Aircraft Structural Maintenance Recommendations Based on Fracture Mechanics Analysis

The paper discusses the implementation of the Force Management (FM) phase of the Aircraft Structural Integrity Program (ASIP) for a USAF utility transport/trainer aircraft. The analyses, performed in support of the Force Management tasks, use the principles of linear elastic fracture mechanics with emphasis on subcritical flaw growth. The durability and damage tolerance lives, generated with a crack growth computer program, are the basis for structural maintenance recommendations and inspection intervals presented to the USAF in the Force Structural Maintenance Plan (FSMP). The case study considered herein shows how the T-39 wing, designed to the fatigue requirements of the late 1950s, performs to the structural criteria of the 1980s as defined by the military specifications MIL-STD-1530 and MIL-A-83444. The initial FM task was the durability and damage tolerance assessment (DADTA) of the structure based on the current service usage as defined by the loads/environment spectrum survey (L/ESS). An outline of the necessary spectrum development is provided, as is the crack growth analyses leading to estimates of the economic life and structural life enhancement recommendations. The DADTA was followed by the institution of the individual aircraft tracking (IAT) program, the purpose of which is to compute the rate at which the available structural life of each aircraft is being used and to establish inspection intervals to ensure safety. Tracking is accomplished by compiling flight records, collected by means of pilot logs containing mission information for each flight. The paper will show the economic crack growth procedure used, both to ascertain the accumulated damage based on the flight records and to estimate the remaining structural life. The conclusion of the paper will discuss the maintenance recommendations and the advantages of using fracture mechanics based aircraft tracking in support of force management.