PLT OF SR22 EXECUTES MISSED APCH FROM AUTO-COUPLED APCH TO DECISION ALT. AUTOPLT FAILS TO DISCONNECT AND HE MUST OVERCOME SERVO LOADS TO MAINTAIN ACFT CTL UNTIL HE IS ABLE TO LOCATE AND PULL THE AUTOPLT CIRCUIT BREAKER.

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ON ILS RWY 20L APCH AT PDK; TWR ADVISED EXPECT TO BREAKOUT AT MINIMUMS. SINGLE PLT; NO PAX. INTERMITTENT RAIN ON FINAL APCH. CONTINUED APCH ON AUTOPLT TO 300 FT AGL (250 FT AGL MINIMUM). AUTOPLT PERFORMED WELL; APCH FULLY STABILIZED; NEEDLES CTRED; AIRSPD NORMAL (APPROX 100 KTS). DID NOT BREAK OUT AT 300 FT AGL; DISENGAGED AUTOPLT AND WENT MISSED. FULL PWR; NOSE UP ATTITUDE; BEGAN CLB. GIVEN VECTORS 330 DEGS BY TWR. BEGAN TURN. APPARENTLY BECAUSE OF NOISE (RAIN); RADIO AND OTHER COCKPIT ACTIVITY; DID NOT IMMEDIATELY NOTICE THAT AUTOPLT HAD FAILED TO DISENGAGE. WHEN REQUIRED BACK PRESSURE TO MAINTAIN CLB BEGAN TO INCREASE; INITIALLY ASSUMED DOWNDRAFT DUE WX; HOWEVER; IT QUICKLY BECAME APPARENT THAT IT WAS THE AUTOPLT TRIMMING DOWN TO 'MAINTAIN THE GS' WHICH WAS THE PROB. ATTEMPTED TO DISENGAGE AGAIN USING THE PLT'S SIDE COOLIE HAT; NO SUCCESS. RECEIVED A FREQ CHANGE TO DEP. LOOKED AWAY FROM INSTS TO CHANGE FREQ; WHEN RETURNED TO INSTS; NOSE DOWN ATTITUDE AND DSNDING. HARD TO ACCURATELY MAINTAIN REQUIRED (AND INCREASING) BACK-PRESSURE WITHOUT CONSTANT ATTN TO INSTS. RECOVERED CORRECT PITCH ATTITUDE AND RESUMED CLB ON ASSIGNED HEADING. ATTEMPTED TO DISENGAGE AUTOPLT WITH CIRCUIT BREAKER; LOCATED NEAR PLT'S R LEG. INITIAL ATTEMPT TO LOCATE BREAKER UNSUCCESSFUL; AGAIN; SOME LOSS OF ALT WHILE TRYING TO LOCATE CORRECT BREAKER. AUTOPLT BREAKER IS IDENTED WITH VELCRO ON THE TOP FOR JUST THIS REASON; BUT WITH 3 ROWS OF BREAKERS AND PRESSURE TO QUICKLY DISENGAGE AUTOPLT; DIDN'T IDENT AT FIRST. RECOVERED CORRECT PITCH ATTITUDE AND HEADING. ON SECOND ATTEMPT; LOCATED CORRECT BREAKER USING VELCRO TO CORRECTLY IDENT IT. PULLED BREAKER. AUTOPLT DISENGAGED. REQUESTED VECTORS TO FTK WERE MINIMUMS WERE RPTED 500 FT. GIVEN VECTORS. VFR ON TOP; SO REENGAGED AUTOPLT. DID NOT PROPERLY CAPTURE LOC. FLEW ILS RWY 8 BY HAND. LANDED FTK. CAUSES: PLT FAILURE TO IMMEDIATELY RECOGNIZE THAT AUTOPLT HAD FAILED TO DISENGAGE. COMBINATION OF RAIN; ACTIVITY; ATTN ON FIRST LOCATING ARPT (NO LUCK); THEN FLYING THE MISSED. HAVE CHANGED PERSONAL INST APCH PROCS TO DISENGAGE AUTOPLT EARLIER ON AN AUTOPLT APCH (200 FT ABOVE DECISION HT RATHER THAN AT DECISION HT MINIMUMS). INABILITY TO DISENGAGE USING NORMAL PROC (PLT SIDE COOLIE HAT). DIFFICULTY IN LOCATING BREAKER. IF BREAKER HAD NOT BEEN IDENTED WITH VELCRO ON ITS TOP (PLT ADDITION; NOT SUPPLIED BY MANUFACTURER); IT WOULD HAVE REQUIRED FAR TOO LONG TO IDENT BREAKER VISUALLY. LACK OF EXPERIENCE HAND FLYING THE ACFT WITH AUTOPLT ENGAGED. WHILE CTL COULD BE MAINTAINED; THE EXTREMELY HIGH BACK PRESSURE REQUIRED WAS DISORIENTING; WOULD HAVE BEEN BETTER IF I HAD PRACTICED THIS EMER PRIOR TO ENCOUNTERING IN ACTUAL CONDITIONS.