A JS31 CREW; ON DSCNT TO FRG; OVERSHOT THEIR ASSIGNED ALT.

Narrative

NEW YORK APCH CLRED US TO DSND TO 4000 FT MSL FROM OUR CRUISE ALT OF 7000 FT MSL. THE FO; WHO WAS THE PF; DSNDED THROUGH OUR ASSIGNED ALT OF 4000 FT MSL TO 3000 FT MSL. PRIOR TO THE EVENT; I INFORMED THE FO THAT I WOULD BE OFF THE PRIMARY FREQ TO MAKE A REQUIRED PAX BRIEFING. WHEN I RETURNED TO THE PRIMARY FREQ; I OBSERVED WE WERE DSNDING DOWN TO 3000 FT MSL. I QUESTIONED THE FO IF NEW YORK APCH CLRED US TO A LOWER ALT; HE RESPONDED IN DOUBT. AT THAT TIME; NEW YORK APCH ASKED US WHAT ALT WE WERE DSNDING TO. I REPLIED WE WERE AT 3000 FT MSL; AND ASKED THE CTLR WHAT ALT HE WANTED US AT. HE TOLD US WE WERE ASSIGNED 4000 FT MSL BUT JUST STAY AT 3000 FT MSL. SHORTLY AFTER; WE WERE SWITCHED TO ANOTHER NEW YORK APCH CTLR. THE ALTDEV DID NOT RESULT IN ANY IMMEDIATE ACTIONS TO TAKE PLACE. NO FURTHER INCIDENT. THE OPERATOR HAS A FLEET OF 2 BAE3100 JETSTREAMS. BOTH HAVE NO AUTOPLT. ONLY ONE HAS AN ALT ALERTER. FOR THE SAKE OF STANDARDIZATION WITH OTHER ACFT; THE ALERTER WAS NOT USED. AT THE TIME OF THE EVENT; WE WERE ON DUTY FOR 11 HRS. PRIOR TO THE FLT IN QUESTION; WE HAD AN 8 HR 30 MIN LAYOVER (ON DUTY) WHICH REQUIRED US TO STAY AT THE FBO FACILITY. SUPPLEMENTAL INFO FROM ACN 546540: I FEEL THAT THIS IS A SIT THAT IS VERY EASY TO GET INTO -- ESPECIALLY FLYING AN EXTREMELY COMPLEX AIRPLANE SINGLE PLT WITHOUT AID OF AN AUTOPLT. THIS IS A SHORT FLT IN WHICH WE FLY ABOUT 5 TIMES A WK; AND THE FACT THAT ONE FLT IS A CARBON COPY OF THE LAST; ONE GETS USED TO THE SAME THINGS TIME AND AGAIN. EVEN THOUGH THE CTLR SAID CLRED TO 4000 FT; I HEARD 3000 FT; THE ALT GIVEN 99% OF THE TIME. I CAN PREVENT THIS BY COMPLETING ALL DSCNT CHKLIST ITEMS BEFORE THE DSCNT. THIS WAY; BOTH PLTS CAN BE WATCHING THE PROGRESS OF THE ACFT IN SUCH A CRITICAL PHASE OF FLT.