ACR LGT RPTR'S ALLUDE TO AN ACFT EQUIP PROBLEM WITH TCASII TA BEING PRONOUNCED FROM DEP TFC ON RWY AHEAD AS POTENTIAL SAME DIRECTION TFC CONFLICT.

Narrative

I WAS THE CAPT AND THE PF OUR FLT FROM DCA TO DTW; ON AN LGT. WE WERE ON AN ILS APCH; IN VISUAL CONDITIONS; OUTSIDE THE OM; FOLLOWING ANOTHER LGT TO RWY 21R. A WDB WAS TAXIING ONTO THE RWY FOR DEP. AT THIS POINT MY FO NOTICED A TCASII TARGET JUST AHEAD OF THE ACFT IN FRONT OF US. HE ASKED THE CTL TWR IF THERE WAS ANOTHER ACFT JUST AHEAD OF THE ACFT WE WERE FOLLOWING. THE TWR CTLR INDICATED THAT WE WERE NUMBER 2 FOR THE RWY; AND THAT NO ONE WAS AHEAD OF THE LFT IN FRONT OF US. AS THE DEPARTING WDB BECAME AIRBORNE HIS TCASII INDICATION; IN OUR ACFT; JUMPED AHEAD TO APPROX HIS CORRECT POS. AFTER LNDG THE GND CTLR ASKED US TO TELEPHONE THE TWR. THE CHIEF CTLR WITH WHOM I SPOKE; WAS FILLING OUT A NEW FORM PERTAINING TO TCASII ISSUES. I EXPLAINED THE ABOVE AND ADDED THAT OUR CONCERN WAS FOR A POSSIBLE MISSED APCH. OUR CONCERN WAS NOT FOR OUR SAFETY; BUT WE WERE CONCERNED ABOUT THE ACFT AHEAD OF US WITH A TCASII TARGET QUITE CLOSE TO HIM. AFTER FURTHER DISCUSSION; I REMEMBERED AND ADDED THAT APPROX 3 MONTHS AGO I EXPERIENCED A LIKE TCASII INDICATION ON APCH TO LAX WITH A WDB DEPARTING AHEAD OF US. THAT DEPARTING TARGET ALSO JUMPED ON THE INDICATOR WHEN HE BECAME AIRBORNE. I NOW BELIEVE THE TCASII SYS MAY GIVE A FALSE DISTANCE INDICATION WHEN AN ACFT IS STILL ON THE GND. I AM FILING THIS RPT; BECAUSE IT IS IMPOSSIBLE TO GUESS WHAT MAY TRANSPIRE WHEN A JOB JUSTIFYING BUREAUCRAT GETS HIS HANDS ON THE TCASII RPT FILED BY THE DTW CHIEF CTLR.