relative to separation of gill hole rows and
radially by gill or cooling holes.
Craze cracking.
These cracks are superficial
surface cracks, caused by high temperature.
They are random lines that are very thin in
appearance with tight lines (no depth or width to
the cracks).
There is no limit against this
condition.
Nicks, scores, scratches, or dents. These defects
are allowed by the service limit and may be
present on any area of the nozzle vanes.
Cracks in the airfoil fillet at the platform. There
is no limit restricting these cracks, except at the
leading edge area.
Metal splatter. Aluminum and combustor liner
metal, when liberated by the compressor or
combustor, frequently splatter the surface areas
of the stage 1 HP turbine nozzle vanes. There is
no limit for these deposits; however, abnormal
amounts of this splatter is reason to inspect the
compressor.
Platforms. Cracking in the HP turbine nozzle
stage 1 platforms is difficult to see from the combustor
borescope ports. When this area is viewed through port
No. 12, extreme magnification is afforded even with
probe No. 2. This is due to the closeness of the surface
to the distal end of the probe. Record the origin and end
of the cracking and assess the magnitude using trailing
edge slots and gill hole rows for radial and axial
dimensions.
Nicks, scores, scratches, and dents on platform
surfaces 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 the
defect using the geometry of the trailing edge, gill hole
rows, and gill hole separation for comparative
dimensions.
You must record burns on vane platform areas and
use probe No. 1 to assess the conditions. If a
burn-through occurs, the inner and outer surface edge of
the platform should be seen. This difficult assessment
can be done with the aid of a fiberscope. Any
incomplete or doubtful evaluation should be the subject
of a followup check after a specified amount of
operating time.
HP TURBINE BLADE DAMAGE. W h e n
inspecting the HP turbine blades, you should use probe
No. 2 with the 150-watt light source. The following
paragraphs describe some of the damage you may find.
Cracks in the Leading Edge. The leading edge
of the stage 1 turbine rotor blades is the area forward of
the gill holes. Cracks in the leading edge can be caused
by DOD impact (combustion liner pieces) or thermal
stress. An indication on the leading edge open enough
to show depth is defined as a crack. Some conditions
may mislead you in the determination of the presence of
cracks. Dirt and debris buildup inlayers on the leading
edge, as shown in figure 2-26, are not cracks. When this
buildup begins to flake off, the edge of the area where
the flake came off causes visible lines. These lines are
irregular and appear to be cracks. The other common
point of cofusion on leading edge cracks is on the
convex side of the leading edge tip area. This area is
subject to scratching by the small pieces of combustor
metal that pass through the HP turbine.
Cracks in the Trailing Edge. The trailing edge is
the flat surface with cooling holes that forms the after
edge of the blade airfoil. Trailing edge cracks are
difficult to see, but if a crack is suspected, use probe No.
1 for increased magnification. Record the location
relative to a cooling hole and the magnitude of the crack.
Record any plugged trailing edge cooling holes.
Cracks in Concave and Convex Surfaces. The
airfoil surfaces are the areas aft of the gill holes back to
the trailing edge. The tip area is further restricted to that
area above the tip cap. When you evaluate the airfoil
serviceability, do not consider the tip as a part of that
area. Cracks in the airfoil surfaces are very tight, but
can readily be seen with probe No. 2. Airfoil surface
cracks are irregular in edge appearance and are not
usually confined with streaks, which are usually straight
in appearance. Record the area by the percent of span
or gill hole spacing equivalent for location and
magnitude of the cracking. For axial position, use an
estimate of percent chord and the position relative to the
tip cooling film cooling holes.
Cooling Hole Blockage. The HP turbine rotor
stage 1 blades are film cooled by air that flows out of
the cooling holes. Report plugged holes relative to the
number of blades affected and the position and number
of plugged holes. Ensure the correct callout of the holes
(such as the nose cooling, convex gill, tip film cooling
holes, and so forth.)
Distortion. Heavy impact damage to the leading
edge of the blade usually results in distortion. When the
impact is severe enough, cracking and/or tearing of the
leading edge, adjacent to the impact area, occurs.
2-21