VISUAL APCH TO 18 DCA; PENETRATED P56.

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GOING INTO WASHINGTON NATIONAL ARPT (DCA) IN A LTT; WE WERE INFORMED THAT WE WERE #1 FOR THE APCH INTO DCA AND WERE TOLD TO KEEP OUR SPD UP FOR SPACING. WE WERE CLRED AND STARTED THE RIVER VISUAL APCH TO RWY 18. AT 3.0 DME; WE HAD NOT SIGHTED THE ARPT DUE TO THE HAZE AND RAIN AND THEN WE WERE TOLD TO EXPECT RWY 15 INSTEAD OF 18 AS PREVIOUSLY CLRED. AT ABOUT 2.5 DME WE PICKED UP THE ARPT AND ADVISED TWR WE NEEDED 18. WE WERE THEN CLRED TO LAND 18; BUT I NOTICED THAT THERE WERE WAS AN AIRLINER HOLDING IN POS ON 18 FOR DEP. WE ADVISED TWR WE NEEDED TO MAKE A LEFT 270 TO REENTER FINAL FOR 18 FOR SPACING TO GET THE AIRLINER OFF THE RWY. WE WERE GIVEN A HDG OF 090 DEG AND CLB TO 2000 MSL. DURING SOME POINT IN THE FINAL 3 DME PHASE OF THE APCH; WE WERE LATER INFORMED WE ENTERED PROHIBITED AREA P-56. IN MARGINAL CONDITIONS; THE RIVER VISUAL DOES NOT ALLOW ENOUGH PROTECTED AIRSPACE BTWN THE FINAL APCH COURSE AND PROHIBITED AIRSPACE AS LONG AS THE LDA 15. IT SHOULD BE USED INSTEAD OF FOLLOWING A WINDING RIVER. IN A HIGH WORK ENVIRONMENT WITHIN 3 MI OF THE ARPT; THE TWR SHOULDN'T CHANGE RWYS BECAUSE WE HAVE TO WORK UP LNDG #S FOR THE NEW RWY; ESPECIALLY WHEN IT IS WET AND THE WINDS ARE CHANGING TO A TAILWIND. I ALSO BELIEVE THAT THE TWR RADAR SHOULD BE WATCHING THESE RIVER APCHES SO THEY CAN WARN PLTS IF THEY ARE NEARING A PROHIBITED AREA.