LCL CTLR AT XNA CONCERNED THAT PAPI (PRECISION APCH PATH INDICATOR) GLIDE PATH IS NOT IN ALIGNMENT WITH ILS GLIDE PATH.

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ACFT X ADVISED WHILE FLYING THE PAPI GLIDE PATH FOR RWY 16 AT XNA HIS GLIDE SLOPE ALARM SHOWING HIS PATH 1.5 DOTS BELOW THE ILS GLIDE SLOPE. THE PLT WAS ON A VISUAL APCH FROM THE SOUTH; ENTERING R DOWNWIND FOR A LNDG ON RWY 16. THE FAA MOCC WAS CALLED TO RPT THE PROB. THE FAA ISSUED A NOTAM PLACING THE PAPI OUT OF SERVICE. ON MONDAY; THE FAA CALLED XNA TWR TO ADVISE THAT THE XNA ARPT MAINT HAD ADJUSTED THE PAPI. BASED ON SUBSEQUENT RPTS BY PLTS ABOUT THE PAPI LIGHTS FOR RWY 16 AT XNA ARPT THE PAPI GLIDE PATH IS STILL NOT IN ALIGNMENT WITH THE ILS GS. BELOW ARE OTHER RPTS SINCE THE PROB WAS FIRST RPTED TO THE FAA. OCTOBER/FRI/2004 AT XA:01; ACFT X NOTED THAT THE PAPI WAS STILL LOWER THAN THE GS; SHOWS THREE WHITE LIGHTS WHEN FOLLOWING ILS GS. NOVEMBER/FRI/2004 AT XA:45; ACFT Y; A C172 FLYING THE PAPI (TWO WHITE AND TWO RED LIGHTS) RPTED HIS GLIDE SLOPE INDICATED BETWEEN ONE DOT AND 1.5 DOTS BELOW THE ILS GLIDESLOPE. NOVEMBER/THUR/2004 AT XD:05; ACFT Z AN E145; ADVISED THE PAPI AND ILS GS DO NOT AGREE STATING THAT WHEN FOLLOWING THE ILS GS THE PAPI SHOWS THREE WHITE LIGHTS AND ONE RED LIGHT. ACCORDING TO THE FAA FACILITY; THEY HAVE ALWAYS RECEIVED COMPLAINTS ABOUT THE RWY 16 PAPI LIGHTS AND THAT THE ARPT MAINT IS EITHER UNWILLING OR UNABLE TO MAKE THE NECESSARY ADJUSTMENTS TO CORRECT THE PAPI SIGNAL. MY FEAR IS THAT DURING AN ACFT EMER THE PAPI PROB COULD MEAN THE DIFFERENCE BETWEEN MAKING IT TO THE RWY AND LNDG SHORT! EVEN IF THE ADJUSTMENT IS DIFFICULT THE XNA ARPT SHOULD BE TOLD TO CORRECT THE PROB.