Appendix 3 replacements or modifications that must occur prior to the LOV, must be included in the ALS.
Step 10: Revise the FAA-approved ALS to include all inspections, modifications, and replacements required before the LOV.
Based on the results of Step 9, include, in the ALS of the ICA (per § 25.1529), the inspections or other procedures determined to be necessary to prevent catastrophic failure up to the LOV. The LOV must also be included.
For each required inspection, the operator should include— The structure to be inspected.
The method of inspection.
The inspection threshold (the point in time at which to begin inspections).
The inspection repetitive interval.
For required modifications or replacements, the operator should include— The structure to be modified or replaced.
The method of modification or replacement.
The replacement time or structural-modification point (the point in time to begin the modification), as applicable.
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Appendix 4 Appendix 4
Examples of Alterations that May Require Full-Scale Fatigue Testing
1. The following are examples of types of alterations that may require full-scale fatigue testing:
a. passenger-to-freighter conversions (including addition of cargo doors);
b. gross-weight increases (e.g., increased operating weights, increased zero-fuel weights, increased landing weights, and increased maximum-takeoff weights);
c. installation of fuselage cutouts (e.g., passenger-entry doors, emergency-exit doors or crew-escape hatches, fuselage-access doors, and cabin-window relocations);
d. complete re-engine or pylon alteration;
e. engine-hush kits;
f. wing alterations (e.g., installation of winglets; changes in flight-control settings such as flap droop; and alteration of wing trailing-edge structure);
g. modified or replaced skin splice;
h. any alteration that affects three or more stiffening members (e.g., wing stringers and fuselage frames);
i. an alteration that results in operational-mission change, which significantly changes the original equipment manufacturer’s load/stress spectrum (e.g., extending the flight duration from 2 hours to 10 hours); and
j. an alteration that changes areas of the fuselage from being externally inspectable using visual means to being uninspectable (e.g., installation of a large, external, fuselage doubler that results in hiding details beneath it).
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Appendix 5 Appendix 5
PSE, FCS and WFD-Susceptible Structure
1. Overview. Three key terms used when showing compliance to the damage-tolerance and fatigue requirements of 14 CFR parts 25 and 26 are, “principle structural element” (PSE), “fatigue critical structure” (FCS), and “widespread fatigue damage (WFD) susceptible structure.”
This appendix provides clarification on the intended meanings of these terms and how they relate to one another. This appendix describes the items that should be included in an airplane’s Airworthiness Limitations section based on the evaluation of each PSE, FCS, and WFD- susceptible structure.
2. PSE – Principal Structural Element.
a. The term “principal structural element” (PSE) is defined in this AC as follows:
b. Principal structural element, 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 integrity of the airplane.
c. While this definition does not specifically address the fatigue susceptibility of structure, or environmental or accidental damage, it is intended to address all structure that must be evaluated under § 25.571. Sectio n 25.571(a) states the following:
“This evaluation must be conducted … for each part of the structure that could contribute to a catastrophic failure (such as wing, empennage, control surfaces and their systems, the fuselage, engine mounting, landing gear, and their related primary attachments).”
d. The above reinforces the notion that the identification of PSEs should be based solely on the importance of the structure to assure the overall airplane integrity.
e. Section 6c of this AC provides guidance for identifying PSEs. Many manufacturers use this list as a starting point for their list of fatigue critical structure (FCS), because AC 120-93 specifically refers to this AC for identifying FCS. Section 25.571 is intended to address all structure that could contribute to a catastrophic failure resulting from fatigue, environmental, and accidental damage, and therefore may include some structure that is not considered FCS.
Nevertheless, PSE should be considered when developing a list of FCS.
f. The definitions used by applicants to identify PSEs have not been consistent between applicants and, in some cases, between models produced by the same applicant. The lack of standardization of the usage and understanding of the term “PSE,” and the resultant diversity that exists between type-design PSE lists, have required the FAA to introduce a new term, “fatigue-critical structure,” in the Aging Airplane Safety—Damage Tolerance Data for Repairs and Alterations found in 14 CFR part 26, subpart E, and corresponding advisory material.
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Appendix 5
3. FCS – Fatigue Critical Structure.
a. Under 14 CFR 26.41, fatigue critical structure is defined as airplane structure that is susceptible to fatigue cracking, which could contribute to a catastrophic failure, as determined in accordance with 14 CFR 25.571. Fatigue critical structure includes structure susceptible to fatigue cracking, which could contribute to a catastrophic failure. Fatigue-critical structure also includes structure which, if repaired or altered, could be susceptible to fatigue cracking and contribute to a catastrophic failure. Structure may be susceptible to fatigue cracking when subjected to tension-dominated repeated loads during operation. Such structure may be part of the baseline structure or part of an alteration. “Baseline structure” means structure that is designed under the original type certificate or amended type certificate for that airplane model
(i.e., the as-delivered airplane model configuration).
b. Fatigue critical structure is a subset of principal structural elements, specifically those elements that are susceptible to fatigue damage.
4. Widespread Fatigue Damage Susceptible Structure.
a. Widespread fatigue damage is the simultaneous presence of cracks at multiple structural locations, which are of sufficient size and density such that the structure no longer meets the residual-strength requirements of § 25.571(b).
b. Multiple site damage (MSD) and multiple-element damage (MED) are conditions that, with no intervention, can lead to WFD. The term “WFD-susceptible structure” refers to areas of structure that, under normal circumstances, could be expected to eventually develop MSD and/or MED cracks, which could lead to WFD.
c. Although not explicitly stated, structure susceptible to WFD cannot be inspected reliably to preclude WFD. Unless a flight cycles and/or flight hours limit is placed on an airplane, modifications may be needed to preclude WFD. Structure susceptible to WFD is a subset of FCS, which, in turn, is a subset of PSE.
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Appendix 6 Appendix 6
Acronyms and Abbreviations used in this AC Term Definition AC Advisory circular ALI Airworthiness Limitation item ALS Airworthiness Limitations section FCS Fatigue critical structure ICA Instructions for Continued Airworthiness ISP Inspection start point LOV Limit of validity of the engineering data that supports the structural maintenance program MED Multiple element damage MSD Multiple site damage PSE Principal structural element SMP Structural modification point WFD Widespread fatigue damage A6-1