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AFGROW | DTD Handbook

Handbook for Damage Tolerant Design

  • DTDHandbook
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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 8.1.0. Force Structural Management

Tasks IV and V of the ASIP guidelines of MIL-HDBK-1530 define the force management tasks for preserving the airworthiness of an aircraft throughout its design life.  According to Berens, et al. [1981], force management is the "specification and direction of inspections, preventive maintenance, repairs, modifications, and damage assessments required to economically prevent structural failure and preserve the strength and rigidity of the individual airframe during its useful life."  The basic objective of ASIP is to ensure operational safety and readiness of the aircraft. Force Management objectives are to:

1)      Prevent structural failures through an effective maintenance program of inspections, repairs and modifications.

2)      Preserve structural strength and rigidity through an effective preventive maintenance program of environmental protection and economic repair or replacement of deteriorating parts.

3)      Minimize structural maintenance costs by eliminating unnecessary structural maintenance actions through effective application of data on test and operational failure modes and data on individual aircraft usage.

4)      Provide a basis for planning of system phase-out and future force structure.

The guidelines of ASIP Task IV are directed at the manufacturer generated, force management data package that provides the design usage FSMP and the mechanism for collecting and analyzing data for updating the FSMP as required.  Task V is directed at the implementation of the force management activities by the Air Force.  Figures 8.1.1 and 8.1.2 from MIL-HDBK-1530 are functional flow diagrams of Tasks IV and V, respectively.

Under Task IV of ASIP, the airframe contractor devises a Force Management Plan that contains three essential parts: 1) the Force Structural Maintenance Plan (FSMP), 2) the Loads/Environment Spectra Survey (L/ESS), and 3) the Individual Aircraft Tracking (IAT) Program.

The initial FSMP presents the schedule for inspections and maintenance actions for aircraft that are accumulating damage according to the design loads spectra usage predictions.  It is updated when the baseline operational load spectra are developed.

The L/ESS is a data collection and analysis program designed to provide the data to develop the baseline operational load spectra.  A number of the force aircraft, usually about twenty percent, are fitted with data measuring and recording equipment.  Parameters such as accelerations, angular rates, airspeed, altitude, weight and other load indicative quantities are obtained in a time history form as the aircraft are flown.  The data are categorized by mission type and segment, and load histories are calculated for the critical areas of the aircraft.  These are the same areas which were identified in the critical parts list and which will be subjected to subsequent inspection and possible repair or modification during maintenance actions.  The new baseline operational damage accumulation rates based on the L/ESS data are used to update the FSMP.

The IAT program is also a data collection and analysis effort that is applied to each aircraft of the force.  The minimum amount of data is collected that will allow the estimation of the damage being accumulated.  Comparison with the baseline damage accumulation predictions allows modification of the FSMP to account for the differences in usage of each aircraft.

The planning for these three parts of the FSMP should begin with the initial design studies and the fracture control plan.  Crack growth techniques used during the design are also those used in the IAT and FSMP portions of the program and should be formulated to permit easy incorporation. Studies made for evaluation of the effect of different load parameters on the loads computation and subsequently on crack growth calculations should be used in development of the parameter list for the L/ESS program. Accuracy requirements and parameter ranges should be selected to be commensurate with the methods of analysis.

Figure 8.1.1.  Functional Flow Diagram of ASIP Task IV from MIL-HDBK-1530


Figure 8.1.2.  Functional Flow Diagram of ASIP Task V from MIL-HDBK-1530

The following subsections present general descriptive comments for the three major elements of force management.  See Berens, et al. [1981] for more complete descriptions and discussions of these topics.