A B757 FLC FLYING A CHARTED VISUAL APCH ON RADAR VECTORS IS TOLD THEY HAVE DSNDED BELOW THE PROPER ALT AND TO CLB TO 3000 FT. THEY COMPLY.

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ON RADAR VECTORS FOR BAY VISUAL APCH RWY 16R SEA; CLRED BAY VISUAL 'INSIDE' SEA 320 DEG RADIAL; 10 DME; AT APPROX 3000 FT WITH A SPD RESTR OF 170 KTS TILL BOEING FIELD. A DSCNT FOR LNDG WAS STARTED WITH A TURN TOWARD THE FINAL APCH COURSE. AT APPROX 2400 FT; THE CTLR COMMANDED A CLB TO 3000 FT; POINTING OUT VFR TFC; AND THEN STATED WE WERE SUPPOSED TO BE AT 3000 FT. THE VFR TFC WAS SIGHTED AND NO CONFLICT WAS NOTED. THE SUBSEQUENT APCH AND LNDG WAS UNEVENTFUL. APCH CTL WAS CONTACTED BY PHONE. A SUPVR STATED THAT THE CTLR EXPECTS A BASE LEG (90 DEGS TO INTERCEPT TO FINAL) AND TO MAINTAIN 3000 FT UNTIL INTERCEPTING FINAL -- PRESUMABLY TO PREVENT CONFLICTS WITH BOEING FIELD TFC. THERE ARE NO ALT RESTRS ON THE BAY VISUAL TO REFLECT THIS; NOR DID IT APPEAR THAT OUR ACFT CAME IN CONFLICT WITH TCA LIMITS. IF A TRUE CONFLICT WITH BOEING VFR TFC IS POSSIBLE; I SUGGEST THE ALT RESTRS BE DISPLAYED AND THE COURSE DEPICTED TO SHOW A 90 DEG INTERCEPT ANGLE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THAT THERE HAS BEEN NO FURTHER FOLLOW-UP. SINCE THE FLT WAS UNDER RADAR VECTORS; THERE WAS NO WAY TO MAKE A 90 DEG INTERCEPT TO THE FINAL APCH. THE CREW DID SEE THE BOEING FIELD TFC BELOW AND ABOUT 1 MI TO THE R. THEY FELT COMFORTABLE WITH THE SEPARATION. RPTR SAID THAT THE VISUAL APCHS FOR TFC FROM THE S DO HAVE ALT RESTRS INDICATED. HE FEELS MORE GUIDANCE IN WRITING IS NEEDED IF CTLRS 'EXPECT' CERTAIN ALTS TO BE USED. THERE ARE VISUAL APCHS WHERE ONE CAN DSND BELOW THE CLASS B AIRSPACE WHEN ON A VISUAL. LAS IS ONE OF THESE; AND IF EXPECTED TO REMAIN ABOVE THE CLASS B; IT IS SO STATED. SUPPLEMENTAL INFO FROM ACN 432603: APPROX 9 NM FROM SEA VORTAC; DSNDED TO 2400 FT MSL. AT APPROX 7 DME; CTLR POINTED OUT VFR TFC AT 1 O'CLOCK AND 1500 FT. THEN HE STATED WE SHOULD HAVE BEEN AT 3000 FT MSL. WE CLBED TO 3000 FT MSL; THE MADE NORMAL VISUAL APCH/LNDG AT SEA. DURING A PHONE CONVERSATION WITH THE CTLR; HE STATED THAT APCH 'EXPECTS' A BASE TURN TO FINAL AND 'EXPECTS' ACFT TO MAINTAIN 3000 FT UNTIL INTERCEPTING FINAL. THE SEA BAY VISUAL APCH TO RWY 16R HAS NO ALT RESTRS; AND APCH SHOULD NOT CLR AN ACFT BASED ON EXPECTATIONS CONTRARY TO REQUIREMENTS.