PLT OF CIRRUS SR22 DSNDED BELOW GS ON APCH TO BNA WHILE TROUBLESHOOTING MALFUNCTIONING GPS-HSI. ATCT LCL CTLR RECEIVED MSAW AND ALERTED PLT WHO SALVAGED THE APCH AND LANDED SAFELY.

Narrative

ON AN ILS FINAL APCH TO BNA; #1 NAV (GPS); WHICH IS LINKED TO AN HSI; DID NOT ACTIVATE AND CAPTURE THE LOC AND GS. I HAD SET UP #2 NAV (GPS); WHICH IS LINKED TO A CONVENTIONAL CDI; TO FUNCTION AS A NORMAL LOC RECEIVER; WHICH DID CAPTURE THE LOC AND GS. REALIZING I HAD CONFLICTING INFO BTWN THE 2 NAVS; I TURNED ONTO THE FINAL APCH COURSE USING THE #2 NAV AND WAITED FOR THE #1 NAV TO ACTIVATE AND CAPTURE THE LOC AND GS. IT NEVER DID. WHILE ATTEMPTING TO RESOLVE THE INFO CONFLICT BTWN THE 2 NAVS; I INADVERTENTLY DSNDED BELOW THE GS. JUST AS I REALIZED I WAS LOW AND BEGAN TO CLB BACK UP TO THE GS I WAS INFORMED BY BNA TWR THAT I HAD SET OFF THE LOW ALT ALERT. I REGAINED THE GS USING THE #2 NAV AND LANDED UNEVENTFULLY. AFTER LNDG I ANALYZED WHAT HAD HAPPENED AND DETERMINED THAT ALTHOUGH THE #1 NAV WAS PROPERLY SET FOR THE APCH IT DID NOT AUTOMATICALLY SWITCH TO THE PROPER LOC FREQ AND TO LOC MODE AS IT HAS DONE NUMEROUS TIMES BEFORE; AND AS I HAD EXPECTED IT TO DO. AS A RESULT; ON ALL ILS AND LOC APCHS I NOW MANUALLY SWITCH IT TO THE CORRECT FREQ; AND MODE; WHEN APPROPRIATE TO DO SO DURING THE APCH.