MOONEY 20J PLT HAS A TRACK AND HDG DEV DURING APCH TO HDC.

Narrative

WE WERE INTERCEPTING THE LOC FOR RWY 18 IN HAMMOND AT 2000 FT MSL WHEN ATC (NEW ORLEANS) ADVISED THAT I COULD SWITCH TO CTAF TO COMPLETE THE APCH. ALTHOUGH WE HAD BROKEN OUT OF THE WX AT ABOUT 2500 FT; I DECIDED TO CONTINUE ON WITH THE APCH FOR PRACTICE; AND THOUGHT IT BEST TO CANCEL IFR FROM THE GND. MY COUSIN; AN INST RATED PVT PLT; WAS WITH ME; AND HE HAD NEVER USED AN HSI BEFORE. I THOUGHT I COULD GIVE HIM A DEMO WITH THIS APCH. I HAD POINTED OUT THE SWITCH TO CHANGE THE HSI FROM SENSING THE GPS TO THE NAV; AND MUST HAVE SOMEHOW MENTALLY CHKED THAT ITEM OFF OF MY CHKLIST. THE SWITCH DID NOT GET SET TO 'VOR' SO THE CDI DID NOT INDICATE THAT WE WERE BLOWING RIGHT THROUGH THE LOC. THE ARPT WAS NOT YET IN SIGHT; BUT WE WERE IN VMC. THE NEW ORLEANS CTLR NOTICED THAT WE WERE OUT OF POS -- SW OF THE INBOUND COURSE BY A COUPLE OF MI -- AND NOTIFIED US ON THE CTAF. THERE WAS A MOMENT OF CONFUSION HERE. YOU SEE; IT APPEARS THE HSI WAS PICKING UP THE GS AND INDICATING WE HAD NOT YET REACHED THE LOC. THE LATTER COULD POSSIBLY BE EXPLAINED BY VECTORING; BUT THE FORMER; A WORKING GS WHILE THE HSI WAS SET TO 'GPS' WAS SOMETHING I HAD NEVER BEFORE EXPERIENCED. I KNEW THERE WAS VERY LITTLE DANGER OF HITTING AN OBSTACLE BECAUSE THE LAND IS VERY FLAT SURROUNDING HDC AND I HAD OVER 4 MI VISIBILITY. THE FIRST THOUGHT THAT OCCURRED TO ME WAS THAT THE NAV WAS OUT; AND WE WOULD HAVE TO SWITCH TO THE OTHER NAV AND INDICATOR. I RPTED THIS TO NEW ORLEANS (NOW ON THEIR FREQ); DECLARED A MISSED APCH AND REQUESTED VECTORS. THE CTLRS OBLIGED. I NOTICED DURING OUR SECOND APCH THAT I HAD LEFT THE SWITCH ON 'GPS' (A FIRST FOR ME). THEN I THOUGHT THAT THE SWITCH MUST HAVE BEEN STUCK BTWN THE TWO OR SOMETHING; BUT UPON FURTHER INVESTIGATION I MUST ADMIT THIS IS IMPOSSIBLE. THE SWITCH WORKS PERFECTLY.