A COMMUTER CAPT RPTS EXCEEDING PRESCRIBED SPD LIMITATIONS AFTER DSNDING BELOW CLASS B AIRSPACE DURING DSCNT INTO PHL. THE APCH CTLR QUERIED THE CREW ABOUT THE ACFT'S SPD BEFORE THE FO COULD SLOW TO THE MAX 200 KIAS RESTR. THIS RPT ALSO CONTAINS RPTR COMMENTS ABOUT COMMUTER ATC HANDLING AT PHL AND BWI.

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MY NEWLY HIRED FO HAD THE CTLS AS WE APCHED THE OUTER BOUNDS OF THE PHL CLASS B AIRSPACE. WE WERE IN A DSCNT TO 4000 FT AT AN IAS OF APPROX 230 KTS. UPON RECEIVING A CLRNC TO CONTINUE DSCNT TO 3000 FT; I ALERTED THE FO THAT WE WOULD BE BENEATH CLASS B AIRSPACE WHEN BELOW 4000 FT AND WITHIN 20 NM OF THE FIELD. I ASSUMED HE WOULD RECOGNIZE THE NEED TO SLOW TO THE REQUIRED 200 KT AIRSPD FOR FLT BENEATH THE CLASS B AIRSPACE. SEEING NO RESPONSE AS WE APCHED 4000 FT; I REMINDED HIM THAT WE NEEDED TO SLOW TO 200 KTS FOR FLT BELOW THE CLASS B AIRSPACE. HE GRADUALLY REDUCED THE PWR; THE ACFT WAS SLOW TO RESPOND; AND WE PASSED THROUGH 4000 FT STILL INDICATING NEARLY 230 KTS. AS WE PASSED THROUGH APPROX 3700 FT; ATC ASKED US TO RPT OUR AIRSPD TO WHICH I RESPONDED '210 KTS AND SLOWING.' SHORTLY THEREAFTER WE SLOWED THROUGH THE 200 KT LIMIT. I PROBABLY SHOULD HAVE BEEN MORE PRECISE IN MY INITIAL COM WITH MY FO; STATING RIGHT AWAY THAT WE WOULD NEED TO SLOW TO 200 KTS OR LESS; PRIOR TO DSNDING BELOW 4000 FT. AS AN ADDITIONAL NOTE; WE ARE FREQUENTLY PLACED BELOW CLASS B AIRSPACE UPON ARR AT BOTH PHL AND BWI. IT WOULD SEEM THAT; IN THE INTEREST OF SAFETY; LARGE TURBOPROPS CAPABLE OF NEARLY 250 KIAS SHOULD ALWAYS BE KEPT WITHIN CLASS B AIRSPACE DURING ARR AT SUCH LOCATIONS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: ACFT WAS ON A VECTOR TO INTERCEPT THE LOC. THIS CAPT STATES THAT ANY SIMILAR CLRNCS BELOW CLASS B PLACE THE HIGHER SPD DH8 IN A VULNERABLE AND DANGEROUS MIX WITH SLOW VFR TFC SKIRTING THE CONFINES OF THIS AIRSPACE. WITH GROWING TFC CONGESTION; THIS RPTR HAS NOTED THAT GREATER PRESSURE IS PLACED UPON CTLRS TO USE TECHNIQUE/ATC METHODS THAT ARE NOT NECESSARILY SAFE. THIS RPTR'S GREATEST CONCERN IS OVER THE USE OF VISUAL SEPARATION CLRNCS; 'WHEN THE OTHER ACFT IS DOING THE MANEUVERING; YOU CANNOT JUDGE WELL; WHAT THE OTHER ACFT IS ACTUALLY DOING' IN ORDER TO MAINTAIN VISUAL AND WAKE SEPARATION.