CHAPTER 2 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.

OBJECT DAMAGE 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).

HAZARDS 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.

PREVENTION 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 creens. Correct any abnormal conditions. The frequency of inspection will depend on the operating conditions, PMS requirements, and engineering department instructions. 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 manufacturer's 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 occurrences. 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 INSPECTIONS 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 an inspection. Borescope inspections are usually performed semiannually or when the engine has been operated beyond the allowable limits listed on the MRC.

Figure 2-1.- Geometric orientation of the LM2500 GTE The following section discusses the borescope procedures used to inspect the LM2500 GTE. The inspection procedures and the knowledge gained from damage evaluation may also be applied to the borescope inspection of the Allison 501-K17 GTE.

GENERAL INSPECTION PROCEDURES It is a good engineering practice to review the machinery history of an engine before you conduct an inspection Various component improvement programs will eventuallyy effect all engines in service. A rebuilt or modified engine may contain improved parts that differ from the original. An example of this is the first-stage compressor midspan damper that may have its original coating, an improved coating, or a carboloy shoe welded on at the midspan damper interface. If you review the machinery history, you will discover the status of those parts that have been changed or modified.

Assuming that the engine history is normal and FOD is not suspected, you should be aware of the following factors when conducting a borescope inspection:

l Know your equipment. l Locate all inspection areas and ports.

Figure 2-2.- Example of radial and axial cracking.

Establish internal reference points. Scan the inspection area thoroughly orderly manner.

Note any inconsistencies. Evaluate the inconsistencies. Report your conclusions.