CHAPTER 2GAS TURBINE MAINTENANCEThis chapter will cover object damage, borescopeinspection, troubleshooting, and maintenance of theLM2500 and the Allison 501-K series of GTEs. Themajority of this chapter deals with the LM2500 GTEdamage evaluation. The last part of this chapter is onproper preservation and corrosion control methods formaintaining all GTEs in peak operating status.OBJECT DAMAGEThere are two basic types of object damage GSsupervisors see. One of the most damaging gas turbinecasualties, and one of the easiest to prevent, is foreignobject damage (FOD). In this section we will discussthe hazards of FOD and some of the ways to prevent it.The other type of object damage that can cause failureof a GTE is domestic object damage (DOD).HAZARDSThe effects of object damage and the hazardsinvolved vary greatly with the size and location of theobject ingested. Small dents and abrasions may causelittle or no damage. However, if a large enough objectis ingested by the engine, severe internal damage willresult. Large, soft items (such as paper) can clog theFOD screen, causing a loss of power and elevatedturbine inlet temperatures. The other type of damagethat was mentioned is DOD. DOD occurs when aninternal object from the engine breaks loose and causesimpact damage to the engine.PREVENTIONTo prevent FOD to engines while working in andaround intake and plenum areas, you and your personnelmust observe the following safety precautions:—When performing maintenance inside the intakeareas, always-follow all written guidelines found in theEOP. Remember to remove all loose objects from yourperson. You must also account for all tools andequipment used in the intake. After completing yourwork, inspect the intake for cleanliness, and reinventorythe tools and equipment before securing the accesses.—Periodically inspect all intakes for cleanliness,the state of preservation, and the condition of the FODscreens. Correct any abnormal conditions. Thefrequency of inspection will depend on the operatingconditions, PMS requirements, and engineeringdepartment instructions.Remember, the PMS onlyprovides minimum standards. PMS can always beexceeded if you or your superiors deem it necessary.—When inspecting the intakes, be sure that theareas around the blow-in doors are kept clear of loosegear and debris that could be ingested if the blow-indoors are activated.To prevent DOD damage, you and your personnelneed to follow a strict regiment of cleaning andinspections (internal and external). This attention todetail, as described in the next two paragraphs, isabsolutely necessary to avoid DOD damage.—Make sure the engine is properly cleaned insideand out. Always following the standards in the PMS andthe manufacturer’s technical manual. Cleanliness is animportant factor in the fight against corrosion.Corrosion control (discussed later in this chapter) alsocan reduce the chances of component failures that canlead to DOD.—Perform frequent external and internal GTEinspections to reduce the possibilities of DODoccurrences.GTE external inspections are veryimportant. Locating loose, missing, or broken externalcomponents (VSV retaining nuts) during theseinspections is a significant factor in preventing damage.—Using borescope inspections aids in determiningthe extent and prevention of DOD. The most frequentdamage is identified as potential component failures(blade stress cracks).BORESCOPE INSPECTIONSBorescope inspection requirements and proceduresare found on the maintenance requirement card (MRC).These cards contain all the basic information necessaryto conduct an inspection. Included on the MRCs are theserviceability limits and a list of conditions that requirean inspection.Borescope inspections are usuallyperformed semiannually or when the engine has beenoperated beyond the allowable limits listed on the MRC.2-1
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