FK100 CREW ENCROACHED ON PROHIBITED AREA P56-B DEPARTING DCA.

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FLT DEPARTED DCA ON STANDARD NOISE ABATEMENT PROC 328 DEGS DCA. THE FMS HAD PROGRAMMED DCA -- DCA/328 DEGS 20 DME FOR A COURSE LINE FOR THE PRESCRIBED PROC. ACFT DEPARTED RWY 36. AT 400 FT; NAV SELECTED AND A TURN WAS INITIATED. AT 600 FT AUTOPLT SELECTED AND COURSE CAPTURED BY 1000 FT AGL WITH 0.0 DEV DISPLAYED. THIS COURSE WAS FOLLOWED FOR APPROX 12-15 MI FOLLOWED BY A TURN (BY ATC) TO A 090 DEG HDG. CREW WAS ADVISED TO CALL DCA RADAR WHEN ON THE GND. WE WERE ADVISED THAT WE HAD ENCROACHED ON P56-B AIRSPACE. WITH NO VISUAL REF TO THE GND; ONLY OUR INSTS WERE AVAILABLE FOR COURSE GUIDANCE AND ALL INDICATIONS; AS PREVIOUSLY STATED; WERE THAT OUR ACFT WAS ON COURSE. OUR HDG WAS CONSISTENT WITH A HDG NECESSARY TO TRACK THE 328 DEG COURSE AS WELL AS RAW DATA INDICATIONS. THE 328 DEG RADIAL DCA; WHEN FOLLOWED; DOES POS AN ACFT VERY NEAR P56-B. WITHOUT VISUAL REF TO FOLLOW THE POTOMAC RIVER; THE RADIAL MUST BE USED. WITH ALL COCKPIT INDICATIONS NORMAL AND 0.0 ERROR FOR TRACKING DISPLAYED; I CANNOT UNDERSTAND HOW AN ENCROACHMENT COULD HAVE OCCURRED.