SMT ON ILS APCH TOLD LOW ALT ALERT ACTIVATED.

Narrative

WHILE SERVING AS A PNF ON A BEECH KING AIR 200 (BE20) ON AN IFR FLT PLAN FROM DESTIN; FL (81J) TO MEMPHIS; TN (MEM); WE WERE INSTRUCTED TO FLY THE ILS 27 APCH TO MEM. THE AUTOPLT WAS ENGAGED AND THE APCH MODE WAS SELECTED WE COUPLED AND UP TO THE LOC AND WERE TRACKING IT INBOUND. APPARENTLY THE GS NEVER COUPLED; HOWEVER THE ACFT HAD BEGUN A CONSTANT RATE OF DSCNT AT WHAT I WOULD ESTIMATE TO BE A DISTANCE OF 3/4-1 MI HORIZ FROM THE THRESHOLD; MEM TWR ADVISED 'LOW ALT ALERT. I SHOW YOU PRESENTLY AT 1300 FT.' I RESPONDED ; 'IDENT IS VISUAL.' GIVEN THE FACT THAT IT WAS VFR AT THIS TIME WITH VISIBILITIES IN EXCESS OF 10 MI; THIS SHOULD HAVE PROVIDED CONFIRMATION TO THE CTLR THAT WE DID HAVE THE RWY IN SIGHT. WE PROCEEDED AND LANDED WITHOUT INCIDENT. I BELIEVE THAT IMPROPER VIGILANCE IS PARTIALLY TO BLAME FOR THIS ON BOTH THE PLTS PART AS WELL AS MINE. EVEN THOUGH I WAS NOT ACTUALLY MANIPULATING THE CTLS; VIGILANCE MIGHT HAVE PREVENTED THIS SIT. ANOTHER FACTOR THAT I FEEL CONTRIBUTED TO THIS IS POOR ANNUNCIATION OF THE CONDITION WE EXPERIENCED -- LOC COUPLED AND GS NOT. I SUSPECT THE GS RECEPTION ABILITY OF THE #1 NAV AND WILL FURTHER INVESTIGATE ANY OTHER GS DEVS ASAP. THE RADIO MAY BE IN NEED OF REPAIR.