19.
Aircraft which operate at high altitudes can become contaminated with radio-active particles held in the atmosphere, this radio-activity is retained in the dirt and carbon deposits in the engine.
20.
If contamination is suspected the radio-activity level of the engine must be determined to ensure the limitations agreed by the health authorities are not exceeded, Evidence of contamination will entail additional cleaning in a designated region, separate from the overhaul area, to safeguard the health of personnel in the workshop. Arrangements have to made with the health authorities for disposal of the waste radio-active cleaning material.
Inspection
21. After cleaning, and prior to inspection, the surfaces of some parts, e.g. turbine discs, are etched. This process removes a small amount of material from the surface of the part, leaving an even
Overhaul
matt finish which reveals surface defects that cannot normally be seen with the naked eye. The metal removal is normally achieved either by an electrolyt-ic process in which the part forms the anode, or by immersing the part for a short time in a special acid bath. Both methods must be carefully controlled to avoid the removal of too much material.
22.
After the components have been cleaned they are visually and, when necessary, dimensionally inspected to establish general condition and then subjected to crack inspection. This may include binocular and magnetic or penetrant inspection techniques, used either alone or consecutively, depending on the components being inspected and the degree of inspection considered necessary.
23.
The non-dimensional inspections can be divided into visual examination for general condition and inspection for cracks. The visual examination depends on the inspector's judgement, based on experience and backed by guidance from the manu-facturer. Although the visual examination of many parts of the engine conform to normal engineering practice, for some parts the acceptance standards are specialized, for example, the combustion chambers, which are subjected to very high temper-atures and high speed airflows in service.
24.
Dimensional inspection consists of measuring specific components to ensure that they are within the limits and tolerances laid down and known as 'Fits and Clearances'. Some of the components are measured at each overhaul, because only a small amount of wear or distortion is permissible or to enable the working clearances with mating components to be calculated. Other components are measured only when the condition found during visual inspection requires dimensional verification. The tolerances laid down for overhaul, supported by service experience, are often wider than those used during original manufacture.
25.
The detection of cracks that are not normally visible to the naked eye is most important, particular-ly on major rotating parts such as turbine discs, since failure to detect them could result in crack propagation during further service and eventually lead to component failure. Various methods of accentuating these are used for inspection, the two principal techniques being penetrant inspection for nonmagnetic materials and electro-magnetic inspection for those parts that can be magnetized.
26.
Two forms of penetrant inspection in common use are known as the dye penetrant and the
Overhaul
fluorescent test. With the dye test, a penetrating coloured dye is induced to enter any cracks or pores in the surface of the part. The surface is then washed and a developer fluid containing white absorbents is applied. Dye remaining in cracks or other surface defects is drawn to the surface of the developer by capillary action and the resultant stains indicate their locations.
27.
Fluorescent testing is based on the principle that when ultra-violet radiation falls on a chemical compound, known as fluorescent ink, it is absorbed and its energy re-emitted as visible light. If a suitable ink is allowed to penetrate surface cavities, the places where it is trapped will be revealed under the rays of an ultra-violet lamp by brilliant light emissions.
28.
Magnetic crack testing (fig. 25-3) can only be applied to components which can be magnetized. The part is first magnetized and then sprayed with, or immersed in, a low viscosity fluid which contains ferrous particles and is known as 'ink'. The two walls of a crack in the magnetized part form magnetic poles and the magnetic field between these poles attracts the particles in the ink, so indicating the crack (fig. 25-4). In some instances, the ink may contain fluorescent particles which enable their build-up to be viewed under an ultra-violet lamp, A part that has been magnetically crack tested must be de-magnetized after inspection.
29.
Chromic acid anodizing may be used as a means of crack detection on aluminium parts, e.g. compressor blades. This process, in addition to providing an oxide film that protects against corrosion, gives a surface that reveals even the smallest flaws.
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