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AFGROW | DTD Handbook

Handbook for Damage Tolerant Design

  • DTDHandbook
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    • Sections
      • 1. Introduction
        • 0. Introduction
        • 1. Historical Perspective on Structural Integrity in the USAF
        • 2. Overview of MIL-HDBK-1530 ASIP Guidance
        • 3. Summary of Damage Tolerance Design Guidelines
          • 0. Summary of Damage Tolerance Design Guidelines
          • 1. Summary of Guidelines
          • 2. Design Category
          • 3. Inspection Categories and Inspection Intervals
          • 4. Initial Damage Assumptions
          • 5. Residual Strength Guidelines
          • 6. Required Periods Of Safe Damage Growth
          • 7. Illustrative Example Of Guidelines
        • 4. Sustainment/Aging Aircraft
        • 5. References
      • 2. Fundamentals of Damage Tolerance
      • 3. Damage Size Characterizations
      • 4. Residual Strength
      • 5. Analysis Of Damage Growth
      • 6. Examples of Damage Tolerant Analyses
      • 7. Damage Tolerance Testing
      • 8. Force Management and Sustainment Engineering
      • 9. Structural Repairs
      • 10. Guidelines for Damage Tolerance Design and Fracture Control Planning
      • 11. Summary of Stress Intensity Factor Information
    • Examples

Section 1.3.3. Inspection Categories and Inspection Intervals

For each individual aircraft system, the Air Force is obligated to specify the planned major depot and base level inspection intervals to be used in the design of the aircraft.  Typically, these intervals will be approximately 1/4 of the design service life.  The types and extent of inspection (i.e., equipment, accessibility, necessity for part removal, etc.) required at each of these major inspections is dependent upon the specific aircraft design and modifications resulting from development and full-scale tests or service experience.  The Air Force wants its contractors to design a damage tolerant structure that will minimize the need for extensive non-destructive depot or base level inspections.  Primary emphasis should therefore be placed on obtaining designs for which significant damage sizes can readily be found by visual inspection.  However, where periodic inspections are required to satisfy the damage tolerance guidelines, the contractor must recognize that the USAF will probably conduct the inspections.  The in-service damage sizes associated with the inspection categories of JSSG-2006 paragraph 3.12 reflect the estimated capability of the Air Force to find damage.

Guidelines for degree of inspectability are contained in JSSG-2006 paragraph 6.1.15.  The degree of inspectability of safety of flight structure is established in accordance with the following definitions:

·        In-flight evident inspectable - If the nature and extent of damage occurring in flight will result directly in characteristics which make the flight crew immediately and unmistakably aware that significant damage has occurred and that the mission should not be continued.

·        Ground evident inspectable - If the nature and extent of damage will be readily and unmistakably obvious to ground personnel without specifically inspecting the structure for damage.

·        Walkaround inspectable - If the nature and extent of damage is unlikely to be overlooked by personnel conducting a visual inspection of the structure.  This inspection normally shall be a visual look at the exterior of the structure from ground level without removal of access panels or doors without special inspection aids.

·        Special visual inspectable - If the nature and extent of damage is unlikely to be overlooked by personnel conducting a detailed visual inspection of the aircraft for the purpose of finding damaged structure.  The procedures may include removal of access panels and doors, and may permit simple visual aids such as mirrors and magnifying glasses.  Removal of paint, sealant, etc. and use of NDI techniques such as penetrant, X-ray, etc., are not part of a special visual inspection.

·        Depot or base level inspectable - If the nature and extent of damage will be detected utilizing one or more selected nondestructive inspection procedures.  The inspection procedures may include NDI techniques such as penetrant, X-ray, ultrasonic, etc.  Accessibility considerations may include removal of those components designed for removal.

·        In-service non-inspectable structure - If either damage size or accessibility preclude detection during one or more of the above inspections.

The specified frequency of inspections for each of the inspectability levels is indicated in Table 1.3.1 and is based on estimates of typical inspection intervals.  As previously mentioned, the typical depot or base level frequency is once every one quarter of the design lifetime but may be otherwise specified in the appropriate contractual document.  Special visual inspection requires Air Force approval before being considered as a design constraint but, if approved, shall not be required more frequently than once per year.  The justification for this restriction is cost and maintenance schedule guidelines.

Table 1.3.1.  Summary of In-Service Inspections from JSSG-2006 Appendix Table X

Degree of Inspectability

Typical Inspection Interval

In-Flight evident inspectable

One flight*

Ground evident inspectable

One day (two flights)*

Walk-around inspectable

Ten flights*

Special visual inspectable

One year

Depot or base level inspection

¼ Design service lifetime

In-Service non-inspectable structure

One design service lifetime

                                * Most damaging mission


The design of some aircraft components for intermediate special visual inspections, typically once per year, may be advantageous from a performance or cost standpoint and may be used by the contractor in satisfying the guidelines.  Normally, special visual inspections will not be specified by the Air Force in the design and development stage but may be dictated, subsequent to design, by the results of testing or service experience.

The assumed Air Force depot or base level inspection capabilities depend on the type of inspection performed.  In special cases where potential benefits justify it, the contractor may recommend to the Air Force that specific components be removed from the aircraft and inspected during scheduled depot or base level inspections.  If approval is given, the recommendations may be incorporated during design.  In these cases, the assumed initial damage sizes subsequent to the inspection shall be the same as those in the original design providing the same inspection procedures are used and certified inspection personnel perform the inspection.

Conventional NDI procedures such as X-ray, penetrant, magnetic particle, ultrasonic, and eddy current are generally available for depot or base level inspections.  Such inspection procedures will be performed as dictated by the specific aircraft design inspection guidelines, or as modified because of subsequent tests and service experience.  In establishing the design inspection guidelines, the contractor should attempt to minimize the need for such NDI, and should not plan on nor design for general fastener pulling inspections.