Airfoil Surface Defects.- Surface defects are the result of object damage or adjacent blade interference (tip clang). Impacts in the center section of the airfoil are not common. Tip clang damage is the result of a blade leading edge tip contacting the adjacent blade tip at approximately one-third of the chord length forward of the trailing edge on the low-pressure (convex) side of the blade (fig. 2-13). This is the result of compressor stall and is observed in stages 3 through 6. You should report any observed defect on the airfoil surfaces in the inspection record. Your report should contain

Figure 2-13.- Compressor rotor blade tip clang damage.

information relative to the stage, location on the blade (estimate the percent of chord and span), and the condition of the surrounding airfoil. You do not have to record the appearance of the defect (sharpness and contour). Compressor stall is one of the worst things that can happen to an engine. tip clang damage is difficult to spot and gives the appearance of minor damage. The V-shaped notch on the top of a blade caused by tip clang is only an indicator; it in itself is not the damage. The damage is at the blade root and normally cannot be seen. If a blade has been overstressed, it must be replaced.

Platform Distortion.- Compressor blade platform fretting or shingling (fig. 2-14) can be observed on some after stage blades. These distortions are the overlapping of one blade platform mating edge with the adjacent platform edge. When shingling is found, the platforms will be distorted and bowed (fig. 2-15). When the platforms are shingled, only the locking lug blades will exhibit this defect. Monitor this condition to see if a platform crack develops. Also look for missing pieces around the locking lugs. You must report and record any cracks in the platform. Be sure you have included the following information:

l The stage Q The number of blades

Figure 2-14.- Platform fretting or shingling.

Figure 2-15.- Platform bowing. The spacing of the blade numbers separating the shingled blade platforms he platform gap observation (estimate gap as percent of circumferential span of the platform) he condition of the shingled edge (bent, fretted, or stepped as per table 2-1)

Midspan Shroud Wear.- Some stage 1 compressor blades show wear at the mating surfaces of the midspan damper shrouds. (See figs. 2-16 and 2-17.) Wearing of the tungsten-carbide wear coat causes the mating face contour to change from a straight line to a stepped line. This occurs at the after edge of the clockwise blade midspan (trailing edge) and the forward edge of the counterclockwise blade midspan shroud (leading edge). In the step area, some metal maybe turned or protruding from the midspan shroud mating line (mushrooming). This protrusion is indicative of wear-through. A missing pad on one face would initiate an accelerated failure of the mating surfaces.

BLADE DEPOSITS.- Compressor blades and stator vanes exhibit varying degrees of cleanliness. Variables such as air-inlet configuration, ambient atmospheric conditions, and air contaminants (chemicals, salt, dirt, water, and so forth) all tend to affect the surface condition of the compressor rotor and stator blades.

Aluminum Deposits.- Two areas in the compressor assembly are coated with aluminum, the

Figure 2-16.- Compressor blade midspan shroud wear.

Figure 2-17.- Compressor blade midspan damper carboloy pad.

shrouding over the blade tips and the rotor drum area under the stator vanes. tip rubs of either the blades or the vanes will rub off the aluminum coating. As time is accrued on the compressor assembly, the after stages of the rotor release or flake the aluminum coating. This deterioration is a normal progression. Flaking occurs because of the differences in thermal expansion of dissimilar metals and the differences in the size and configuration of the various parts. The released aluminum flakes enter the airstream, impact the rotor blades or vanes, and splatter the airfoils. Aluminum splatter observed forward of stage 11 can be caused by object damage or aluminum flakes that are rubbed out of the compressor case coating. This condition requires a thorough inspection of the forward compressor stages.