ALTDEV ALT OVERSHOT IN DSCNT.

Narrative

DURING OUR ARR INTO NEWBURGH; NY (SWF); ZNY GAVE US A DSCNT CLRNC TO 6000 FT. I WAS THE PF; AND THE FLT DIRECTOR AND AUTOPLT WERE IN USE AT THE TIME. PASSING 7000 FT; I CHOSE TO MAINTAIN OUR 1000 FPM RATE OF DSCNT (RATHER THAN DECREASE IT TO A NORMAL 500 FPM) IN ORDER TO GET MORE QUICKLY BELOW A BROKEN CLOUD LAYER THAT WAS PROVIDING LIGHT TO MODERATE TURB. AS WE PROCEEDED THROUGH OUR ARR CHKLIST; ATC SUDDENLY ALERTED US TO 'MAINTAIN 6000 FT.' AS MY FO ACKNOWLEDGED; I REALIZED WE WERE DSNDING THROUGH 5600 FT. I IMMEDIATELY DISCONNECTED THE AUTOPLT AND RETURNED THE ACFT TO 6000 FT. WE ACHIEVED A MINIMUM ALT OF 5500 FT. ASSUMING THE AUTOPLT DID NOT MALFUNCTION; I APPARENTLY HAD FAILED TO SELECT THE ALT SELECT MODE ON THE FLT CTLR (OR HAD SELECTED IT TWICE; CAUSING THE MODE TO BE CANCELLED); RESULTING IN A FAILURE TO CAPTURE THE SELECTED ALT. FACTORS THAT MAY HAVE CONTRIBUTED TO THE INCIDENT INCLUDE: 1) THE TURB WE EXPERIENCED MAY HAVE CAUSED ME TO FAIL TO PRESS THE ALT SELECT BUTTON HARD ENOUGH TO ENGAGE IT; OR MAY HAVE CAUSED ME TO INADVERTENTLY PRESS IT TWICE; THEREBY CANCELLING THE MODE. 2) BOTH PLTS WERE OCCUPIED WITH EXECUTION OF THE ARR CHKLIST AT THE TIME. 3) BOTH PLTS WERE TIRED AND FATIGUED. IT WAS THE 5TH LEG OF THE DAY; OUR ORIGINATING FLT HAD BEEN DELAYED 30 MINS; AND WE HAD BEEN PLAYING CATCH UP ALL DAY WITH NO BREAKS. 4) THE FO HAD BEEN MAKING REQUIRED ALT CALLOUTS ONLY INTERMITTENTLY DURING THE DAY; AND IN ORDER TO AVOID CONFLICT; I HAD NOT DEMANDED THAT HE BEGIN CONSISTENTLY MAKING THEM. SUPPLEMENTAL INFO FROM ACN 246123: RADIO CALL FROM ATC AND THEN ALT SELECT WAS NOT PRESSED. ACFT DSNDED TO 5600 FT WHEN ATC REMINDED US OF OUR ALT. THAT RADIO CALL FROM ATC WHICH LED TO OTHER CONVERSATIONS BTWN CAPT AND MYSELF WHICH LED TO THE ALT SELECT NOT BEING PUSHED.