FLC OF AN LTT TURBOPROP EXCEEDED MAX AIRSPD BELOW CLASS B AIRSPACE. APCH CTLR INTERVENED; AFTER APPROVING THEIR REQUEST TO DSND SLOWLY TO A LOWER ALT; TO ALERT THEM REGARDING THEIR SPD EXCEEDING THAT PERMITTED.

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DURING A SCHEDULED PAX FLT FROM PAH TO MEM; ACFT WAS FLOWN AT 250 KTS BELOW CLASS B AIRSPACE. THE ACFT INVOLVED HAD BEEN DISPATCHED WITHOUT A PRESSURIZATION SYS PER MEL. DUE TO PAX COMFORT I DECIDED TO BEGIN AN EARLY DSCNT TO ALLOW FOR A 300 FPM DSCNT RATE. AFTER GETTING ATC CLRNC TO 3000 FT THE FO PROCEEDED TO DSND FROM 5000 FT TO 3000 FT AT 250 KTS THEN CONTINUED WHILE ENTERING THE CLASS B AIRSPACE VEIL. I WAS IN THE PROCESS OF TALKING TO MEM COMPANY OPS ON THE OTHER RADIO SINCE WE HAD NOT BEEN ABLE TO GET THEM AT THE LOW ALTS. AFTER RETURNING TO ATC I WAS QUESTIONED ABOUT OUR SPD. AFTER RPTING 250 KTS ATC THEN ADVISED WE WERE BELOW CLASS B AIRSPACE. I IMMEDIATELY HAD THE FO SLOW TO 200 KTS PER FARS. APPROX 1 MIN LATER ATC ADVISED WE HAD RE-ENTERED CLASS B AND TO RESUME NORMAL SPD. FLT CONTINUED TO NORMAL LNDG. WITH MULTIPLE DISTRACTIONS THE FACT THAT WE WERE BELOW CLASS B WENT UNNOTICED UNTIL ADVISED BY ATC. THESE FACTORS INCLUDED ACFT COMPONENT INOP: REQUIRING NON STANDARD ARR LOWER THAN NORMAL ALTS; WHICH ALSO MADE RADIO RECEPTION A PROB LEADING TO ANOTHER DISTR DURING APCH. THIS PROB COULD BE PREVENTED BY REQUIRING ATC TO ADVISE PRIOR TO OPERATING UNDER CLASS B AIRSPACE EITHER PENETRATING FROM OUTSIDE THE RADIUS OR DSNDING BELOW THE FLOOR. ALSO CLASS B AIRSPACE AND FLOORS COULD BE DEPICTED ON SIDS; STARS AND APCH CHARTS INSTEAD OF A SEPARATE CHART TO MAKE THEM MORE OBVIOUS. OR THE AIRSPD LIMITS COULD BE MADE UNIFORMLY IN CLASS B AND BELOW.