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DTD Handbook

Handbook for Damage Tolerant Design

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    • Sections
      • 1. Introduction
      • 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
        • 0. Force Management and Sustainment Engineering
        • 1. Force Structural Management
          • 0. Force Structural Management
          • 1. Force Structural Management Plan (FSMP)
          • 2. Loads/Environment Spectra Survey (L/ESS)
          • 3. Individual Aircraft Tracking (IAT)
        • 2. Sustainment Engineering
        • 3. References
      • 9. Structural Repairs
      • 10. Guidelines for Damage Tolerance Design and Fracture Control Planning
      • 11. Summary of Stress Intensity Factor Information
    • Examples

Section 8.1.3. Individual Aircraft Tracking (IAT)

The Individual Aircraft Tracking (IAT) plan is an integral element of MIL-HDBK-1530.  The plan is constructed by the airframe contractor as part of the Task IV, Force Management Data package.  The plan is implemented by the Air Force under Task V, Force Management.  The objective of IAT is to provide data on each aircraft that reflects differences in usage from that of the baseline spectra of the FSMP.

The basic concept of the IAT plan is as follows.  The FSMP specifies the timing of required structural inspections and modifications and estimates the costs for repairs and inspections.  These times and quantities are based on the FSMP crack growth curves as calculated from the relevant baseline (average) spectrum.  Since the baseline stress histories that were used to generate these crack growth curves are not necessarily representative of the actual experience of individual aircraft, a method is needed to account for the individual differences.  This is done in the IAT Program by collecting, processing and accumulating data descriptive of every flight of each airframe in a fleet.  There is considerable variability in the degree of complexity of the necessary data systems required for different fleets of aircraft.

From the parameters measured in the IAT program, a crack growth increment per flight or per flight hour is computed and accumulated for each aircraft in the fleet.  Comparing the observed crack growth plot with the predicted plot provides a determination of equivalent flight units for the current usage level.  Figure 8.1.6 from Berens, et al. [1981] shows this comparison.  The baseline usage life remaining until damage size af is reached is (t*-t).  The life t* defined a specified maintenance action time.  At any specific time, the total fleet can be viewed as having a distribution of remaining life as expressed in terms of the baseline flight hours.  Such information is then used for scheduling the maintenance activity.

 

Figure 8.1.6.   Relating Individual Aircraft Usage to FSM Plan Usage [Berens, et al., 1981]

Establishing the IAT plan involves the following steps:

1)      the selection of the aircraft flight condition descriptions or parameters,

2)       the development of a method to translate these parameters into incremental crack growth,

3)      the translation of this crack growth into a measure of time which can be projected to a future date for the scheduling of some maintenance activity, and

4)      the definition of a data processing system for maintaining and updating all of the analyses and record keeping.

There are many approaches to IAT as driven by the use and structural complexity of the fleet.  Generally, in the past bomber/transport type aircraft have been tracked using crew reporting forms while attack/fighter/trainer aircraft have used load and flight parameters to reflect the more variable usage.  See Clay, et al. [1978] for a description of the crack growth tracking methods developed during the 1970’s.  Many of these methods are still in use but the modern micro-processor based data recording systems are permitting the use of more sophisticated methods.  See Selder & Liu [1997] for an example that calculates damage based on cycle-by-cycle crack growth analysis at each control point.  These processors are also blurring the distinction between data collection for L/ESS and IAT.