ALTDEV ALT OVERSHOT IN DSCNT WHEN FO INADVERTENTLY DISARMS ALT AUTO CAPTURE BY SELECTING 'VERT SPD' ON AUTOPLT MODE SELECTOR.

Narrative

FLT TO BOS; ZNY ISSUED CLRNC FOR DSCNT; TO SLOW TO 300 KTS; TO CROSS A FIX N OF JFK AT FL240. WE HAD A TAILWIND OF 80-90 KTS. FO PROGRAMMED FMS FOR XING RESTR. DSCNT WAS BEGUN. ACFT WAS ON VERT NAV PATH TO XING RESTR. TAILWIND WAS MAKING IT DIFFICULT FOR ACFT TO MAINTAIN BOTH VNAV PATH AND AIRSPD. ACFT MAINTAINED PATH AND SPD INCREASED TO 313 KTS. AS ACFT APCHED FL240; CTR ISSUED CLRNC TO FL230. FO SET NEW ALT IN AUTOPLT ALT SELECT WINDOW. FO NOTICED SPD HAD INCREASED ABOVE 300 KTS; AND SELECTED VERT SPD MODE OF AUTOPLT TO REDUCE SPD. APPARENTLY SELECTING VERT SPD DISARMED ALT CAPTURE FOR FL230. AT 200 FT ABOVE FL230 I NOTICED ACFT WAS DSNDING AT 3300 FPM AND WOULD NOT LEVEL OFF AT FL230. I ANNOUNCED THIS TO FO WHO BEGAN TO LEVEL OFF ACFT. ACFT DSNDED ABOUT 400 FT BELOW FL230 BEFORE RETURNING TO FL230. PREOCCUPATION WITH FMS PROGRAMMING AND ACFT NOT ABLE TO MAINTAIN BOTH AIRSPD AND DSCNT PATH WITH TAILWIND LED TO SELECTION OF AUTOPLT VERT SPD MODE AND LOSS OF ALT ARM AND CAPTURE DURING AUTOPLT DSCNT.