The importance of predicting the durability and damage tolerance capability of aircraft structures has underscored the need to derive a realistic flight-by-flight stress spectrum. This stress spectrum is generated from an analysis of the projected usage (mission profiles), combined with load factor exceedance curves that are obtained either from MIL-A-8866B or recorded data. The mission profiles are defined by the using command (the organization which flies the aircraft) and contain such information as mission length, number of times flown per aircraft lifetime, fuel load, weapons configuration, and a breakdown of the mission by segment, for example, takeoff, climb, cruise, descent, etc.

This paper describes two different approaches for obtaining load factor exceedance curves: (1) MIL-A-8866B and (2) data recorded from a different aircraft flying a similar mission. In the first example, the spectrum is derived for a ground-attack fighter that is in the design phase (that is, has never flown) and in the second case, the spectrum is derived for an aircraft that has already been built but is being used in a new mission (flight demonstration squadron). In this second case, a large amount of recorded load factor exceedance data exists from a different aircraft flying the same mission. Using either exceedance curve approach, these data are combined with the mission profile data to develop a stress-exceedance curve for each mission. The stress-exceedance information then is input to a computer program, along with some additional data such as the number of times a given mission is flown. The computer program first generates a random mission ordering and then randomly orders the stresses within each mission. The final result is a random flight-by-flight stress spectra suitable for use in performing damage tolerance and durability analyses and tests.