AN MD90 INCURRED A DELAY DUE TO PLT'S RPT OF NOSE GEAR INSPECTION MIRROR DIRTY. MIRROR CHKED OK PRIOR TO FLT ON OVERNIGHT CHK.

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ON APR/WED/03; THE MAIN LNDG GEAR DOWN-LOCK VIEWING MIRROR WAS FOUND DIRTY; DURING A PREFLT INSPECTION BY THE PLT. THE MIRROR WAS CLEANED; AND A LOGBOOK WRITE-UP WAS GENERATED. ON APR/TUE/03; THE PREVIOUS NIGHT; I WAS WORKING ON THIS ACFT PERFORMING AN OVERNIGHT LAYOVER CHK. THE 'ACFT GENERAL' SECTION (EXTERIOR) WAS WHAT I WAS WORKING ON. THE ITEM CALLED FOR A GENERAL VISUAL INSPECTION OF THE LNDG GEAR; GEAR DOORS; WHEEL WELL AREAS; SPRAY DEFLECTORS; AND THE MAIN LNDG GEAR DOWN-LOCK VIEWING MIRROR AND INDICATORS. THIS PAPERWORK REQUIRES CHKING THE MIRROR FOR CLEANLINESS; CONDITION; AND SECURITY. IN THE 5 DAYS SINCE MY INITIAL INSPECTION; I HAVE BEEN INFORMED THAT THIS ITEM WAS POSSIBLY MISSED. THIS IS AN IMPORTANT ITEM. IF THERE HAPPENED TO BE A GEAR INDICATION PROB; THE MIRROR WOULD BE REQUIRED TO VIEW THE INDICATORS. THIS ALSO STRESSES THE IMPORTANCE OF PERFORMING A DOUBLE CHK OF THE CHKLIST TO MAKE SURE NOTHING WAS MISSED. DURING A NORMAL INSPECTION OF THE 'DOWN-LOCK INSPECTION MIRROR;' I USE A FLASHLIGHT TO VIEW THE MIRROR FROM BELOW; IN LINE WITH THE DOWN-LOCK INDICATORS. I LOOK THROUGH THE MIRROR TO THE BASE OF THE INSIDE COVER. IF THIS IS VIEWABLE; IT IS CLEAN ENOUGH TO VIEW THE INDICATORS ON THE GEAR ASSEMBLY; AND WOULD NOT REQUIRE CLEANING. THIS IS UP TO THE DISCRETION OF THE PERSON INSPECTING THE MIRROR. IN THE FUTURE; IT WOULD BE BEST TO CLEAN THE MIRROR REGARDLESS OF THE CONDITION; THEN IT WILL ALWAYS BE CLEAN. I INTEND TO DO THIS FROM NOW ON. TO PREVENT FUTURE OCCURRENCES; I HAVE PASSED THE EXPERIENCE ON TO MY FELLOW MAINT TECHNICIANS.