DC9 ACFT CLRED DIRECT TO DCA ENTERED P56 INADVERTENTLY. APPARENT RPTR RADIO PROB PREVENTED ATC FROM VECTORING ACFT AROUND THE PROHIBITED AREA.

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FLYING AS FO IN R SEAT; CAPT IN L SEAT; OBSERVER IN JUMP SEAT (JUMP SEATING HOME). CAPT FLYING ACFT; MYSELF ON THE RADIOS. WHILE ENRTE TO DCA FROM EWR; WAS CLRED BDL DIRECT TO DCA. WAS DSNDED TO 5000 FT MSL INBOUND TO DCA. ATIS HAD GIVEN MT VERNON VISUAL AS APCH IN USE AND APCH PLATE WAS OUT IN ANTICIPATION. CAPT (FLYING) HAD MENTIONED THE PROHIBITED AIRSPACE AND A RECENT ARTICLE ABOUT A CPR PLT VIOLATING IT. JUMP SEATING CAPT WAS POINTING OUT DC AREA LANDMARKS OF P56 AREA; AS I HAD NEVER APCHED DCA FROM THE NE. I HAD A VISUAL ON WASHINGTON MONUMENT; WHITE HOUSE; BUT HAD NOT YET SEEN THE CAPITOL BUILDING. AT THIS TIME WE WERE 5000 FT ON ABOUT A 230 DEG HDG. JUMP SEATING CAPT MADE A COMMENT THAT APCH CTLR SHOULD BE TURNING US SOON TO AVOID P56 AND THEN FOLLOWED UP THAT WE SHOULD NOT GO ANY CLOSER AS WE WERE ABOUT 3 DME. I CALLED APCH TO ASK IF THEY 'HAD A TURN FOR US.' NO RESPONSE. I MADE ANOTHER CALL; NO RESPONSE. CAPT BEGAN A TURN TO THE S. I SWITCHED RADIOS; MADE ANOTHER CALL; 'DO YOU HAVE A TURN FOR US?' ATC RESPONDED; 'WHERE HAVE YOU BEEN? I'VE BEEN TRYING TO REACH YOU; TURN L HDG 210 DEGS.' I REPLIED WE WERE ALREADY THERE (CAPT HAD TURNED). REMAINDER OF APCH WAS UNEVENTFUL. UPON LNDG; GND CTL TOLD US TO CALL ATC. UPON FURTHER REVIEW; IT IS EVIDENT THE MT VERNON VISUAL DOES NOT DICTATE A TURN POINT TO AVOID THE P56 AREA. A LOST COM ACFT; WHICH WE WERE; WOULD FLY DIRECT TO DCA AND OVERFLY THE EASTERN EDGE OF P56. THERE IS NO TURN SPECIFIED IN THE MT VERNON VISUAL APCH.