AN ACR MLG MISSED AN ALT ON DSCNT ON A STAR.

Narrative

ENRTE TO NEW YORK'S LGA ARPT WE WERE GIVING A XING RESTRICTION TO CROSS SOMTO INTXN AT FL260. I WAS THE PF AND THE CAPT HAD GONE TO THE FORWARD LAV WHEN CLRNC WAS ISSUED. I PROGRAMMED THE FMC WITH THE XING RESTRICTION BUT FAILED TO ENTER THE FL260 ALT IN THE MODE CTL PANEL; CAUSING THE ACFT NOT TO START DOWN ON TIME MISSING THE ALT BY APPROX 1000 FT OR 4 MI. THIS PROBLEM COULD HAVE BEEN AVOIDED IF; ON THE CAPT'S RETURN TO THE COCKPIT; A BRIEFING WOULD HAVE BEEN CONDUCTED OF EVENTS THAT HAD OCCURRED WHILE A PLT WAS OFF THE FLT DECK. DURING THE REST OF OUR 4 DAY TRIP WE PRACTICED THIS CHK OF BRIEFING EACH OTHER IF ONE PLT LEFT THE FLT DECK; INCLUDING ANY CHANGES IN RTE; ALT; REQUEST OR GENERAL INFO RELAYED BY ATC; WITH EMPHASIS ON SET UP OF THE FMC AND MODE CTL PANEL WITH THE AUTOPLT CONNECTED. POSSIBLY ANOTHER SOLUTION TO THIS WOULD BE THAT CERTAIN FMC COMMANDS THAT APPEAR IN THE MESSAGE PAD BE FOLLOWED BY AN AURAL WARNING OR CHIME; ESPECIALLY THE COMMAND OF RESET MCP; FMC FAIL; VERIFY POS; OR OTHER CRITICAL FMC MESSAGES. IN THE CASE OF BRIGHT SUNLIGHT; THE FMC PROMPTS ARE NOT REALLY EYE CATCHING.