MIL-HDBK-1530B(USAF)
APPENDIX B
APPENDIX B
ADDITIONAL GUIDANCE FOR AGING AIR VEHICLES
B.1 Aging air vehicles. For a new air vehicle, the development of the Force Structural Maintenance Plan (FSMP) in Task IV provides the basis for the maintenance costs expected to be incurred for the air vehicle during its design service goal. When the FSMP needs to be changed because the air vehicle: 1.) has overflown its design service goal, 2.) is corroded, 3.) has reached the time of onset of widespread fatigue damage (WFD), or 4.) has been repaired; then the air vehicle is said to be aging.
B.1.1 Operations beyond the design service goal of the air vehicle. If the air vehicle has flown or is
expected to be flown beyond its design service lifetime, the service inspection program should be updated to include necessary additional structural locations and/or inspection intervals to ensure structural integrity. This is accomplished through a damage tolerance assessment to search for new structural areas which may need to be inspected or modified. This would include a review of inspection results from operational air vehicles and a review of findings from previous durability testing. If an air vehicle can be removed from operational service, then a teardown inspection should be performed to determine if there are any fatigue cracking or corrosion problems not predicted earlier through design and test.
B.1.2 Corrosion. Inspections of individual air vehicles should be accomplished to ascertain the condition of the airframes with respect to corrosion. Emphasis should be placed on corrosion detection through nondestructive inspections and prevention. For those areas found to be corroded, the preferred approach is to eliminate the corrosion by removing it or replacing the structural elements in question. This may not be feasible in rare cases because of near-term operational requirements. In these cases, an assessment should be accomplished to determine the change in the inspection program that will account for the influence of corrosion on structural integrity.
B.1.3 Widespread fatigue damage. An initial prediction of the time of onset of WFD should be made based on the results from the durability test, air vehicle inspections, and usage tracking. Before the predicted time of the onset of WFD, a teardown inspection of a high-time air vehicle needs to be accomplished to validate the crack distribution function. This refined distribution should be used to recalculate the time of onset of WFD. Before operational air vehicles reach this time, a detailed nondestructive inspection program is needed to validate the prediction. When the air vehicle has reached the time of onset of WFD, then it should be modified to remove the problem since routine inspections are inadequate to protect flight safety.
B.1.4 Repairs. Both metallic and composite repairs should be designed to be damage-tolerant in addition to satisfying the strength and aeroelastic requirements. Further, they should be added to the FSMP so that they can be properly tracked to determine the inspection times. Additionally, a database needs to be established to maintain configuration control of air vehicle repairs. Any interaction of repairs should be taken into consideration in the damage tolerance assessment of them.
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