ACR ON ILS APCH DEVIATES OFF COURSE. RECEIVES TCASII RA; TWR ADVISORY AND VECTOR.

Narrative

FLYING THE GHM1 ARR TO BNA; WE ENCOUNTERED SEVERAL AREAS OF LIGHT TO MODERATE TURB; MODERATE TO HVY RAIN; AND EMBEDDED TSTMS WHICH REQUIRED SOME COURSE DEV. ALSO SUFFERED INTERMITTENT STATIC RADIO INTERFERENCE AND A STATIC DISCHARGE/LIGHTNING STRIKE OFF THE RADOME. WE WERE TOLD TO EXPECT ILS TO RWY 20R AND BROKEN OFF ARR FOR APCH VECTORS. I WAS FLYING AND BRIEFED THE APCH CORRECTLY; LEAVING THE APCH PLATE IN A BINDER ON THE MAP TABLE TO MY R. DURING APCH VECTORS COCKPIT WORKLOAD BECAME QUITE HIGH AS WX DETERIORATED SOMEWHAT AND AUTOPLT HAD AN UNCOMMANDED DISCONNECT WHILE IN A 30 DEG BANK DSNDING TURN. UPON RECEIVING APCH VECTORS I TUNED IN ILS FREQ; BUT MISTAKENLY TUNED IN THE FREQ FOR RWY 31 AT BNA. AT SOME POINT THE CAPT SAID 'IDENTED'; WHICH I TOOK TO MEAN THE ILS HAD BEEN IDENTED WHEN IN FACT ONLY THE LOM HAD BEEN IDENTED. CAPT'S NAV RADIO REMAINED TUNED TO BNA VOR FOR DME IDENT. THE MOMENT I TUNED MY ILS BOTH LOC AND GS INDICATIONS WERE INTERMITTENT; WHICH I DID NOT CONSIDER ABNORMAL GIVEN OUR ANGULAR DISPLACEMENT FROM THE RWY. WHEN CLRED FOR LOC INTERCEPT MY LOC/GS INDICATIONS REMAINED INTERMITTENT. I WAS ABLE TO INTERCEPT LOC COURSE BY FOLLOWING TFC AHEAD ON APCH; LESS THAN 3 MI IN FRONT OF US ACCORDING TO TCASII DISPLAY AND VISIBLE THROUGH THIN CLOUD LAYER. I NOTIFIED THE CAPT OF UNRELIABLE LOC/GS INDICATIONS AND QUESTIONED OUR ABILITY TO CONTINUE THE APCH. AT ABOUT THIS TIME APCH CTL ADVISED US TO 'SWITCH TO TWR'; WHICH CAUSED MORE CONFUSION AS WE WERE SURE THAT WE WERE NEVER ACTUALLY CLRED FOR THE APCH. WE CONTINUED INBOUND AS THE CAPT TUNED HIS NAV RADIO TO MATCH THE FREQ ON MY SIDE AND CHK THE ACCURACY OF THE SIGNALS AND QUERIED THE TWR AS TO WHETHER THE ILS WAS WORKING PROPERLY. RUNNING THROUGH MY MIND WAS THE POSSIBILITY THAT MY LOC RECEIVER HAD BEEN AFFECTED BY THE STATIC DISCHARGE OR THAT THE SIGNAL WAS BEING AFFECTED BY THE ACFT AHEAD; WHICH APPEARED TO BE CLOSER THAN NORMAL. I STAYED ABOVE THE GS NOT WANTING TO COMMIT TO THE APCH WITH STILL INTERMITTENT LCL/GS SIGNAL. THE CAPT TOLD ME THAT HIS SIDE WAS WORKING AND I WAS SLIGHTLY R OF COURSE. ON MAKING A SLIGHT L CORRECTION I BELIEVE THE CAPT LOST HIS LOC/GS SIGNAL AND WE GOT A TCASII 'TFC -- MONITOR VERT SPD -- CLR OF CONFLICT' RA. THE TCASII DISPLAY SHOWED TFC 400 FT BELOW US; SLIGHTLY TO OUR L; AND DSNDING; OBVIOUSLY AN ACFT ON THE ILS TO 20L. THE CAPT THEN REALIZED THAT WE HAD TUNED IN THE WRONG LOC FREQ. THE TWR CALLED US NOTIFYING US THAT WE WERE L OF COURSE AND REQUESTING AN IMMEDIATE R TURN. I TURNED R; WE RETUNED THE PROPER FREQ; ESTABLISHED A NORMAL APCH TO AN UNEVENTFUL LNDG. IN RETROSPECT; THE BEST WAY TO AVOID THIS INCIDENT WOULD HAVE BEEN STRICT ADHERENCE TO COMPANY PROCS; INCLUDING POSITIVE IDENT OF NAVAIDS AND A MORE TIMELY AND FORTHRIGHT NOTIFICATION TO ATC OF OUR INABILITY TO TRACK THE LOC. ADDITIONALLY; THE ENTIRE INCIDENT COULD HAVE BEEN AVOIDED HAD I PLACED THE PROPER APCH PLATE ON THE YOKE IN FRONT OF ME RATHER THAN THE MAP TABLE TO MY R; MINIMIZING THE RISK OF READING THE WRONG PLATE. FROM NOW ON; IN BAD WX OR HIGH WORKLOAD SITUATIONS; THAT IS EXACTLY WHAT I WILL DO.