UNAUTH PENETRATION OF CLASS B AIRSPACE BY AN EMB120 FLC IN A NIGHT OP. ALTDEV ALT UNDERSHOT IN DSCNT.

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I WAS CAPT OF A PART 135 FLT FROM QRH TO EWR. WHILE ON RADAR VECTORS AND OPERATING VFR; I RECEIVED CLRNC FROM NY APCH CTL TO 'ENTER THE CLASS B AT 2000 FT AND REMAIN CLR OF THE WESTCHESTER CLASS D.' (WE WERE 'GOING DOWN THE RIVER.' A COMMON APCH INTO EWR WHEN IT IS VFR. WE FLY DOWN THE HUDSON RIVER AT 2000 FT TO THE STATUE OF LIBERTY AND THE TURN INTO EWR. IT IS SPECTACULAR FOR THE PAX AS WELL AS THE CREW.) ADDITIONALLY; WE RECEIVED A TA REGARDING A SAAB 340 WHO WAS DSNDING INTO WESTCHESTER. THE SAAB WAS AT OUR 5 O'CLOCK POS AND ABOUT 1000 FT BELOW US. THE FO WAS FLYING AND WAS CONCERNED; AND JUSTIFIABLY SO; ABOUT DSNDING TOO CLOSE TO THE SAAB. I WAS CONCERNED ABOUT DSNDING INTO THE WESTCHESTER CLASS D. (THE TOP OF WHICH IS 2900 FT MSL.) WE LEVELED OFF AT 3000 FT AND WE WERE WATCHING THE SAAB AS WELL AS ANOTHER ACFT THAT WAS ON FINAL APCH TO THE S INTO WESTCHESTER. I WAS TRYING TO DETERMINE IF WE WERE CLR OF WESTCHESTER AIRSPACE WHEN THE CTLR BARKED 'OUR CALL SIGN AND ASKED WHAT IS YOUR ALT!?' I RESPONDED '3000 FT' TO WHICH THE CTLR; IN A VERY STERN AND UPSET VOICE; REPLIED 'I TOLD YOU TO ENTER THE CLASS B AT 2000 FT NOT 3000 FT. I HAVE IFR TFC DEPARTING WESTCHESTER CLBING TO 3000 FT. DSND NOW TO 2000 FT.' WE STARTED OUR DSCNT AND I CHKED THE TCASII AND SAW NO TFC ON THE TCASII. I ADVISED THE CTLR THAT WE WERE ATTEMPTING TO REMAIN CLR OF THE CLASS D AND OF THE APCHING TFC. I REALIZED AT THAT TIME THAT I HAD MADE A MISTAKE. I DID NOT MONITOR OUR POS RELATIVE TO THE CLASS B. THE BASE OF THE CLASS B IN OUR AREA WAS 3000 FT. WE WERE 18.6 NM FROM THE LGA VOR. THE EDGE OF THE CLASS B IS AT 20 NM. HUMAN FACTORS CONSIDERATIONS: 1) I WAS TIRED FOR SOME REASON. I HAD HAD PLENTY OF SLEEP THE NIGHT BEFORE BUT I WAS STILL TIRED. 2) MY FO WAS RELATIVELY NEW. HE HAD NEVER GONE 'DOWN THE RIVER.'