shrouding over the blade tips and the rotor drum areaunder the stator vanes. tip rubs of either the blades orthe vanes will rub off the aluminum coating. As time isaccrued on the compressor assembly, the after stages ofthe rotor release or flake the aluminum coating. Thisdeterioration is a normal progression. Flaking occursbecause of the differences in thermal expansion ofdissimilar metals and the differences in the size andconfiguration of the various parts. The releasedaluminum flakes enter the airstream, impact the rotorblades or vanes, and splatter the airfoils. Aluminumsplatter observed forward of stage 11 can be caused byobject damage or aluminum flakes that are rubbed outof the compressor case coating. This condition requiresa thorough inspection of the forward compressor stages.Leading Edge Buildup.— Aluminum buildup onthe leading edges of blades is usually observed in stages11 through 16. The buildup changes the contour of theairfoil and can alter the stall margin. You should reportthe presence of leading edge buildup in the inspectionreport. This type of buildup may occur on low-timecompressors.The compressor blades tend to “selfclean” or lose this leading edge buildup as the assemblyaccrues time.Airfoil Powdering.— Compressor rotor blades mayhave aluminum particles visible on the airfoils invarying degrees (from stage to stage). This powder isindicative of a possible compressor stall or a hard bladetip rub.Combustion SectionInspect the combustor for eroded or burned areas,cracks, nicks, dents, hot streaks, flatness of liners causedby hot spots, blocked air passages, and carbon buildup.If damage is found in the combustion section, it usuallyconsists of a burn-through in the dome area adjacent toa fuel nozzle. The problem can usually be traced to aloss of film-cooling air caused by upstream debris or toa faulty fuel nozzle.Cracking is not normally aproblem, but you should photograph and report anysuspected or confirmed cracks. Carbon deposits aroundthe fuel nozzles occur on all engines and are notconsidered serious. These deposits build up only on theventuri and swirl cup rather than on the shroud ordischarge orifice. They do not usually interfere with thefuel spray pattern. If you find cracking, evaluate it toensure that no pieces will detach and cause anysecondary damage to the HP turbine. For reference toparts nomenclature used in the following section, referto figure 2-11, sections B and C.COMBUSTION SECTION DAMAGE.— In thefollowing paragraphs, we describe some of the damagethat you might find during a borescope inspection of thecombustion section. Because the dark surfaces in thecombustion section absorb light, you will need a1,000-watt light source for a proper inspection.Discoloration. — Normal aging of the combustorcomponents will show a wide range of color changes.This is not a cause for concern. As operating time isaccrued on the combustor assembly, an axial streakingpattern running aft of every other circumferential fuelnozzle will occur.On low-time assemblies, thecoloration is random and has little or no information toaid you during the inspection. As operating timeincreases on the assembly, you will observe significantdeterioration at the edges of the streaking patterns.Cracking will begin in the forward inner liner panels andwill propagate aft. The axial cracks tend to follow thelight streaks. Panel overhang cracking and liberationusually occur at the edge of the streaks.Riveted Joints.— The dome band and the inner andouter liner assemblies are joined by rivets as shown infigure 2-18. The presence and condition of the rivetheads and rivet holes are easily assessed because of theirposition in relationship to the borescope ports. Recordany missing rivets and torn or cracked hole edges.Dome Assembly. — Distortion of the trumpetsand/or swirl cups is random and occurs on high-timeassemblies. Record the distortion (in percent) of theedge and/or span of the trumpet and the percent ofcircumference versus diameter of the swirlers.Figure 2-18.—Combustion liner dome rivet joint.2-15
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