relative to separation of gill hole rows andradially by gill or cooling holes.Craze cracking.These cracks are superficialsurface cracks, caused by high temperature.They are random lines that are very thin inappearance with tight lines (no depth or width tothe cracks).There is no limit against thiscondition.Nicks, scores, scratches, or dents. These defectsare allowed by the service limit and may “bepresent on any area of the nozzle vanes.Cracks in the airfoil fillet at the platform. Thereis no limit restricting these cracks, except at theleading edge area.Metal splatter. Aluminum and combustor linermetal, when liberated by the compressor orcombustor, frequently splatter the surface areasof the stage 1 HP turbine nozzle vanes. There isno limit for these deposits; however, abnormalamounts of this splatter is reason to inspect thecompressor.Platforms.— Cracking in the HP turbine nozzlestage 1 platforms is difficult to see from the combustorborescope ports. When this area is viewed through portNo. 12, extreme magnification is afforded even withprobe No. 2. This is due to the closeness of the surfaceto the distal end of the probe. Record the origin and endof the cracking and assess the magnitude using trailingedge slots and gill hole rows for radial and axialdimensions.Nicks, scores, scratches, and dents on platformsurfaces are again masked from the combustor ports,except for the forward areas. Viewed via port No. 12,the area is magnified. Record the magnitude of thedefect using the geometry of the trailing edge, gill holerows, and gill hole separation for comparativedimensions.You must record burns on vane platform areas anduse probe No. 1 to assess the conditions. If aburn-through occurs, the inner and outer surface edge ofthe platform should be seen. This difficult assessmentcan be done with the aid of a fiberscope. Anyincomplete or doubtful evaluation should be the subjectof a followup check after a specified amount ofoperating time.HP TURBINE BLADE DAMAGE.— W h e ninspecting the HP turbine blades, you should use probeNo. 2 with the 150-watt light source. The followingparagraphs describe some of the damage you may find.Cracks in the Leading Edge.— The leading edgeof the stage 1 turbine rotor blades is the area forward ofthe gill holes. Cracks in the leading edge can be causedby DOD impact (combustion liner pieces) or thermalstress. An indication on the leading edge open enoughto show depth is defined as a crack. Some conditionsmay mislead you in the determination of the presence ofcracks. Dirt and debris buildup inlayers on the leadingedge, as shown in figure 2-26, are not cracks. When thisbuildup begins to flake off, the edge of the area wherethe flake came off causes visible lines. These lines areirregular and appear to be cracks. The other commonpoint of cofusion on leading edge cracks is on theconvex side of the leading edge tip area. This area issubject to “scratching” by the small pieces of combustormetal that pass through the HP turbine.Cracks in the Trailing Edge.— The trailing edge isthe flat surface with cooling holes that forms the afteredge of the blade airfoil. Trailing edge cracks aredifficult to see, but if a crack is suspected, use probe No.1 for increased magnification. Record the locationrelative to a cooling hole and the magnitude of the crack.Record any plugged trailing edge cooling holes.Cracks in Concave and Convex Surfaces.— Theairfoil surfaces are the areas aft of the gill holes back tothe trailing edge. The tip area is further restricted to thatarea above the tip cap. When you evaluate the airfoilserviceability, do not consider the tip as a part of thatarea. Cracks in the airfoil surfaces are very tight, butcan readily be seen with probe No. 2. Airfoil surfacecracks are irregular in edge appearance and are notusually confined with streaks, which are usually straightin appearance. Record the area by the percent of spanor gill hole spacing equivalent for location andmagnitude of the cracking. For axial position, use anestimate of percent chord and the position relative to thetip cooling film cooling holes.Cooling Hole Blockage.— The HP turbine rotorstage 1 blades are film cooled by air that flows out ofthe cooling holes. Report plugged holes relative to thenumber of blades affected and the position and numberof plugged holes. Ensure the correct callout of the holes(such as the nose cooling, convex gill, tip film coolingholes, and so forth.)Distortion. — Heavy impact damage to the leadingedge of the blade usually results in distortion. When theimpact is severe enough, cracking and/or tearing of theleading edge, adjacent to the impact area, occurs.2-21
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