AN MD88 FLC REQUIRED RADAR VECTORS TO AVOID DCA AIRSPACE.

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N OF EMI VOR; ZDC SAID TURN 30 DEGS R; WE WERE 10 NM OFF COURSE. TURN ACCOMPLISHED AND WE RPTED THAT OUR NAV DISPLAY SHOWED ON COURSE. NAV DISPLAY DID SHOW DEAD RECKONING AND NO NAVAIDS AVAILABLE FOR AHARS POSITIONING. RAW DATA USED FOR NAV DIRECT TO EMI WHEN CLRED AND LATITUDE/LONGITUDE FOR EMI MANUALLY ENTERED INTO ACFT POS. GIVEN A FREQ CHANGE AND NEXT CTLR TOLD US TO PROCEED DIRECT MOL VOR. WE TURNED TO THIS COURSE AND AFTER A SHORT TIME WE WERE AGAIN GIVEN A HDG CHANGE BECAUSE WE WERE ENCROACHING ON DCA ARR AIRSPACE. ONCE AGAIN DEAD RECKONING WAS DISPLAYED ON NAV DISPLAY AND RAW DATA WAS USED TO NAV TO MOL VOR WHEN CLRED. ABOUT 50 NM N OF MOL VOR; NUMEROUS VORS APPEARED ON NAV SYS SO AHARS UNIT WAS ABLE TO NOW LOCATE POS ACCURATELY. LATITUDE/LONGITUDE POS WAS VERIFIED PASSING OVER MOL VOR. THIS SIT EXISTS IN THE BAL/DCA AREA FREQUENTLY IN GLASS COCKPIT ACFT; WHERE DEAD RECKONING ANNUNCIATES WITH NUMEROUS VORS AVAILABLE FOR POSITIONING. FOR SOME REASON THE AUTOMATION IS CONFUSED. WITH IRU EQUIPPED ACFT THIS IS NOT A BIG PROB; HOWEVER; AHARS ACFT BECOME COMPLETELY CONFUSED. THIS WAS THE WORST CASE I HAVE EXPERIENCED; HOWEVER. ADDITIONALLY; IF YOU DON'T CONTINUOUSLY MONITOR RAW DATA; THE DISPLAY INDICATES THAT EVERYTHING IS PERFECT. IT IS AN INSIDIOUS PROB THAT IS DIFFICULT TO RECOGNIZE. AHARS PERFORMS POORLY IN THIS CORRIDOR! CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATES THAT THE AHARS UNIT ROUTINELY DEGRADES TO DEAD RECKONING NAV IN THE NE CORRIDOR. IT IS HIS OPINION THAT THE AHARS UNIT IS INCAPABLE OF ASSIMILATING DATA FROM THE NUMEROUS SOURCES AVAILABLE. WHEN THE UNIT DEGRADES TO DEAD RECKONING; WIND CORRECTIONS ARE NOT PROVIDED; RESULTING IN OFF COURSE FLT. THE DEG OF DISPLACEMENT DEPENDS ON THE DIRECTION AND STRENGTH OF THE WINDS ALOFT. THE NAV DISPLAYS INDICATE ON COURSE UNLESS RAW DATA SELECTED. AHARS UNITS WILL NORMALLY FUNCTION IN AREAS OUTSIDE OF THE NE CORRIDOR. DEGRADATION TO DEAD RECKONING IS VERY SELDOM EXPERIENCED; OR LASTS ONLY MOMENTARILY; IN THESE AREAS.