CAN/CGSB-48.5-95 Part III: Chap. 11

11.1 Introduction The radiography of aircraft structures requires that the radiographer have general knowledge of the structure of an aircraft, the principles of flight, the terms used to describe components and their function, and the radiographic procedures and techniques unique to aircraft construction and maintenance. This chapter will attempt to provide this general knowledge without going into detail on the myriad differences between present and past aircraft, civilian and military aircraft or problems associated with aerospace programs. The radiographer must be prepared to initiate new radiographic procedures or modify present ones in order to provide proper quality control for all types of aircraft with which he/she may be associated. Before reading further, it is recommended that you the reader become familiar with the terms in the glossary. See par. 11.12. Radiography of critical aircraft structural members and component parts has proven to be an effective and economical inspection method when properly applied in an aircraft maintenance program. In the interest of public safety, routine radiography of critical aircraft structural members is carried out at most busy airports. The primary advantage of radiography is that it facilitates the inspection of inaccessible structures or closed assemblies without the need for time consuming, and therefore expensive, disassembly. As well, dissassembly of structural members can be detrimental to the continued airworthiness through enlargement of fastener holes and scratches or other induced damage to critical areas. Radiography has the potential to inspect such critical, hard-to-reach areas without added risks. The application of radiographic inspection to aircraft structures must be approached with care; the part to be inspected may not be readily accessible for ideal placement of the film. The radiographic beam may be obstructed by other structural members and components in its vicinity. The best focal and part-to-film distances may be impossible to attain. The presence of sealants, trapped fuel, wiring harness and similar items will interfere with the radiographic work and hence quality of the radiograph. The effect of these and other obstacles must be carefully evaluated during the development of a radiographic procedure. Since the defects are contained in the hard-to-reach structural members or components, it may become very difficult to place the penetramenter at the appropriate location, hence the best radiographic sensitivity may be hard to achieve. Often the best available guide to sensitivity is the structure itself. If, for example, it is possible to differentiate between components of different thicknesses, this may be used effectively as a measure of sensitivity. It is important to remember that radiographs of airframes, which normally possess high contrast are, for this reason, very deceptive when evaluating the sensitivity obtained. It is easy to be optimistic in considering what can be successfully accomplished with radiography. Fatigue cracks in large fittings, for example, should never be investigated by means of radiography as they are normally too small to be located by this method. While determining a radiographic technique for the inspection of a definite aircraft-structure-failure area, a surface indication such as a crack, when confirmed visually, becomes the most efficient penetrameter. Knowledge thus gained may be used as future reference for similar work.