ALTDEV ALT OVERSHOT IN CLB. POTENTIAL CONFLICT AVOIDED BY ALERT TRACON DEP CTLR INTERVENTION WITH A RADAR VECTOR AND DSCNT CLRNC.

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IT WAS THE 4TH LEG OF THE DAY FROM STEWART FIELD (SWF) TO SYRACUSE (SYR). I WAS THE PF: DEP FROM SWF WAS ON RWY 1; UP TO 6000 FT AND L TO 340 DEGS. CLBING THROUGH 5500 FT; CTLR GAVE US DIRECT HANCOCK VOR (HVR) AND I THOUGHT I HEARD 'CLB AND MAINTAIN 10000 FT.' AT THIS TIME IN THE COCKPIT; THE CAPT WAS REACHING OVER IN THE COCKPIT FOR SOME REASON; EITHER CHANGING FREQS ON COM 2 OR CHANGING THE ALT IN THE ALT ALERTER. IN ANY CASE; I COULD NOT SEE THE VALVE IN THE ALT ALERTER. CLOSING ON 6000 FT; I QUERIED THE CAPT AS TO THE ALT; AS I COULD NOT USE IT IN THE ALERTER. BY THIS TIME WE WERE AT 6000 FT AND THE CAPT CALLED THE CTLR TO CONFIRM ALT. UNFORTUNATELY; SEVERAL FACTORS CAUSED A POSSIBLE ALTDEV OF APPROX 500 FT (6500 FT). I HAD A HIGH PWR SETTING WHICH GAVE US BTWN 2000- 3000 FPM CLB -- SINCE I THOUGHT I HEARD A CLRNC TO 10000 FT; I KEPT THE CLB GOING BUT REDUCED IT SLIGHTLY UNTIL I CONFIRMED THE ALT WITH THE CAPT. AFTER TRYING TO CONFIRM THE ALT WITH THE CAPT; HE WAS UNSURE AS WELL AND QUERIED THE CTLR. THE CAPT HAS A SLIGHT STUTTER PROB AND THE EXCHANGE OF WORDS TOOK LONGER THAN IT SHOULD HAVE. BY THIS TIME I WAS CLOSING ON APPROX 6500 FT. THE CTLR IMMEDIATELY TOLD US TO DSND TO 6000 FT AND GAVE US A VECTOR. I CUT THE PWR AND DSNDED. THE INAPPROPRIATE (ALTHOUGH USUAL) USE OF COM 2 TO CALL COMPANY BLOW 10000 FT ALSO COULD HAVE BEEN A FACTOR. THE CAPT'S HAND WAS BLOCKING THE ALT ALERTER -- I WOULD HAVE STAYED AT 6000 FT IF I SAW IT IN THE BOX. I WAS ON MY 6TH DAY OF FLYING AND WAS SLIGHTLY FATIGUED. ALTHOUGH I CANNOT BLAME THE PROB ON BEING TIRED; IT MAY HAVE BEEN A FACTOR. ON FEB/XX/95 I CALLED NEW YORK TRACON AND SPOKE WITH SUPVR AS WELL AS MR X; WHO WAS THE CTLR ON FEB/XX/95 FOR THE SECTOR. THEY SAID WE PASSED WITHIN 3 3/4 MI OF AN ACR JET -- SAID IT WAS NO PROB AND THAT THERE WAS NO CONFLICT. I ALSO FOUND OUT THAT WE WERE IN THE MIDDLE OF BEING HAULED OFF TO ZBW AND THE CTLR DIDN'T KNOW IF WE WERE WITH HIM.