MLG FLC MISREADS APCH CHART; DSNDS TOO SOON.

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ENRTE FROM DFW TO HOU WE WERE VECTORED SW AND THEN BACK TO NE TO APCH ARPT FROM SW DUE TO LINE OF NUMEROUS TSTMS APCHING HOUSTON AREA. ON DSCNT WE ENCOUNTERED HVY PRECIPITATION; LIGHT RIME ICING AND CONTINUOUS LIGHT TURB WITH OCCASIONAL MODERATE CHOP. APCH CTL VECTORED US TO INTERCEPT THE LOC BACK COURSE TO RWY 22 AT HOU. WHILE ON HDG OF 230 DEGS AND AT 2000 FT WE WERE CLRED TO INTERCEPT LOC AND CLRED FOR APCH (219 DEG LOC BACK COURSE; 039 SET IN HDG SELECT FOR BACK COURSE APCH). LOC STARTED TO DRIFT OFF TO R AND I TURNED R TO HDG OF 250 DEGS TO INTERCEPT. LOC SUDDENLY DEFLECTED FULL SCALE TO THE L. WHEN IT DID; I DISCONNECTED AUTOPLT AND MANUALLY TURNED L TO INTERCEPT. APCH CTL ASKED IF WE WERE RECEIVING LOC. COPLT RPTED THE RAPID OSCILLATIONS BUT AURAL IDENT WAS ALRIGHT. CTLR RPTED THAT WE WERE CORRECTING BACK AND ABOUT 1/4 MI FROM LOC. I TURNED TO HDG 200 DEGS AND LOC SNAPPED BACK TO ON-COURSE. DUE TO WX CONDITIONS AND LOC PROBS WE WERE DISTRACTED IN COCKPIT. HAVING PASSED THE 7.5 DME I STARTED MY DSCNT. I ASKED COPLT TO CONFIRM THAT WE WERE CLRED FOR APCH. HE AGREED AND SAID YOU ARE CLRED TO DSND. IN DSCNT WHILE CONCENTRATING ON LOC FOR RELIABILITY AND RADAR FOR TSTM AVOIDANCE; I BEGAN DSCNT FOR MDA OF 480 FT. I NOTICED ALT ALERTER LIGHT AND ASKED COPLT TO SET MDA OF 480 FT IN ALT SELECT. HE REFERRED TO APCH PLATE AND REALIZED THAT WE HAD STARTED DSNDING AT 7.5 MI INSTEAD OF 4.5 MI. I LEVELED OFF AT 1200 FT TO 1100 FT AND STARTED TO CLB BACK TO ALT WHEN CTLR ADVISED US TO RECHK ALT. UPON REACHING 2000 FT HE ADVISED US THAT WE WERE AT FAF (4.5 DME). WE CONCURRED AND THEN BEGAN DSCNT TO MDA. WE BROKE OUT OF CLOUDS AT 600 FT; LEVELED AT MDA 480 FT. WE SAW RWY 22 AND VASI LIGHTS AND CONTINUED THE APCH VISUALLY TO A LNDG. NUMEROUS FACTORS ARE INVOLVED IN THIS EVENT. THE WX PROB: RAIN; ICE; TURB AND NUMEROUS TSTMS IN AREA. THIS WAS COMPOUNDED BY ERRATIC LOC DEVS PRIOR TO AND DURING INTERCEPT. HVY WORKLOAD IN THE COCKPIT SUCH AS RADAR OBSERVANCE FOR TSTMS; WATCHING FOR LOC RELIABILITY AND TALKING TO APCH CTL ABOUT EVENTS AS THEY UNFOLDED. THE ALTDEV HAPPENED BECAUSE I DSNDED AT 7.5 DME WHICH I HAD IN MY MIND AS THE FAF WHEN IT WAS ACTUALLY 4.5. TIME; TOO; IS A FACTOR SINCE ALL OF THIS OCCURRED IN A MATTER OF SECONDS AND AMONG ALL OF THE DISTRS GOING ON. CORRECTIVE ACTION WAS TAKEN WHEN THE EARLY DSCNT WAS DISCOVERED AND I REALIZED THAT WHAT I HAD IN MY MIND AS THE FAF WAS ACTUALLY 3 MI TOO SOON. HUMAN FACTORS INCLUDE STRESS FROM WX CONDITIONS; LOC VARIATIONS AND INCORRECTLY THINKING THAT FAF POINT HAD BEEN OBTAINED. OTHER FACTORS OF CONCERN ARE MAJOR ARPTS THAT HAVE NONPRECISION APCHS SUCH AS LOC BACK COURSE APCHS AS A PRIMARY APCH TO A RWY WHEN AN ILS COULD BE MADE AVAILABLE.