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 screens.   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. 2-1