A320 FLC HAS ERRONEOUS LOC GS INDICATIONS DURING VISUAL APCH TO MDT.

Narrative

FLT WAS CLRED FOR VISUAL APCH ABEAM MDT ARPT FOR L TFC RWY 31. WE ENTERED A L BASE ARCING TURN TO FINAL OUTSIDE OF THE COOLING TWRS. WHILE ROLLING OUT ON FINAL APCH; I NOTICED GS INDICATION WAS JUMPY THEN SETTLED DOWN AS WE CAPTURED IT. THE CAPT HAND FLEW THE APCH WITH THE PROPER 800 FPM DSCNT. ALMOST IMMEDIATELY WE NOTICED THAT GLIDE PATH SEEMED TOO STEEP; THEN VERIFIED HIGH ON VASI; ALL WHITE INDICATIONS. ILS WAS ABANDONED AS I SET UP 1000 FPM TO RECAPTURE VASI. AT VASI CAPTURE; EGPWS SOUNDED 'GS' ONCE; THEN EGPWS GS SW ON OVERHEAD WAS TURNED OFF. APCH WAS CONTINUED; ALWAYS STABILIZED; TO A NORMAL LNDG. DURING TAXI IN; TWR WAS ASKED OF ANY GS PROBS. THEY RPTED NO NOTAMS OR FLAGS ON THE SYS. WE ADVISED OF THE GS ERRORS AND TWR SAID THAT HE SAW THE PROB. SOMEONE HAD SWITCHED THE LOC GS TO THE OPPOSITE RWY. I ALWAYS USE AN ILS FOR EVERY APCH WHERE AVAILABLE. FROM NOW ON I NEED TO VERIFY ILS IDENTS ON EVERY VISUAL APCH JUST AS I DO FOR IFR APCHS. THE FALSE GS TRACKED AT JUST THE RIGHT FLT INDICATIONS BUT WAS ABOUT 1-2 DOTS HIGH...COULD EASILY HAVE BEEN 1 OR MORE DOTS LOW TO THE ACTUAL GLIDE PATH. CONSIDERING THE VERTICAL DEVIATIONS AND VISUAL CUES; BACKED UP BY THE VASI; SAVED THE APCH. ONE BAD INDICATION; EVEN THE ILS ITSELF; DOESN'T OVERRIDE GOOD JUDGEMENT AND 3 INDICATIONS THAT ALL AGREE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED HE INTERCEPTED THE LOC COURSE INSIDE THE FAF WHICH GAVE HIM AN ABNORMAL PICTURE OF THE RWY ENVIRONMENT. THIS LED HIM TO CHASE THE GS INDICATIONS; THINKING HE WAS BELOW GS ON FINAL. ADDITIONALLY; HE DID NOT ARM THE 'APPR' MODE ON THE FCU; WHICH WOULD HAVE HELPED VERIFY HIS REFS.