FLC OF MDT CHARTER ACFT ON VISUAL APCH AT DCA IS TOLD AFTER LNDG THAT THEY ENTERED RESTR AREA P56.

Narrative

WE WERE CLRED FOR THE RIVER VISUAL APCH FOR RWY 18. FROM A HDG OF 180 DEGS; ASSIGNED BY ATC; WE INTERCEPTED THE RIVER APPROX 9-10 DME FROM DCA. WHILE WE WERE ON THE APCH AT APPROX 4 DME FROM DCA; ATC TOLD US TO FLY ON THE L SIDE OF THE RIVER. WE ADJUSTED OUR FLT PATH ACCORDINGLY SO THAT WE WERE ON THE L SIDE OF THE RIVER. AT APPROX 3 DME ATC ADVISED US TO SQUARE THE BASE TO FINAL FOR SPACING. AS WE CONTINUED ATC ASKED; 'DO YOU HAVE THE TFC ON FINAL FOR RWY 18 AND RWY 21?' WE RESPONDED TO ATC REQUEST THAT WE HAD THE TFC FOR BOTH RWYS IN SIGHT. WE CONTINUED THE APCH AND LANDED WITHOUT INCIDENT. ABOUT 45 MINS AFTER WE LANDED (XX45Z) WE WERE CONFRONTED BY THE LCL AUTHS AND SECRET SVC AGENTS. AT THAT TIME WE WERE TOLD WE MIGHT HAVE ACCIDENTALLY PENETRATED PROHIBITED AREA P56. CONTRIBUTING FACTORS: 1) FLYING ON THE L SIDE OF THE RIVER THAT WAS ASSIGNED BY ATC. 2) WITHIN 1-2 MI FROM LNDG WE WERE LOOKING FOR TFC THAT WAS LNDG ON RWY 18 AND RWY 21 AS WELL AS NAVING AND MAKING FINAL PREPARATIONS TO LAND. 3) INADEQUATE SPACING THROUGHOUT THE ENTIRE APCH. CORRECTIVE ACTION: THE MOST SIGNIFICANT FACTOR THAT COULD HAVE PREVENTED THIS SIT WOULD HAVE BEEN FOR ATC NOT TO HAVE ALTERED THE RIVER VISUAL APCH TO RWY 18. IF THERE WAS INADEQUATE SPACING FOR SIMULTANEOUS LNDGS ATC SHOULD HAVE MADE US GAR OR DO NOT ALLOW SIMULTANEOUS LNDGS ON INTERSECTING RWYS. IF THE APCHING ACFT HAD BEEN PROPERLY SPACED BY ATC; MY ACFT WOULD NOT HAVE PENETRATED THE PROHIBITED AREA. SUPPLEMENTAL INFO FROM ACN 388503: CORRECTIVE ACTION: ATC SHOULD NOT ALTER AN ALREADY 'BUSY' APCH. SPACING WAS PROBABLY INADEQUATE AND TWR WAS CREATING A TFC JAM. WE HAD A SLOT TIME FOR XX00Z AND TOUCHED DOWN AT THAT TIME. WHY ISSUE SLOT TIMES IF SPACING IS INADEQUATE?