ASTM F3498-21

Standard Practice for Developing Simplified Fatigue Load Spectra

SIGNIFICANCE AND USE
4.1 This standard practice provides one means for determining fatigue load spectra for aeroplane durability assessments. This information can be used in conjunction with Specification F3115/F3115M, Section 5, Load Considerations.  
4.1.1 Users of this practice may propose alternate spectra, subject to the approval of their CAA.  
4.2 The methods are applicable to the durability evaluation of wings of small aeroplanes. Additional calculation (such as methods noted in ACE-100-01) are needed to properly develop load spectra for fatigue evaluation of empennage and/or configurations with canards (or forward wings) and/or winglets (or tip fins), fuselage, and potentially other components, with approval from appropriate regulatory agency.  
4.3 Much of the material presented herein is directly taken from AC 23-13A. The FAA developed the flight load spectra, presented herein, based on a statistical analysis of the data presented in DOT/FAA/CT-91/20. The ground load spectra are directly from AFS-120-73-2.  
4.4 The flight load spectra, presented in Section 7, includes an adjustment (1.5 standard deviations) to the average measured load frequency. The adjustment accounts for the variability in the loading spectra experienced by individual aeroplanes, as well as across aeroplane types. The magnitude of the adjustment was selected to maintain the probability that a component will reach its safe-life without a detectable fatigue crack established by scatter factor (see paragraph 2–15 of AC 23-13A).
SCOPE
1.1 This practice provides data to develop simplified loading spectra that can be used to perform structural durability analysis for aeroplanes, specifically for wings of small aeroplanes. The material was developed through open consensus of international experts in general aviation. The information was created by focusing on Level 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 The values stated in inch-pound units are to be regarded as standard. No other units of measurement are included in this 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 applicability of regulatory limitations prior to use.  
1.5 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.

General Information

• Status: Published
• Publication Date: 31-Dec-2020
• ICS: 49.035 - Components for aerospace construction
• Technical Committee: F44 - General Aviation Aircraft
• Drafting Committee: F44.30 - Structures

Relations

• Refers: ASTM E1049-85(2023) - Standard Practices for Cycle Counting in Fatigue Analysis
• Effective Date: 01-Nov-2023
• Refers: ASTM F3060-20 - Standard Terminology for Aircraft
• Effective Date: 01-Jan-2020
• Refers: ASTM E1049-85(2017) - Standard Practices for Cycle Counting in Fatigue Analysis
• Effective Date: 01-Jun-2017
• Refers: ASTM F3060-16a - Standard Terminology for Aircraft
• Effective Date: 01-Nov-2016
• Refers: ASTM F3060-16 - Standard Terminology for Aircraft
• Effective Date: 01-Apr-2016
• Refers: ASTM F3060-15b - Standard Terminology for Aircraft
• Effective Date: 15-Sep-2015
• Refers: ASTM F3115/F3115M-15 - Standard Specification for Structural Durability for Small Airplanes
• Effective Date: 01-Jun-2015
• Refers: ASTM F3060-15a - Standard Terminology for Aircraft
• Effective Date: 01-May-2015
• Refers: ASTM F3060-15 - Standard Terminology for Aircraft
• Effective Date: 01-Mar-2015
• Refers: ASTM F3060-14 - Standard Terminology for Aircraft
• Effective Date: 01-Dec-2014
• Refers: ASTM E1049-85(2011)e1 - Standard Practices for Cycle Counting in Fatigue Analysis
• Effective Date: 01-Oct-2011
• Refers: ASTM E1049-85(2005) - Standard Practices for Cycle Counting in Fatigue Analysis
• Effective Date: 01-Jun-2005
• Refers: ASTM E1049-85(1997) - Standard Practices for Cycle Counting in Fatigue Analysis
• Effective Date: 10-Oct-1997

Overview

ASTM F3498-21: Standard Practice for Developing Simplified Fatigue Load Spectra defines a recognized methodology for establishing fatigue load spectra used in the structural durability analysis of aeroplane wings, specifically for small aircraft. Developed through the consensus of international experts in general aviation, this standard provides data and procedures suited for Level 1, 2, 3, and 4 Normal Category aeroplanes, ensuring that fatigue assessments are consistent, reproducible, and support regulatory compliance.

The standard is closely tied to regulatory guidance, referencing FAA and EASA documents that inform the development of both flight and ground load spectra. While calibrated for aircraft wings, its methods can be adapted to other components with appropriate validation and regulatory oversight.

Key Topics

• Fatigue Load Spectra Development: Offers stepwise procedures to create simplified loading spectra based on statistical analysis of gust, maneuver, ground (taxi and landing), and special usage loads.
• Applicability: Focus is on small aeroplane wings, with potential adaptation to empennage, canards, winglets, and fuselage using additional methodologies and regulatory approval.
• Mission Profiles: Outlines typical mission assumptions (flight durations, speeds, weight distributions) for common aircraft usages such as instruction, personal, executive, utility, acrobatic, agricultural, and survey operations.
• Cycle Counting: Recommends recognized cycle counting practices (like ASTM E1049) for evaluating and re-ordering load sequences throughout an aircraft’s operational life.
• Adjustment for Variability: Includes statistical adjustments (using scatter factors) to account for differences in load experiences between aircraft and ensure a high degree of confidence in safe-life predictions.
• Regulatory Compliance: Guidance for proposing these spectra as Means of Compliance requires coordination with civil aviation authorities (CAAs) and awareness of oversight acceptance through ASTM Committee F44.

Applications

Practical uses of ASTM F3498-21 include:

• Aircraft Design and Certification: Used by airframe manufacturers and engineers to establish standardized, validated load spectra in support of structural fatigue life evaluations for new or modified aeroplane designs.
• Regulatory Submissions: Provides recognized documentation and supporting data when demonstrating compliance with 14 CFR 23, EASA CS-23, or equivalent national standards for aircraft certification.
• Maintenance Planning: Informs the development of inspection intervals and the structuring of continuing airworthiness programs by providing a defensible basis for anticipated fatigue life and safe-life estimates.
• Operational Assessment: Helps fleet operators evaluate the impact of different mission types, such as instructional versus executive usage or agricultural versus survey roles, by comparing relevant spectra.
• Adaptability: Methods may be extended to non-wing components (empennage, canards, winglets, etc.) pending additional assessment and approval, making the standard a versatile tool in fatigue analysis.

Related Standards

• ASTM F3115/F3115M: Specification for structural durability in small aeroplanes; complements F3498-21 by addressing broader load and durability considerations.
• ASTM E1049: Cycle counting in fatigue analysis; provides methodological support for load spectrum development and sequence ordering.
• ASTM F3060: Aircraft terminology reference, ensuring clarity and standardization of definitions used within F3498-21.
• FAA AC 23-13A: Contains significant foundational material and is referenced directly, especially regarding statistical bases for spectrum development.
• FAA Report DOT/FAA/CT-91/20: Source for statistical data underpinning flight load spectra.
• FAA ACE-100-01: Guidance for fatigue evaluation of empennage and related structures, helpful when extending applicability beyond wings.
• EASA CS-23 / 14 CFR 23: Regulatory standards whose requirements F3498-21 is designed to support.

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Keywords: fatigue load spectra, aeroplane durability, structural fatigue, load spectrum, fatigue analysis, small aeroplanes, ASTM F3498, aircraft structural evaluation, airworthiness compliance, cycle counting, safe-life prediction, ASTM aviation standards.

Standards Content (Sample)

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: F3498 −21
Standard Practice for
Developing Simplified Fatigue Load Spectra
This standard is issued under the fixed designation F3498; 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 2. Referenced Documents
1.1 This practice provides data to develop simplified load- 2.1 ASTM Standards:
ing spectra that can be used to perform structural durability E1049 Practices for Cycle Counting in Fatigue Analysis
analysis for aeroplanes, specifically for wings of small aero- F3060 Terminology for Aircraft
planes.The material was developed through open consensus of F3115/F3115M Specification for Structural Durability for
international experts in general aviation. The information was Small Aeroplanes
created by focusing on Level 1, 2, 3, and 4 Normal Category
2.2 EASA Standard:
aeroplanes. The content may be more broadly applicable; it is
CS-23 Normal, Utility,Aerobatic and CommuterAeroplanes
the responsibility of the applicant to substantiate broader
2.3 FAA Documents:
applicability as a specific means of compliance.
14 CFR 23 Airworthiness Standards: Normal, Utility,
Acrobatic, and Commuter Category Airplanes
1.2 An applicant intending to propose this information as
AC 23-13A Fatigue, Fail-Safe, and Damage Tolerance
Means of Compliance for a design approval must seek guid-
Evaluation of Metallic Structure for Normal, Utility,
ance from their respective oversight authority (for example,
Acrobatic, and Commuter Category Airplanes
published guidance from applicable civil aviation authorities,
ACE-100-01 Fatigue Evaluation of Empennage, Forward
or CAAs) concerning the acceptable use and application
Wing and Winglets/Tips Fins on Part 23 Airplanes
thereof. For information on which oversight authorities have
Report No. AFS-120-73-2 Fatigue Evaluation of Wing and
accepted this standard (whole or in part) as an acceptable
Associated Structure on Small Airplanes
Means of Compliance to their regulatory requirements (here-
DOT/FAA/AR-96/46 User’s Guide for FAR23 Loads Pro-
inafter “the Rules”), refer to the ASTM Committee F44 web
gram
page (www.astm.org/COMMITTEE/F44.htm).
DOT/FAA/CT-91/20 General Aviation Aircraft – Normal
1.3 The values stated in inch-pound units are to be regarded
Acceleration Data Analysis and Collection Project
as standard. No other units of measurement are included in this
standard.
3. Terminology
1.4 This standard does not purport to address all of the
3.1 The following are a selection of terms relevant to this
safety concerns, if any, associated with its use. It is the
practice. See Terminology F3060 for more definitions and
responsibility of the user of this standard to establish appro-
abbreviations.
priate safety, health, and environmental practices and deter-
mine the applicability of regulatory limitations prior to use.
1.5 This international standard was developed in accor-
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
dance with internationally recognized principles on standard-
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
ization established in the Decision on Principles for the
the ASTM website.
Development of International Standards, Guides and Recom-
Available from European Aviation Safety Agency (EASA), Konrad-Adenauer-
mendations issued by the World Trade Organization Technical
Ufer 3, D-50668 Cologne, Germany, https://www.easa.europa.eu/.
Barriers to Trade (TBT) Committee. Available from Federal Aviation Administration (FAA), 800 Independence
Ave., SW, Washington, DC 20591, http://www.faa.gov.
Available from U.S. Government Publishing Office (GPO), 732 N. Capitol St.,
This practice is under the jurisdiction of ASTM Committee F44 on General NW, Washington, DC 20401, http://www.gpo.gov.
Aviation Aircraft and is the direct responsibility of Subcommittee F44.30 on Available from National Technical Information Service (NTIS), 5301 Shawnee
Structures. Rd., Alexandria, VA 22312, http://www.ntis.gov.
Current edition approved Jan. 1, 2021. Published February 2021. DOI: 10.1520/ Available from Clearinghouse for Federal Scientific andTechnical Information,
F3498-21. Springfield, VA 22151.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F3498 − 21
3.2 Definitions of Terms Specific to This Standard: 5.1.4 Single-engine acrobatic (non-pressurized).
3.2.1 fatigue, n—the process of progressive, localized, per- 5.1.5 Twin-engine general usage (non-pressurized).
manent structural change occurring in a material subjected to 5.1.6 Twin-engine instructional usage (non-pressurized).
conditions that produce fluctuating stresses and strains at some 5.1.7 Pressurized usage.
point or points, which may result in damage or complete 5.1.8 Special usage (including survey and aerial applica-
fracture after a sufficient number of fluctuations. tion).
3.2.2 safe-life, n—the safe-life of a structure is that number
5.2 If the wing center section skin panels are affected by
of events, such as flights, landings, or flight hours, during
cabin pressurization or external aerodynamic pressure, the
which there is a low probability that the strength will degrade
wing loading spectrum should include these effects.
below its design ultimate value due to fatigue-induced damage.
5.3 Loading spectra should include the ground-air-ground
3.2.3 scatter factor, n—statistically derived divisor applied
cycle (GAG), where applicable. The GAG cycles represent the
to fatigue test results to account for the variation in fatigue
range of the maximum and minimum loads expected to occur
performance of built-up or monolithic structures and usage
on a per flight basis. Typically, the minimum load results from
variability. A scatter factor can also be used in a fatigue
landing or taxi conditions, and the maximum load results from
analysis to address the uncertainties inherent in a fatigue
the gust or maneuver spectra. For typical aeroplanes certifi-
analysis; also called “life reduction factor.”
cated in the normal category, two-thirds to three-fourths of the
total fatigue damage on the wing may be caused by the GAG
4. Significance and Use
cycle.
4.1 This standard practice provides one means for determin- 5.3.1 Cycle counting methods to re-order the load sequence
ing fatigue load spectra for aeroplane durability assessments. across an entire flight history may be employed instead of
This information can be used in conjunction with Specification using the GAG cycle for each flight.The cycle counting should
F3115/F3115M, Section 5, Load Considerations. be performed on a single flight spectrum with all appropriate
4.1.1 Users of this practice may propose alternate spectra,
flight and ground cycles included.
subject to the approval of their CAA. 5.3.2 Cycle counting may be performed in accordance with
Practices E1049.
4.2 The methods are applicable to the durability evaluation
of wings of small aeroplanes. Additional calculation (such as 5.4 While positive and negative load cycles are considered
methods noted inACE-100-01) are needed to properly develop to occur randomly in service, the high positive and negative
load spectra for fatigue evaluation of empennage and/or loads of a given type of repeated loading tend to occur close
configurations with canards (or forward wings) and/or winglets together during the flight.
(or tip fins), fuselage, and potentially other components, with
6. Mission Profile
approval from appropriate regulatory agency.
6.1 Indevelopingthemissionprofile,thefollowingassump-
4.3 Much of the material presented herein is directly taken
tions should be considered:
from AC 23-13A. The FAA developed the flight load spectra,
6.1.1 Flight Time:
presented herein, based on a statistical analysis of the data
6.1.1.1 Single-engine and twin-engine (non-pressurized) –
presented in DOT/FAA/CT-91/20. The ground load spectra are
0.65 h.
directly from AFS-120-73-2.
6.1.1.2 Pressurized – 1.10 h.
4.4 The flight load spectra, presented in Section 7, includes
6.1.1.3 Single-engine special usage (low level survey) –
an adjustment (1.5 standard deviations) to the average mea-
2.00 h.
sured load frequency.The adjustment accounts for the variabil-
6.1.1.4 Twin-engine special usage – 3.00 h.
ity in the loading spectra experienced by individual aeroplanes,
6.1.2 Aeroplane Speed:
as well as across aeroplane types. The magnitude of the
6.1.2.1 Thespeedfordeterminingmilesflownshouldnotbe
adjustment was selected to maintain the probability that a
less than 0.9V or 0.9V .
NO MO
component will reach its safe-life without a detectable fatigue
6.1.2.2 For special usage, the speed for determining miles
crack established by scatter factor (see paragraph 2–15 of AC
flown may be 100 knots or 0.9V , whichever is less.
A
23-13A).
6.1.3 Gross Weight and Load Distribution:
6.1.3.1 Estimates of the gross weight and distribution of
5. Load Spectra
disposable load should be based on conservative estimates of
5.1 The flight load (that is, gust and maneuver) along with typical operating conditions.
ground load (that is, landing impact and taxi) spectra presented 6.1.3.2 Itisacceptabletousetheweightconditionthatgives
herein are for the following types of aeroplanes and usages: the highest 1-g stress for 1-g and per-g calculations (a
5.1.1 Single-engine executive usage (non-pressurized, en- segment-by-segment analysis would, therefore, not be neces-
gine size greater than 185 hp). sary).
5.1.2 Single-enginepersonalusage(non-pressurized,engine 6.1.4 The mission profile of certain aeroplanes should
size less than or equal to 185 hp). consider any unique aspects of the usage. Some examples are
5.1.3 Single-engine instructional usage (non-pressurized). as follows:
F3498 − 21
6.1.4.1 Instructional Usage—Takeoff and landing training,
=
2.67
or “touch-and-go” training, is a significant portion of the
1.33 2 for W/S.16lb/ft ,
W 4
student pilot training curriculum. Incorporate “touch-and-go”
S D
S
training into the mission profiles of any aeroplane used for
instructional purposes. Each “touch-and-go” should be treated
W/S = wing loading at maximum gross weight, lb/ft ,
as a short duration flight, 6 min to 10 min in length.
V = aeroplane structural design cruise speed, V , knots
C
6.1.4.2 Mixed Usage—Some aeroplanes are designed for
equivalent air speed (KEAS), and
more than one type of usage. In general, the spectrum should –1
m = wing lift curve slope, C , rad .
Lα
be based on the usage that results in the shorter life. It is
7.3 Method to determine maneuver limit load factor. The
acceptable to estimate a mix of missions, the percent of time
maneuver limit load factor depends on the aircraft usage types,
spent operating in the different usages, as a way of accounting
as prescribed in Section 7.1 of DOT/FAA/AR-96/46.
for the overall usage spectrum. In using this approach, it is not
Negative
acceptable to use a mix of missions within a single flight. For
Usage Type Positive Load Factor
Load Factor
purposesoffatigueevaluation,asingleflightisoperatedwithin
a single category and within a single type of usage. Normal –0.4 × positive
2.1 1 24 000 ⁄ Maximum Gross Weight
h f s
load factor
1 10 000 j#3.8
6.1.4.3 Severe Usage—Some aeroplanes may be used more dg
Special/Utility 4.4 –0.4 × positive
severely than assumed in the fatigue evaluation. Common
load factor
examples of this include aeroplanes normally considered to be
Acrobatic 6.0 –0.5 × positive
in the single-engine personal or executive usage type, but are load factor
used for pipeline and utility patrol, for instruction, or for short 7.4 Method to “normalize” the gust and maneuver load
duration commuter and air taxi flights. Such severe usage may
spectra.
be addressed by placing a statement in the Limitations Section 7.4.1 Normalize the gust spectra to the incremental limit
of the Instructions for ContinuedAirworthiness, in accordance
(see Note 1) gust load factor computed in 7.2 to derive the gust
with Specification F3115/F3115M, noting that certain types of spectra in terms of the gust load factor ratio:
usage require a re-evaluation of the structure by the type
a
n
certificate (TC) holder. The instructions list these types of 5 (2)
a
nLLF
usage and requests the owner to contact the TC holder
regarding the usage.
Incremental Gust Load Factor at Operating Weight
Incremental Design Limit Gust Load Factor at Maximum Gross Weight
7. Flight and Ground Load Spectra
7.4.2 Normalize the maneuver spectra to the incremental
7.1 Table 1 summarizes the flight and ground load spectra
maneuver limit load factor computed in 7.3 to derive the
presented herein. maneuver spectra in terms of the maneuver load factor ratio:
7.2 Method to compute limit (see Note 1) gust load factor a
n
5 (3)
used in the gust load equation.
a
nLLF
NOTE 1—The limit, in gust load factor, indicates being a subset of the
Incremental Maneuver Load Factor at Operating Weight
envelope/design limit load case.
Incremental Design Limit Maneuver Load Factor at Maximum
7.2.1 The limit (see Note 1) gust load factor (for use in
Gross Weight
developing fatigue loading spectra only) must be calculated
8. Correlation of Standard-Content and Rules
using the same equation used to derive the gust spectra.
8.1 Means of Compliance Correlation Sorted by Standard:
7.2.2 Equation for computing the incremental limit (see
Note 1) gust load factor:
14 CFR 23 CS-23
4 §2240(a) §2240(a)
UKVm
5 §2240(a) §2240(a)
a 5 (1)
nLLF
W 6 §2240(a) §2240(a)
7 §2240(a) §2240(a)
S
8.2 Means of Compliance Correlation Sorted by Rule:
where:
14 CFR 23 CS-23
U = 30.0, nominal gust velocity in ft/s,
§2240(a) §2240(a) 4, 5, 6, 7
K = 1
1 W 4 9. Keywords
for W/S,16lb/ft
S D
2 S
9.1 airframe structure; fatigue; fatigue spectra; gust; load;
maneuver; spectra; taxi
F3498 − 21
TABLE 1 Flight and Ground Load Spectra
Description Graphic Data Tabulated Data Comments
Flight Spectra, General Usage, Gust spectra for single-engine, Fig. 1 Table 2 These spectra should also be used for
Single-Engine Unpressurized unpressurized operations, including pressurized single-engine aeroplanes that
basic instruction, personal, spend a significant amount of flight time at
executive, and acrobatic usage low altitude.
Maneuver spectra for single-engine Fig. 2 Table 3 No comments.
basic instruction usage
Maneuver spectra for single-engine Fig. 3 Table 4 An aeroplane in the personal usage category
personal usage has a single, reciprocating engine with 185
hp or less.
Maneuver spectra for single-engine Fig. 4 Table 5 An aeroplane in the executive usage
executive usage category has a single, reciprocating engine
with more than 185 hp. The executive usage
category also includes unpressurized, single-
engine turboprop aeroplanes.
Maneuver spectra for single-engine Fig. 5 Table 6 Applicable to typical acrobatic category
acrobatic usage aeroplane, n = +6g/–3g.
z
Flight Spectra, General Usage, Gust spectra for twin-engine, Fig. 6 Table 7 These spectra should also be used for
Twin-Engine Unpressurized unpressurized operations, including pressurized twin-engine aeroplanes that
instruction and general usage spend a significant amount of flight time at
low altitudes, 7000 ft and below. An example
of this type of operation is short flight
duration commuter and air taxi operations.
Maneuver spectra for twin-engine Fig. 7 Table 8 No comments.
instruction usage
Maneuver spectra for twin-engine Fig. 8 Table 9 These spectra should also be used for
general usage pressurized and unpressurized twin-engine
aeroplanes that may be operated in short
flight duration commuter and air taxi
operations.
Flight Spectra, General Usage, Gust spectra for pressurized usage Fig. 9 Table 10 No comments.
Single-Engine and Twin-Engine
Pressurized Maneuver spectra for pressurized Fig. 10 Table 11 No comments.
usage
Flight Spectra, Special Usage – Gust spectra for agricultural or aerial Fig. 11 Table 12 No comments.
Agricultural (Aerial Application) application usage
Maneuver spectra for agricultural or Fig. 12 Table 13 No comments.
aerial application usage
Flight Spectra, Special Usage – Gust spectra for low-level survey or Fig. 13 Table 14 No comments.
Survey (Pipeline Patrol) pipeline patrol usage
Maneuver spectra for low-level Fig. 1
...