C172 INSTRUCTOR PLT RECEIVES TERRAIN WARNINGS FROM ATC ON THE VOR-A APCH INTO IXD ON 2 SEPARATE FLTS.

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I WAS FLYING WITH AN INST STUDENT TO SHOOT APCHS INTO IXD AND OJC. THE WX WAS VMC AND WE WERE ON AN INST FLT PLAN. THE FIRST APCH WE SHOT WAS THE VOR-A AT IXD. AFTER PASSING THE VOR; WE DSNDED TO THE PUBLISHED MDA OF 1620 FT. AFTER LEVELING OFF AT THIS ALT; WE WERE ASKED WHAT OUR ALT WAS BY IXD TWR. AFTER TELLING THEM THAT WE WERE AT 1620 FT; THE TWR TOLD US THAT KANSAS CITY APCH HAD GOTTEN AN ALT WARNING ON OUR PLANE. I DIDN'T ASK WHY AT THAT TIME BECAUSE THE TFC PATTERN WAS FULL AT NEW CENTURY AND WE WERE TRYING TO DO A CIRCLE-TO-LAND. I WAS FLYING WITH ANOTHER INST STUDENT ON THE SAME FLT PLAN AND ON THE VOR-A INTO IXD. IXD TWR AGAIN ASKED ME ABOUT MY ALT AND STATED THAT KANSAS CITY APCH HAD GOTTEN AN ALT WARNING. THIS TIME MY STUDENT HAD LEVELED OFF AT 1700 FT SO WE WERE 80 FT ABOVE THE PUBLISHED MDA. THIS TIME I ASKED THE CTLR AT NEW CENTURY TO VERIFY THAT THE MDA WAS 1620 FT AND AFTER A POSITIVE RESPONSE; I ASKED WHY WE WERE CAUSING AN ALT WARNING. HIS RESPONSE WAS THAT KANSAS CITY APCH EITHER HAD THEIR EQUIP SET TO SENSITIVE OR THAT WE HAD ARRIVED AT THE MDA EARLY. THE LAST STATEMENT CONFUSED ME BECAUSE THE APCH DOES NOT HAVE A STEP-DOWN FIX AND AFTER PASSING THE VOR YOU ARE CLRED DOWN TO THE MDA WHICH WE WERE AT OR ABOVE ON BOTH OCCURRENCES. MY CONCERN IS THAT THERE IS AN OBSTACLE THAT IS NEAR THE APCH OR THAT THE MDA IS SET TOO LOW. I THINK IT WOULD NOT BE A WASTE TO LOOK INTO THIS FURTHER.