Structural Integrity, Ageing Aircraft (Considerations)
What does the FAA Say about Limit of Validity?
The limit of validity (LOV) represents an operational limit based on fatigue test evidence that supports the maintenance program. The FAA defines the LOV as “the period of time (in flight cycles, flight hours, or both) up to which it has been demonstrated by test evidence, analysis and, if available, service experience and teardown inspections, that widespread fatigue damage will not occur in the airplane structure.” It is further defined as the point in the structural life of an airplane at which there is significantly increased risk of uncertainties in structural performance and probable development of WFD.
Once the airworthiness limitations containing the LOV are approved by the FAA, an airplane may not operate beyond the LOV.
Does Design Service Goal Limit the Operation of an Aircraft?
The original DSG is generally established at the time of type certification and is NOT intended to limit the life of the structure, or to define the point at which the aircraft cannot continue its operation.
Example - Airbus has developed new, extended service goals (ESG) To justify a further period of operation up to the new ESG, it is necessary to review service experience and re-assess the existing inspection programmes.
This may lead to a modification of the maintenance strategy, including the inspection of additional items or an increased level of surveillance in some areas.
The following activities are typical of the process associated with the Full Life Extension.
- Fatigue and damage tolerance analysis of the original structure and modifications including:
- Detailed identification of the concerned area
- Review of Full Scale Fatigue Test and in-service experiences
- Loads comparison for all variants
- Review of former fatigue justifications
- Review of Service Bulletins and current inspection programme
- Widespread Fatigue Damage Analysis
- Update of all inspection programmes incl. MRB, SSIP and the definition of new programmes
- Definitions of modifications or replacement of structure including embodiment threshold
In addition to these activities all repairs and in-service problems that were monitored by the operator must be considered in the ESG analysis. Therefore a complete review of all repairs, in-service problems and of the Structural Repair Manual (SRM) is required.
Review of Essential Terms
Design Service Goal (DSG) is the period of time (in flight cycles/hours) established at design and/or certification during which the principal structure will be reasonably free from significant cracking including widespread fatigue damage.
Fatigue Critical Structure (FCS) is structure that is susceptible to fatigue cracking that could lead to a catastrophic failure of an aircraft. For the purposes of this AMC, FCS refers to the same class of structure that would need to be assessed for compliance with § 25.571(a) at Amendment 25-45, or later. The term FCS may refer to fatigue critical baseline structure, fatigue critical modified structure, or both.
Limit of validity (LOV) is the period of time, expressed in appropriate units (e.g. flight cycles) for which it has been shown that the established inspections and replacement times will be sufficient to allow safe operation and in particular to preclude development of widespread fatigue damage.
Multiple Element Damage (MED) is a source of widespread fatigue damage characterised by the simultaneous presence of fatigue cracks in similar adjacent structural elements.
Multiple Site Damage (MSD) is a source of widespread fatigue damage characterised by the simultaneous presence of fatigue cracks in the same structural element (i.e., fatigue cracks that may coalesce with or without other damage leading to a loss of required residual strength).
LOV Establishment
The establishment of the LOV is based on the fatigue test evidence held by the manufacturer. Sources of this information include:
1. Full-scale fatigue test.
2. Full-scale component tests.
3. Teardown and refurbishment of a high-time airplane.
4. Less than full-scale component tests.
5. Statistical fleet-proven life techniques.
6. Evaluation of in-service problems/test data experienced by this model or other airplanes with similar design concepts.
7. Analysis methods that have been parametrically developed to reflect fatigue test and service experience
LOV – Operators Responsibility
Incorporate mandatory service actions into their maintenance programs.
Adopt the LOV values provided by the manufacturer
MSD / MED Damage
Examples of airplane structure susceptible to multiple-site damage (MSD) and/or multiple-element damage (MED). This list is not meant to be inclusive of all structure that might be susceptible on any given airplane model, and it should only be used for general guidance. It should not be used to exclude any particular structure.
Longitudinal Skin Joints, Frames, and Tear Straps (MSD/MED)
Circumferential Joints and Stringers (MSD/MED)
Lap Joints with Milled, Chem-milled, or Bonded Radius (MSD)
Fuselage Frames (MED)
Stringer to Frame Attachments (MED)
Shear Clip End Fasteners on Shear Tied Fuselage Frames (MSD/MED)
Aft Pressure Dome Outer Ring and Dome Web Splices (MSD/MED)
Skin Splice at Aft Pressure Bulkhead (MSD)
Abrupt Changes in Web or Skin Thickness — Pressurized or Unpressurized Structure (MSD/MED)
Window Surround Structure (MSD/MED) Identifying WFD-susceptible structure
Wing or Empennage Chordwise Splices (MSD/MED)
Rib-to-Skin Attachments (MSD/MED)
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