A CL65 FLT CREW MAY HAVE DSNDED TO THE WRONG ALT WHILE ON APCH TO IAD.

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ON A VECTOR AND DSCNT INTO WASHINGTON DULLES; WE WERE ASKED WHAT ALT WE WERE DSNDING TO. I RESPONDED 3000 FT; AS OUR ALT PRESELECT WAS SET TO. APCH (POTOMAC) RESPONDED; WE SHOULD CLB TO 4000 FT. THE CAPT SWORE WE WERE GIVEN 3000 FT. I; THE PNF; DO NOT REMEMBER THE ALT APCH HAD GIVEN; BUT THE PRESELECT WAS SET AT 3000 FT; SO WE MUST HAVE VERIFIED THE ALT WAS CORRECT. WE; I RESPONDED TO ATC; 'YOU GAVE US 3000 FT.' HE SAID; 'I MUST HAVE TAKEN THE WRONG ALT FROM ANOTHER ACFT.' 2 PROBS HERE: FIRST; WE ARE FREQUENTLY GIVEN ALTS BY APCH THAT PUT US UNDER THE CLASS B AIRSPACE WITH A SPD RESTR ABOVE 200 KTS AND LEFT TO US TO SOMEHOW FIGURE OUT WHERE THE AIRSPACE IS. SECOND; WE ARE REQUIRED BY OUR COMPANY TO CALL 'IN RANGE' WITH OUR ARPT FACILITIES. BOTH TAKE AWAY TIME AND ATTN OF THE MOST IMPORTANT THING 'FLYING THE ACFT.' WE NEED SOME SORT OF GRAPHICAL DEPICTION ON OUR FMS AS TO AIRSPACE AND ALTS AND WE SHOULD ONLY CALL OUR COMPANY FOR ISSUES THE COMPANY DOES NOT KNOW ABOUT. ALSO; THE COM SYS MUST BE IMPROVED. IF WE HAD TAKEN ANOTHER ALT FOR ANOTHER ACFT; SOMEONE SHOULD HAVE KNOWN.