Structural Life Management in a Combat Aircraft

The paper briefly describes some of the underlying concepts, issues and strategies for ensuring efficient management of structural integrity of an airframe throughout the useful life of an aircraft—especially that of a military combat aircraft. Much of the discussion in the paper is based on the experience of the authors during the design and development phase of the Indian light combat aircraft, namely, LCA Mk-I Tejas, and the information available about the LCA in the open literature or public domain. Even though not an ageing aircraft, LCA is taken as an example to facilitate bringing out various issues and strategies to resolve them, particularly with relevance to the Indian aeronautical scenario. The use of both metallic and composite materials in modern combat aircraft structures necessitates different design strategies to be adopted for their structural integrity throughout their lifetime. In this respect too, the LCA serves as a good example. Within the limited scope of the paper, the description and discussions are primarily limited to some of the important aspects, such as the fatigue degradation in metallic materials and impact damage in composites—which do form the bulk of the effort of life management of airframe structures. Other issues such as corrosion in metals and environmental degradation in composites are also briefly mentioned. A brief description of the aircraft and its structure is given initially. Major steps in designing for structural life management, such as, estimation of static loads, derivation of fatigue spectra for the intended usage, degradation through fatigue or impact damage, lifing philosophies, estimation of life and monitoring of health and usage of individual aircraft are described. A brief mention is also made about structural health monitoring (SHM) as a future direction in life management. It is expected that the exposition in the paper will help understand issues and strategies that would eventually be required as a combat aircraft gets aged in service.