GAS TURBINE MAINTENANCE
This chapter will cover object damage, borescope
inspection, troubleshooting, and maintenance of the
LM2500 and the Allison 501-K series of GTEs. The
majority of this chapter deals with the LM2500 GTE
damage evaluation. The last part of this chapter is on
proper preservation and corrosion control methods for
maintaining all GTEs in peak operating status.
There are two basic types of object damage GS
supervisors see. One of the most damaging gas turbine
casualties, and one of the easiest to prevent, is foreign
object damage (FOD). In this section we will discuss
the hazards of FOD and some of the ways to prevent it.
The other type of object damage that can cause failure
of a GTE is domestic object damage (DOD).
The effects of object damage and the hazards
involved vary greatly with the size and location of the
object ingested. Small dents and abrasions may cause
little or no damage. However, if a large enough object
is ingested by the engine, severe internal damage will
result. Large, soft items (such as paper) can clog the
FOD screen, causing a loss of power and elevated
turbine inlet temperatures. The other type of damage
that was mentioned is DOD. DOD occurs when an
internal object from the engine breaks loose and causes
impact damage to the engine.
To prevent FOD to engines while working in and
around intake and plenum areas, you and your personnel
must observe the following safety precautions:
When performing maintenance inside the intake
areas, always-follow all written guidelines found in the
EOP. Remember to remove all loose objects from your
person. You must also account for all tools and
equipment used in the intake. After completing your
work, inspect the intake for cleanliness, and reinventory
the tools and equipment before securing the accesses.
Periodically inspect all intakes for cleanliness,
the state of preservation, and the condition of the FOD
screens. Correct any abnormal conditions. The
frequency of inspection will depend on the operating
conditions, PMS requirements, and engineering
Remember, the PMS only
provides minimum standards. PMS can always be
exceeded if you or your superiors deem it necessary.
When inspecting the intakes, be sure that the
areas around the blow-in doors are kept clear of loose
gear and debris that could be ingested if the blow-in
doors are activated.
To prevent DOD damage, you and your personnel
need to follow a strict regiment of cleaning and
inspections (internal and external). This attention to
detail, as described in the next two paragraphs, is
absolutely necessary to avoid DOD damage.
Make sure the engine is properly cleaned inside
and out. Always following the standards in the PMS and
the manufacturers technical manual. Cleanliness is an
important factor in the fight against corrosion.
Corrosion control (discussed later in this chapter) also
can reduce the chances of component failures that can
lead to DOD.
Perform frequent external and internal GTE
inspections to reduce the possibilities of DOD
GTE external inspections are very
important. Locating loose, missing, or broken external
components (VSV retaining nuts) during these
inspections is a significant factor in preventing damage.
Using borescope inspections aids in determining
the extent and prevention of DOD. The most frequent
damage is identified as potential component failures
(blade stress cracks).
Borescope inspection requirements and procedures
are found on the maintenance requirement card (MRC).
These cards contain all the basic information necessary
to conduct an inspection. Included on the MRCs are the
serviceability limits and a list of conditions that require
Borescope inspections are usually
performed semiannually or when the engine has been
operated beyond the allowable limits listed on the MRC.