FMS MODE CONFUSION. INCORRECT PROGRAMMING.

Narrative

CAPT FLYING; AUTOPLT ON; AUTOTHROTTLES ON; DIGITAL FLT GUIDANCE SYS #1 USED FOR VERT AND LATERAL NAV; SPD 310 KTS AT FL240 PWR MGMNT SYS PROGRAMMED BUT NOT ENGAGED. ATC GAVE DSCNT CLRNC TO FL220. CAPT SELECTED 'PERF' ON THE DIGITAL FLT GUIDANCE SYS TO ENGAGE THE PWR MGMNT SYS. ON THE DSCNT PAGE OF THE PWR MGMNT SYS HE SELECTED VERT SPD. TO START 1000 FPM DSCNT THE PWR MGMNT SYS RECALCULATED THE OPTIMUM SPD TO BE 320 KTS. AS THE THROTTLES BEGAN TO ADVANCE HE TURNED THEM OFF TO PREVENT THE SPD INCREASE. HE THEN TRIED TO CHANGE THE SPD IN THE PWR MGMNT SYS TO 310 KTS; BUT IT WOULD NOT ACCEPT IT. DURING THIS TIME EITHER THE CAPT SELECTED OR AUTOPLT AUTOMATICALLY REVERTED TO IAS. I SAW ON THE FMA WE WERE IN IAS AND BECAUSE THE AUTOTHROTTLES WERE OFF BUT HAD BEEN ADVANCED WHEN PWR MGMNT SYS WAS SELECTED; WE WERE IN A CLB OF APPROX 1000 FPM AND AT APPROX 24500 FT. I CALLED OUT ALT AND CAPT INITIATED DSCNT. CONTRIBUTING FACTORS: NORMAL CRUISE SPD 320 KTS/.76 MACH CAPT USED 310 KTS. PWR MGMNT SYS SHOULD HAVE BEEN PROGRAMMED WITH DESIRED SPDS TO PREVENT USE OF OPTIMUM SPDS. PWR MGMNT SYS SHOULD HAVE BEEN SELECTED WHILE IN CRUISE OR AFTER DSCNT STARTED NOT IN THE MIDDLE OF A 'MODE' CHANGE. AUTOTHROTTLES TURNED OFF. ALT ALERTER ARMED FOR FL220.