A B737-300 CLBS 400 FT ABOVE ITS ASSIGNED ALT WHEN THE FO'S MISPROGRAMMING OF THE MCP CAUSES THE ALT HOLD TO DISENGAGE.

Narrative

LEVEL CRUISE AT FL280; WE RECEIVED A NEW ALT OF FL200 WITH XING RESTR; AND SPD INCREASE TO 320 KTS WHEN ABLE. AUTOPLT WAS ON. WHILE CAPT PROGRAMMED THE FMC FOR THE XING AND SPD INCREASE; I ELECTED TO MANUALLY SELECT A HIGHER MACH NUMBER IN THE MASTER CTL PANEL WINDOW TO EXPEDITE THE SPD INCREASE TO HELP ACCOMMODATE ATC. IN DOING SO; I INADVERTENTLY CAUSED THE 'ALT HOLD' MODE TO DISENGAGE SINCE A NEW XING ALT HAD BEEN SET IN THE MCP WINDOW (FL200). THIS AUTOPLT AND FMC ARE NEW TO ME; AND WHILE I OBSERVED THE CAPT COMPLETING THE FMC PROGRAMMING; THE ACFT BEGAN A SLOW CLB. I ALLOWED MYSELF TO BE DISTR; FAILED TO MONITOR THE AUTOPLT DURING THIS SHORT PERIOD; AND SUBSEQUENTLY DID NOT CATCH THE ALTDEV. THE CAPT ALERTED ME AND STOPPED THE CLB APPROX 300-400 FT HIGH. THERE WAS NO CONFLICT WITH ANOTHER ACFT; NO TA/RA; NOR WAS THERE A CALL FROM ATC. (I WAS REMINDED HOW TO NEVER RELY ON AUTOMATION; TO CONTINUALLY MONITOR THE AUTOPLT -- AT LEAST ONE PLT; AND AVOID COMPLACENCY DURING CRUISE FLT WHEN THINGS ARE USUALLY QUIET.) P.S.: THERE WAS NO ALT ALERT TO HELP US; SINCE THE MCP WAS PROGRAMMED FOR THE XING ALT (FL200).