ALTDEV ALT EXCURSION.

Narrative

ACFT WAS DSNDING TO CROSS PVD AT 14000 FT TO ENTER HOLD. BECAUSE ANTI-ICE SYS WAS TURNED ON IN DSCNT THE AUTO THROTTLES HAD ADVANCED PWR AND ACFT WAS NOT HOLDING ASSIGNED AIRSPD. DISCONNECTED A/T'S AND WAS CONCERNED WITH PROPER AIRSPD AND PWR FOR ANTI-ICE. I DID NOT NOTICE IMMEDIATELY THAT ACFT ON AUTOPLT HAD DSNDED BELOW 14000 BY 300 FT. WHEN I REALIZED WE WERE OFF ALT; I IMMEDIATELY DISCONNECTED AUTOPLT AND CLBED 300 FT TO PROPER ALT. ATC ASKED IF WE WERE AT 14000 FT. I REALLY DON'T KNOW WHY THE AUTOPLT DID NOT ENGAGE 'ALT HOLD.' I AM NOW ON THE ACFT AND WAS SOMEWHAT DISTRACTED BY THE PWR PROBLEM/AIRSPD PROBLEM. JUST AS I NOTICED OUR ALT EXCURSION THE ALT ALERTER ON THE ACFT SOUNDED. EVEN THOUGH THIS ACFT HAS A GLASS COCKPIT AND FULL AUTOMATION; I HAVE LEARNED FROM THIS EXPERIENCE THAT EVEN WITH ALL THE MAGIC YOU DON'T DROP YOUR VIGILANCE FOR 1 SECOND. WE FLEW THE ACFT 1 MORE LEG AND FOUND THE ALT CAPTURE FUNCTION WORKED PROPERLY; SO IT MAY HAVE BEEN OPERATOR ERROR ON MY PART BUT AT THIS POINT I JUST CAN'T SAY FOR SURE. I WILL CONTINUE TO REVIEW THE OCCURRENCE AND MY TECHNIQUES AND OF COURSE WATCH THE AUTOFLT SYS MUCH MORE CAREFULLY IN THE FUTURE.