A320 ON APCH TO IAD WAS ISSUED ATC LOW ALT ALERT; ALLEGES ACFT WAS ON GS WITH NO INCREASED SINK RATE; QUESTIONED MSAW EQUIP CALIBRATION.

Narrative

RECEIVED A FALSE 'ALT WARNING' FROM THE IAD TWR AT ABOUT 1100 FT AFE DURING AN ILS APCH TO RWY 19L. THIS WAS A COUPLED ILS APCH AND THE AIRPLANE HAD BEEN ON GS AND AT APCH SPD (STABILIZED) FOR SEVERAL MI ALREADY. THE RWY AND PAPI WERE IN SIGHT AT THE TIME OF THE ALT WARNING AND ALL AVAILABLE CUES (VISUAL AND INST) INDICATED THAT WE WERE ON THE PROPER VERT PATH. I ADVISED TWR THAT THE ILS GS AND PAPI BOTH SHOWED US ON THE PROPER PATH AND THEY NEED TO HAVE THEIR ALT WARNING EQUIP CALIBRATION CHKED. THIS CAUGHT THE TWR BY SURPRISE AND THEY COMMENTED THAT HIGH SINK RATES CAN ALSO SET IT OFF. I INFORMED THEM THAT WE EXPERIENCED NO HIGH SINK RATES AND AGAIN ASKED THEM TO HAVE THE CALIBRATION OF THEIR EQUIP CHKED. I DON'T RECALL EXACTLY HOW THEY REPLIED; BUT IT FAILED TO GIVE ME ANY ASSURANCE THAT THEY WOULD HAVE THEIR EQUIP CHKED. THE ABILITY FOR ATC TO PROVIDE 'ALT WARNINGS' TO AIRPLANES ON FINAL IS A VALUABLE TOOL; IF THEY ARE ACCURATE. I JUST LOST THAT CONFIDENCE. (IMAGINE IF WE HAD BEEN IMC; WHAT TO DO?)