Cracks in all areas of the blade, including radial
cracks in the tips. Cracks generally start at the
FOD/DOD, including nicks and dents.
Aluminum spattering that appears as metallic
deposits on the blade. This results from
compressor tip rubs.
HP turbine blade tip rubs. This results in coating
removal and tip damage.
TURBINE MIDFRAME DAMAGE. T h e
following paragraphs describe damage that you may
find when inspecting the turbine midframe.
Discoloration. On low-time liners, the coloration
is random and sometimes appears as a wavy surface.
T h e c o l o r a t i o n i s r a n d o m b o t h a x i a l l y a n d
circumferentially. On high-time liners you may observe
some axial carbon streaking. There are no service limits
Liner Cracking. Initial deterioration of the
turbine midframe liners occurs at the forward inner liner
flange in the form of axial cracking. It is difficult to
determine the magnitude or length of a crack in this area.
The area is immediately aft of the HP turbine stage 2
blade platforms. Small tight cracks will probably not be
noticed. Of primary interest is that there are no cracks
with visible turned up edges. If cracking is observed in
the forward inner liner flange, you can use a fiberscope
for a closer look to establish the extent of the crack and
the adjacent area condition. Cracking can also occur
around the leading edge weld beads on the strut fairings
at both the inner and outer liner areas.
Liner Distortion. Turbine midframe distortion
most commonly occurs in the 10 to 12 oclock area of
the outer liner forward flange. The only relative gauge
a v a i l a b l e
f o r
c o m p a r a t i v e
a s s e s s m e n t
(roundness/contour) is the HP turbine stage 2 blade tip
arc and the stage 2 shroud contour. A fiberscope is
recommended for the final assessment of any suggested
distortion of the liner. You will need a guide tube to
position the fiberscope.
The most common problem in the power turbine
section is usually a loss of the hard coat on the tip shroud.
Notch wear and subsequent blade bending are direct
results of tip shroud hard coat loss. Notch wear and
blade bending will ultimately lead to fatigue failure of
the airfoil. The actual loss of the hard coat cannot be
confirmed through the borescope. It can be confirmed
by removal of the upper case and actual physical
inspection of the tip shrouds. You can seethe symptoms
through the borescope by looking at the notch with
probe No. 1. Uneven notch wear may indicate loss of
the hard coat. You should carefully inspect for any
transverse cracks in the blade airfoil around the 10
percent span. Any cracking is cause for replacement of
the power turbine.
The power turbine first-stage blades also have a
history of deposit buildup that leads to rotor unbalance
and excessive vibration.
For reference to parts
nomenclature used in this section, refer to figure 2-11,
sections F and G. Power turbine damage that you may
find is described in the following paragraphs.
CRACKS IN BLADES. Inspect the total airfoil,
platform, and tip shrouds for evidence of cracks. If you
suspect a specific area, use the high-magnification
probe. You will see a limited amount of the stage 1
blading when viewing aft from the turbine midframe
liner inspection ports. You can see more detail with a
fiberscope or by viewing forward from the turbine
exhaust duct. Cracks will show depth and under
magnification will show edge material definition. Be
sure to distinguish cracks from false indications such as
smears and carbon streaks.
NICKS AND DENTS. Record these defects in
relation to the percent span and percent chord for
magnitude and location on the blade. Record also the
condition of the blade material adjacent (at the
extremities of the defect) to the observed defect. Record
any cracking or sharpness of nicks or dents. Investigate
smooth impact deformities to determine the origin of
WEAR. Inspect LP turbine rotor blade tip shroud
interlocks or circumferential mating surface for wear at
stage 1. Wear is observable and will appear as shown
in figure 2-31.
D I R T , C O L O R A T I O N , P I T T I N G , A N D
assemblies may show airfoil surface irregularities that
could be dirt accumulation, carbon buildup, surface
pitting from particles in the gas stream, or corrosion of
the blade material. Dirt and coloration are of little
concern; however pitting and corrosion may be