The sum of the load levels that a structure is expected to experience is determined by a projection of the amount of usage expected over the life in the various possible missions; e.g., hours in training, air-to-air combat, reconnaissance, weapons delivery, etc.  The mission mix includes the relative amounts of time spent in each mission.  The most basic information needed is the load factor exceedances at the center of gravity (CG) of the aircraft.  This information is illustrated in Figure 2.4.5.  For new designs, this data is derived from actual measured exceedances from operational aircraft flying similar missions.  The USAF specifications contain such data.  The Air Force Guidelines Handbook for developing Load/Environmental Design Spectra [Giessler, et al., 1981] summarizes the techniques that are currently being utilized to develop the loading and environmental spectra based on these data for various types of structures.

Figure 2.4.5.  Typical Load Factor Exceedance Information Indicating Usage

The specific sequence of loads applied to the structure is necessary to the crack growth damage accumulation analysis.  Current practice is to simulate the overall life on a flight-by-flight basis.  Each flight in the design, analysis, or test load spectrum consists of a series of cycles that combine the deterministic and probabilistic events describing the type of mission.  The deterministic events include takeoff and landing, and certain basic maneuver loads during each flight.  Probabilistic events such as gusts or rough field taxiing occur periodically.  Although it is possible to estimate the number of times these events occur, their position in the load sequences is determined in a probabilistic manner.

In developing the load spectrum for crack growth damage analysis, it is necessary to determine the stress history for each critical area on the airframe.  This is accomplished by determining the relationship between the load history derived above and the stress response.  Figure 2.4.6 schematically illustrates the load factor to stress history transformation.

Figure 2.4.6.  Load Factor to Stress History Transformation

Differences in crack growth resulting from mission mix can be significant.  A fighter aircraft that is used primarily for air-combat or air-combat training typically accumulates more damage than one that is used for the same number of hours on a reconnaissance-type mission.