementation of ASTM F3651/F3651M-23:

• ASTM F3115/F3115M: Specification for Structural Durability for Small Aeroplanes
• ASTM F3498: Practice for Developing Simplified Fatigue Load Spectra
• EASA CS-23: Certification Specifications for Normal-Category Aeroplanes
• FAA AC 23-13A: Fatigue, Fail-Safe, and Damage Tolerance Evaluation of Metallic Structure for Normal, Utility, Acrobatic, and Commuter Category Airplanes
• FAA AC 20-107B / EASA AMC 20-29: Guidance for Composite Aircraft Structure
• 14 CFR Part 23: Airworthiness Standards for Normal Category Airplanes
• DOT/FAA/AR-10/6: Guidance for Determining Fatigue Life of Composite Aircraft Structures

Keywords

airworthiness limitation, aircraft inspection intervals, safe-life, damage tolerance, fatigue analysis, scatter factors, structural durability, maintenance schedule, general aviation, ASTM F3651/F3651M

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: F3651/F3651M − 23
Standard Practice for
Determining Safe-Life, Inspection Threshold and Recurring
Inspection Intervals
This standard is issued under the fixed designation F3651/F3651M; the number immediately following the designation indicates the year
of original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval.
A superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 1.5 This international standard was developed in accor-
dance with internationally recognized principles on standard-
1.1 This practice provides guidance to determine scatter
ization established in the Decision on Principles for the
factors to establish either the safe-life, or inspection threshold,
Development of International Standards, Guides and Recom-
and inspection intervals to be published in the Airworthiness
mendations issued by the World Trade Organization Technical
Limitation section of the maintenance manual in order to
Barriers to Trade (TBT) Committee.
maintain continued airworthiness. The guidance materials
presented herein for a means of compliance based on cyclic
2. Referenced Documents
testing, damage tolerance testing, fatigue analysis, or damage
tolerance analysis, or combinations thereof. The material was
2.1 ASTM Standards:
developed through open consensus of international experts in
F3060 Terminology for Aircraft
general aviation. The information was created by focusing on
F3115/F3115M Specification for Structural Durability for
Levels 1, 2, 3 and 4 Normal Category aeroplanes. The content
Small Aeroplanes
may be more broadly applicable; it is the responsibility of the
F3498 Practice for Developing Simplified Fatigue Load
applicant to substantiate broader applicability as a specific
Spectra
means of compliance.
2.2 EASA Documents:
1.2 An applicant intending to propose this information as CS-23 Certification Specifications for Normal-Category
Means of Compliance for a design approval must seek guid-
Aeroplanes
ance from their respective oversight authority (for example, AMC 20-29 Composite Aircraft Structure
published guidance from applicable civil aviation authorities,
2.3 FAA Documents:
or CAAs) concerning the acceptable use and application
AC 20-107B Composite Aircraft Structure
thereof. For information on which oversight authorities have
AC 23-13A Fatigue, Fail-Safe, and Damage Tolerance
accepted this standard (whole or in part) as an acceptable
Evaluation of Metallic Structure for Normal, Utility,
Means of Compliance to their regulatory requirements (here-
Acrobatic, and Commuter Category Airplanes
inafter “the Rules”), refer to the ASTM Committee F44 web
DOT/FAA/AR-10/6 Determining the Fatigue Life of Com-
page (www.astm.org/COMMITTEE/F44.htm).
posite Aircraft Structures Using Life and Load Enhance-
1.3 Units—This document may present information in either ment Factors
SI units, English Engineering units, or both; the values stated
AC 25.571-1C Damage Tolerance and Fatigue Evaluation of
in each system may not be exact equivalents. Each system shall Structure
be used independently of the other; combining values from the 5
2.4 Federal Standards:
two systems may result in nonconformance with the standard.
14 CFR Part 23 Airworthiness Standards: Normal Category
1.4 This standard does not purport to address all of the
Airplanes
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
priate safety, health, and environmental practices and deter-
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
mine the applicability of regulatory limitations prior to use.
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
Available from European Union Aviation Safety Agency (EASA), Konrad-
This practice is under the jurisdiction of ASTM Committee F44 on General Adenauer-Ufer 3, D-50668 Cologne, Germany, https://www.easa.europa.eu.
Aviation Aircraft and is the direct responsibility of Subcommittee F44.30 on Available from Federal Aviation Administration (FAA), 800 Independence
Structures. Ave., SW, Washington, DC 20591, http://www.faa.gov.
Current edition approved Dec. 1, 2023. Published January 2024. DOI: 10.1520/ Available from U.S. Government Publishing Office (GPO), 732 N. Capitol St.,
F3651_F3651M-23. NW, Washington, DC 20401, http://www.gpo.gov.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F3651/F3651M − 23
3. Terminology 4.1.2 The unfactored test or analytical mean life, or both,
must be determined prior to the usage of this standard practice
3.1 The following are a selection of terms relevant to this
(see 4.5.1).
practice taken directly from Terminology F3060, Specification
F3115/F3115M, and AC 25.571-1C. Refer to these documents 4.2 The material presented herein is derived from the
for the up-to-date definitions, also for additional definitions and
references listed in Section 2.
abbreviations.
4.3 Either the safe-life or inspection thresholds can be
3.2 Definitions:
determined for the entire aeroplane or separately for compo-
3.2.1 airworthiness limitation, n—limitation applicable to
nents such as wing, empennage, landing gear, control surfaces,
an aircraft or article installed on an aircraft in the form of a life etc. Such determinations are based on test(s), similarity to
limit or a maintenance task that is mandatory to maintain the
previous test(s), or analysis supported by tests. Recurring
aircraft in airworthy condition.
inspection intervals are typically determined on the same basis
but may also be supported by in-service data.
3.2.2 continuing airworthiness, n—set of processes by
which an aircraft, engine, propeller, or part complies with the
4.4 The scatter factors described in this practice are appli-
applicable airworthiness requirements and remains in a condi-
cable to cyclic test data that meets the following criteria:
tion for safe operation throughout its operating life.
4.4.1 The cyclic test article must be representative of the
production article. Careful consideration must be given for any
3.2.3 fatigue, n—the process of progressive localized per-
modifications or alterations, or both, made to the test article
manent structural change occurring in a material subjected to
prior to or during testing, or both, for metallic structures.
conditions that produce fluctuating stresses and strains at some
4.4.2 At the completion of full-scale or component fatigue/
point or points, which may result in damage or complete
fracture after a sufficient number of fluctuations. cyclic tests (excluding landing gear), the residual strength
capability must be demonstrated before determining either the
3.2.4 multiple load path, adj—applies to structure, the
safe-life, or inspection threshold.
applied loads of which are distributed through redundant
structural members, so that the failure of a single structural
4.5 The following are not within the scope of this standard:
member does not result in the loss of structural capability to
4.5.1 Methodologies of durability, damage tolerance
carry applied load.
analysis, or test, or combinations thereof.
4.5.2 Structures which use novel fabrication methods such
3.2.5 safe-life, n—of a structure, that number of events, such
as friction stir welding, additive manufacturing, and thermo-
as flights, landings, or flight hours, during which there is a low
plastic welding.
probability that the strength will degrade below its design
4.5.3 Structural bonding (except already proven metal-to-
ultimate value due to fatigue-induced damage.
metal bonding, etc.); for guidance on structural bonding refer
3.2.6 scatter factor, n—statistically derived divisor applied
to AC 20-107B (or AMC 20-29).
to fatigue test results to account for the variation in fatigue
performance of built-up or monolithic structures and usage
4.6 The Finite Element Model (FEM) used for analysis must
variability; a scatter factor can also be used in a fatigue analysis be validated with test data, or other independent analysis
to address the uncertainties inherent in a fatigue analysis; also
methods in accordance with relevant CAA requirements.
called “life reduction factor.”
4.7 The inspection intervals determined using this practice
3.2.7 single load path, adj—applies to structure, the applied
are independent of other inspection intervals that are defined by
loads of which are eventually distributed through a single
other process, such as Maintenance Steering Group (MSG).
structural member, the failure of which would result in the loss
of the structural capability to carry applied load.
5. Procedure
5.1 The mechanics, modes of failure, and structural assess-
4. Significance and Use
ment methods associated with fatigue and damage growth are
4.1 This practice provides one means for determining the
different for metals compared to the non-metallic (that is,
scatter factors to establish either the safe-life, or inspection
composites). A consequence of this is that the methods to
threshold, or recurring inspection intervals, or combinations
define damage inspection techniques and intervals are also
thereof, as a result of aeroplane durability and damage toler-
different. Therefore, the derivations of either the safe-life or
ance assessments. This information can be used in conjunction
inspection intervals are segregated into two different
with Specification F3115/F3115M.
methods—metallic and non-metallic.
4.1.1 This practice defines scatter factors or factors to be
5.2 Table 1 summarizes the means for determining either
used on the unfactored test or analytical mean lives, or both, for
the safe-life, inspection threshold, or recurring inspection
determining factored lives (that is, safe-life, inspection
intervals, or combinations thereof.
threshold, or recurring inspection intervals, or combinations
thereof). These factors may be related to but are different from 5.3 Safe-Life Determination:
other factors such as load enhancement factor, and life factor 5.3.1 The safe-life or life limit, for metallic structures, can
that are used to compensate for long test duration. For guidance be determined based on a full-scale fatigue test (FSFT). After
on life and load enhancement factors, refer to DOT/FAA/AR- completion of FSFT, and demonstration of required residual
10/6 or from relevant CAAs. strength capability, or, fail-safe capability per the guidance
F3651/F3651M − 23
TABLE 1 Means for Determining Safe-Life, Inspection Threshold, or Recurring Inspection Intervals, or Combinations Thereof
Type Material Method Reference Section
Safe-Life Determination Met
...