(24 in.) of the object and positioned at an angle not
less than 30° to the inspection surface, as shown in
Figure 1 [1].
Making a direct visual examination to deter-
mine the condition of internal components in a gas
turbine is physically impossible without significant
amounts of downtime and disassembly. As seen
in Figure 2, a technician can readily inspect the
internal components of a large frame gas turbine.
When knowing their internal condition is required,
this is where RVI becomes indispensable. It is
interesting to note that while RVI is a subdiscipline
of VT in SNT-TC-1A and ISO 9712:2021(en) visual
testing (methods), both direct unaided visual tests
and visual tests conducted during the applica-
tion of another NDT technique are excluded. This
accentuates the importance and value of qualified
and certified NDT personnel who are specifically
using RVI.
RVI enables the visual inspection of otherwise
inaccessible areas or surfaces. The earliest examples
were endoscopes that began to be used for medical
purposes in the early 18th century. With the advent
of cannons, artillery operators would lower a candle
on the end of a stick into a cannon bore to deter-
mine its condition prior to use. You might see why
this could be problematic for the inspector! People
soon realized they could only see in straight lines,
but if mirrors or fiberoptics were used, the light and
image could “go around” corners. From this discov-
ery, the borescope and borescope technology have
evolved.
Dr. George S. Crampton developed the first
industrial borescope, which was used by the
Westinghouse Co. for examining internal turbine
components. Inspecting internal surfaces of a
turbine rotor were some of the first RVI applica-
tions on industrial turbines. While Crampton was
a mechanical “MacGyver” of sorts, he used optical
instruments in his medical practice as an ophthal-
mologist and tinkered with optical instruments in
his spare time. His work led to the founding of the
Lenox Instrument Co. [2].
Today, typical RVI applications with borescopes
are inspecting internal components on aviation and
FEATURE
|
REMOTEVT
Figure 2. Visual
inspection of
a GE Vernova
large frame
gas turbine’s
fuel lines.
60º to 90º =good viewing angle
30º to 60º =noncritical
angle of view
0º to 30º =poor
viewing angle
600 mm
(24 in.)
Test
surface
Figure 1. Direct visual testing viewing angle and distance.
42
M A T E R I A L S E V A L U A T I O N • J U L Y 2 0 2 4
CREDIT:
GE
VERNOVA

industrial turbines, power cylinders, pipes, tubing,
boilers, and heat exchangers, within numerous
industrial applications.
Aerospace and power-generation gas turbine
operators benefit from RVI procedures, commonly
referred to as “borescope inspections.” In fact,
gas turbines in aerospace and industrial applica-
tions are among the largest industry segments that
use borescopes. Small port plugs can be quickly
removed from the external casing, and a borescope
inserted by a technician allows for the inspection
of internal stages or areas of the fan, compressor,
combustor, power turbine, and related accessories.
Borescopes for turbine inspections come in two
basic configurations: one with a flexible insertion
shaft and one with a rigid insertion shaft. Both types
can be configured with or without video capabil-
ity. This article focuses primarily on flexible video
borescopes.
In some cases, industrial turbines were initially
developed as aviation turbines. For instance, Pratt
&Whitney’s FT4000 is the aeroderivative industrial
variant of the PW4000, and Rolls-Royce’s RB211 is
used in both aviation and industrial applications.
Similarly, the GE Vernova LM6000 (LM is a
Land Marine designation) aeroderivative turbine
shown in Figure 3 was developed from the
CF6-80C2 aviation turbine platform. The CF6 has
been in use for over 50 years on long-haul flights
by Boeing and Airbus. A cut-away of the CF6 as
shown in Figure 4 depicts the major section of a gas
turbine.
In power generation, there are also much larger
and heavier frame turbines that have higher power
output. However, both turbine types operate funda-
mentally the same, in that ambient air is compressed,
mixed with fuel and heat in the combustion section,
and then passes through a power turbine section
where the energy is extracted. Notice the scale differ-
ence of the aeroderivative LM6000 in Figure 3 and
the large frame 7HA.03 in Figure 5.
Therefore, it makes sense that RVI inspections
on aeroderivative and large frame turbines would
be comparable to those conducted on aviation
turbines, and indeed they are. A significant dif-
ference is that aviation turbines are inspected on
Figure 3. GE Vernova
readies an LM6000
aeroderivative
turbine for service.
High-pressure
compressor
Low-pressure
compressor
Low-pressure
shaft Low-pressure
turbine Combustion
chamber
Nozzle
Fan
High-pressure
turbine High-pressure
shaft
Figure 4. Cut-away view of the major section of a CF6 gas
turbine used in aviation.
J U L Y 2 0 2 4 • M A T E R I A L S E V A L U A T I O N 43
CREDIT:
GE
VERNOVA