PLT OF AN SMT TWIN FAILED TO START A MISSED APCH AT THE MINIMUM DSCNT ALT DUE TO A DME PROB.

Narrative

THE PROB AROSE ON FINAL APCH (LOC RWY 15) AT UKI; AFTER IDENTING THE 5.5 IUKI FIX WHILE DSNDING TO MINIMUM DSCNT ALT. AT APPROX 4-5 DME OUT; USING MY #1 NAV TUNED TO THE LOC FREQ; WITH #2 NAV TUNED TO THE LOC AS A BACKUP; MY DME READING OFF THE #1 NAV AS WELL; I BEGIN TO GET INTERMITTENT DME SIGNALS (OFF MOMENTARILY THEN BACK ON). AS A BACKUP; I XCHKED MY DIGITAL TIMER TO VERIFY WHEN THE MISSED APCH POINT WOULD BE REACHED. AT APPROX 3 DME; THE DME READOUT WENT OUT; AND I WAS UNABLE TO USE IT TO DETERMINE MY POS ALONG THE COURSE. WHILE CHKING MY DIGITAL TIMER ONCE AGAIN; I SWITCHED THE DME TO THE #2 NAV AND REALIZED I HAD PASSED THE MISSED APCH POINT. AT ABOUT THIS TIME; UKI FSS RADIO WAS ALERTING ME THAT I WAS 2 MI BEYOND THE ARPT. AT THIS POINT; IN THE INTEREST OF SAFETY; I ELECTED TO MISS THE APCH AND PROCEED WITH THE MISSED APCH PROC. INITIALLY; I HAD RESPONDED TO UKI RADIO WITH A 'NEGATIVE' TO INDICATE THAT I HAD NOT BEGUN THE MISSED APCH BUT IMMEDIATELY DECIDED TO PROCEED WITH THE MISSED APCH UNTIL I COULD RECTIFY THE PROB WITH MY DME. CONTRIBUTING FACTORS: THE DME EQUIP IN THIS ACFT HAS A 'HOLD' FUNCTION LOCATED BTWN THE NAV #1 AND NAV #2 POS. IN RETROSPECT; IT MAY BE POSSIBLE THAT THE SWITCH HAD NOT FULLY MOVED FROM THE 'HOLD' POS TO THE NAV #1 WHEN IT WAS LAST SWITCHED. I OFTEN USE BOTH NAVS AS WELL AS DME FROM BOTH NAV SOURCES. ALSO; THE #1 NAV WAS RECEIVING THE SIGNAL FROM THE LOC FAIRLY WELL MOST OF THE TIME WITH OCCASIONAL ERRATIC NEEDLE MOVEMENTS ALONG THE APCH (NO APPARENT CAUSE). TIMING THE APCH WAS POSSIBLY OFF DUE TO HIGHER THAN PLANNED APCH SPD; AS WELL AS A SLIGHT DELAY IN STARTING THE TIMER AT THE FINAL APCH FIX.