HELI PLT IS CLRED THROUGH IAD CLASS D AIRSPACE BUT FAILS TO CROSS AT ATC ISSUED LOCATION.

Narrative

AFTER LEAVING ZZZ ENRTE JYO (APPROX 325 DEG HDG; 22 NM) AT 800-1000 FT MSL; I WAS TALKING TO IAD FOR CLRNC THROUGH ENRTE. I WAS FOLLOWING MY GPS READOUT IN THE HSI. DUE TO THEIR TKOF/LNDGS AND THE HAZY CONDITIONS; DULLES REQUESTED I FLY STRAIGHT FOR THE CTL TWR AND CROSS MIDFIELD AT ALT. I WAS 9 MI ENE OF DULLES AT THIS TIME. I STARTED LOOKING WSW FOR WHERE THE ARPT WOULD HAVE BEEN FROM MY ORIGINAL ROUTING. BECAUSE OF THE HAZE; I COULD NOT MAKE IT OUT; SO I PROGRAMMED IAD INTO OUR GPS. AT THAT TIME; I LOOKED UP TO SEE DULLES 4 MI TO MY N. AFTER MY SHOCK WORE OFF; AND I CALLED DULLES WHO TOLD ME TO JUST MAINTAIN CURRENT HDG; I REALIZED THAT MY HSI WAS GREATER THAN 30 DEGS OFF OF MY MAGNETIC COMPASS HDG. THIS HAS NEVER HAPPENED BEFORE. I IMMEDIATELY LOOKED TO ENSURE I WAS IN THE SLAVED MODE. THERE IS NO PLT FIX OR SYNC SWITCH. ODDLY; EVEN THOUGH DULLES KNEW WHERE I STARTED IN THEIR AIRSPACE (E GOING NE); AND KNEW THEY REQUESTED I FLY OVER THE TOP; AND SAW ME WHEN I WAS 4 MI DUE S; THEY NEVER QUERIED ME. WE ARE CURRENTLY WORKING THE ISSUE WITH MAINT; BUT BELIEVE THE FLUX VALVES WHICH FEED THE HSI INFO NEED REPLACING. THE HAZE PREVENTED ME FROM POSSIBLY PICKING UP THIS ERROR SOONER. I AM ALSO ONLY 1 MONTH NEW TO THIS AREA; SO GND REF POINTS ARE STILL NEW TO ME. THE INCIDENT IS ONLY THAT DULLES ASKED ME TO CROSS OVER THE TWR AND I CROSSED 4 MI S OF THE ACTIVES.