SMA PLT REALIZES VISION DETERIORATION WHEN HE MISSES FLAG ON HSI. HAS TROUBLE WITH AUTOPLT DISCONNECT. FLASHLIGHT ON FLOOR; OUT OF REACH. ALTDEV.

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SINGLE PLT; IFR; ON VECTORS TO ILS 27R. INTERCEPTING LOC TURNED OFF AUTOPLT; DID NOT DISENGAGE. TRYING TO HAND FLY ILS; FIGHTING AUTOPLT; TOLD APCH WAS HAVING AUTOPLT PROBLEM. COULD NOT FIND AUTOPLT MASTER SWITCH. SO I SHUT OFF ACFT MASTER SWITCH TO DISCONNECT AUTOPLT. THIS WORKED; SO I ASKED APCH FOR A VECTOR BACK TO LOC. LOC AND GS INDICATIONS WERE CONFUSING; AS I WAS FLYING A 290 DEG HDG TO INTERCEPT. APCH THEN CALLED AND ADVISED THAT I WAS S OF COURSE; HDG SBOUND. I THEN REALIZED THAT THE DIRECTIONAL GYROSCOPE PORTION OF THE HSI HAD FAILED TO ERECT AFTER SHUTTING OFF MASTER. NO LIGHT ON SECOND DIRECTIONAL GYROSCOPE; FLASHLIGHT HAD FALLEN ON FLOOR; OUT OF REACH. IMMEDIATELY ASKED FOR NO-GYROSCOPE VECTORS TO LOC. DURING ALL THIS; SEVERAL ALT EXCURSIONS OCCURRED (PARTIAL PANEL). THE PROBLEM WAS TWOFOLD. NORMALLY I FLY A DIFFERENT ACFT WITH A DIFFERENT AUTOPLT ALTOGETHER; AND HAD DIFFICULTY LOCATING CTLS ON THIS ACFT. SECONDLY; I HAD RECENTLY ARRANGED AN EYE EXAMINATION; BUT HAD NOT YET GOTTEN IT. CURRENTLY WEARING BIFOCALS; THEY FOCUS EITHER TOO CLOSE OR TOO FAR AWAY; SO I MISSED THE WARNING FLAG ON THE HSI. I DID NOT REALIZE UNTIL THAT VERY MOMENT; HOW MUCH MY VISION HAD DETERIORATED; AND WITHOUT THE FLASHLIGHT TO ADD SUFFICIENT LIGHT TO FIND THE PROPER INSTS AND CTLS I NEARLY EXCEEDED MY CAPABILITIES. I CURRENTLY HOLD A VALID SECOND CLASS MEDICAL; PASSED THE EYE EXAMINATION. I SUCCESSFULLY COMPLETED SINGLE PLT PART 135 IFR PROFICIENCY EXAMINATION IN OCT; BUT THE LACK OF COMPLETE FAMILIARITY WITH THESE ACFT CERTAINLY CONTRIBUTED. (AS A SAFETY FEATURE IF VOLTAGE IS LOW; HSI WILL NOT ERECT. I HAVE HAD THIS HAPPEN ON THE GND; BUT NEVER IN THE AIR.)