MISSED CROSSING RESTRICTION ACCOUNT FMS REVERTED TO VNAV AND WAS NOT NOTICED BY THE FLT CREW.

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AT CRUISE ALT AND AT INTERMEDIATE ASSIGNED ALT OF FL350; ATC CLRED US TO FL270 TO CROSS FL290 IN 2 MINS OR LESS. CAPT INITIATED DSCNT SELECTING 1700 FPM VERT SPD ON AUTOPLT MODE CTL PANEL WHERE UPON HE SELECTED 3000 FPM RATE TO COMPLY WITH THE DSCNT RESTRICTION. HE THEN BEGAN TO PROGRAM; OR ATTEMPT TO PROGRAM; THE FMS FOR THE DSCNT. AFTER DSNDING APPROX 3000' (AT FL320); HE APPARENTLY CHANGED DSCNT MODE FROM VERT SPD TO ANOTHER MODE; WHAT I BELIEVE WAS VNAV ON THE FMS. THE ACFT SLOWED THE RATE OF DSCNT AND ALMOST LEVELED OFF JUST BELOW FL320. AS I STARTED TO MAKE A REF TO OUR RESTRICTION AND THE FACT THAT WE WERE NOT DSNDING; ATC CALLED AND ASKED IF WE WERE GOING TO MAKE THE RESTRICTION. THE CAPT REPLIED THAT HE COULD. I REPLIED TO THE CTLR THAT WE STILL HAD SOME TIME LEFT; BUT WOULD APPRECIATE A VECTOR TO BE SURE. THE CAPT SELECTED VERT SPD AGAIN; THEN FLT LEVEL CHANGE DEPLOYING THE SPOILERS; AND WE DSNDED THROUGH FL240 AFTER TURNING APPROX 30 DEGS TOWARD THE CTLR'S ASSIGNED HDG. FOR BOTH THE CAPT AND AND I; THIS WAS OUR SECOND TRIP ON THE WDB. THE CAPT'S ATTEMPT TO USE ALL THE ELECTRONIC AIDS AVAILABLE TO COMPLY WITH AN INTERMEDIATE DSCNT THAT SHOULD HAVE BEEN LITTLE MORE THAN ROUTINE. THOUGH A DSCNT CLRNC FROM HIGH ALT REQUIRING A 3000 FPM RATE OF DSCNT OR MORE IS A LITTLE UNUSUAL; IT WAS NOT BEYOND THE CAPABILITY OF THE ACFT; EVEN IF NOT THE MOST COMFORTABLE FOR THE PAX. THOUGH THE FMS AND ASSOCIATED CAPABILITIES GREATLY REDUCE WORKLOAD WHEN PARAMETERS CAN BE PREPROGRAMMED OR EASILY ENTERED AND EXECUTED; SOMETIMES IT IS NOT THE MOST EXPEDITIOUS WAY TO CARRY OUT A TASK. THE DSCNT RESTRICTION COULD HAVE BEEN MADE WITH THE INITIAL SELECTION OF V/S. INCORPORATING THE FMS ALMOST CAUSED US TO MISS IT ALTOGETHER.