WDB WITH IOE CAPT HAS ALTDEV ON DSCNT.

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IOE IN PROGRESS; WITH PF IN L SEAT AND CHK AIRMAN IN R SEAT AS PNF. FLT WAS LEVELING AT FL200 AND IN THE PROCESS OF INTERCEPTING THE 225 DEG RADIAL; 15-20 MI S OF KINGSTON FROM A VECTOR ONTO THE KINGSTON 6 ARR INTO KENNEDY. ZBW ISSUED CLRNC (TO THE BEST OF OUR RECOLLECTION) 'IDENT HVY; CLRED DIRECT TO LA GUARDIA; DIRECT KENNEDY; DSND TO FL190.' PNF SET 19000 IN ALT SELECT WINDOW; SELECTED ALT SELECT; AND READ BACK CLRNC; INCLUDING LEAVING FL200 FOR FL190; AS PF DISCONNECTED AUTOPLT; COMMENCED DSCNT; AND STARTED TURN. PNF THEN BEGAN TO SET UP LGA VOR FREQS; HDG; AND COURSES ON GLARE SHIELD MODE CTL PANEL. BEFORE WE REALIZED IT; ALT ALERT GAVE AURAL AND VISUAL DEV ALERT AT FL187. PNF COMMANDED TO PULL UP TO FL190. A FEW SECONDS LATER; ZBW ALSO COMMANDED LEVEL OFF AT FL190. ACFT WAS PROMPTLY CLBED BACK TO FL190 WITHOUT ANY APPARENT TFC CONFLICT. BOTH PLTS IN THIS CASE HAVE HIGH DEG OF ALT AWARENESS AND HIGH COMPLIANCE WITH VERBALIZING ALT CLRNCS AND CALLOUTS. PF HAD MISHEARD ALT CLRNC AS 17000. WHEN HE LOOKED TO CONFIRM VIA THE ALT SELECT SETTING; IT WAS OBSCURED BY HAND OF PNF WORKING ON THE GLARE SHIELD PANEL. BECAUSE THE FLT DIRECTOR WAS NOT IMMEDIATELY GIVING PROPER ROLL COMMAND; PF HAD TURNED OFF FLT DIRECTOR; THEREBY REMOVING ALT SELECT PITCH COMMAND. BOTH THE PNF AND SO MISSED THE FAILURE TO LEVEL OFF AT FL190. (POSSIBLY BECAUSE OF HIGH WORKLOAD OR DISTR? ARR RWY; ATIS?) THIS EVENT TRIGGERED CONSIDERABLE SOUL SEARCHING; AS BOTH PLTS PRIDE THEMSELVES ON THEIR GOOD HABITS AND ALT AWARENESS. SOME LESSONS MIGHT BE: EVER MORE DILIGENT ATTN TO ALT. DISADVANTAGE OF DISCONNECTING AUTOPLT; RELATIVE TO WORKLOAD LEVEL AND ALT PROTECTION. HIGH DEG OF DIFFICULTY ON WDB ON COORDINATING MULTIPLE AUTOPLT AND THROTTLE ACTIONS: A) ESPECIALLY WITH AUTOPLT ENGAGED IN 'MANUAL' INSTEAD OF COMMAND; B) ESPECIALLY FOR NEW PLTS ON ACFT; C) ESPECIALLY FOR PLTS COMING FROM ACFT WITH MORE SOPHISTICATED AUTOPLTS/AUTO FLT SYS. DISADVANTAGE OF TURNING OFF FLT DIRECTOR; ESPECIALLY WITHOUT ANNOUNCEMENT TO OTHER PLTS. LACK OF EXPOSURE OF NEW PLTS TO AUTOPLT PROTOCOL DURING TRAINING.