B737-300 ACFT IN DSCNT WITH XING RESTR ALT; PF; CAPT USING VERT SPD ON FMS WITH A SHALLOW DSCNT RATE; MISSED XING RESTR BY 2000 FT. RPTR FO WAS NOT IN THE LOOP OBTAINING ATIS AND COM WITH COMPANY.

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WE WERE ESTABLISHED ON THE MINKS ONE ARR FOR LGA; AT A CRUISE ALT OF FL290. CLRNC WAS RECEIVED TO CROSS KERNO AT FL260; STANDARD FOR THIS ARR. JUST SW OF KERNO I TOLD THE CAPT THAT I WOULD BE OFF FREQ FOR A FEW MINS TO GET ATIS AND TALK TO COMPANY OPS. (WE HAD BEGUN A SLOW DSCNT JUST ABOUT THIS TIME -- CAPT SELECTED VERT SPD MODE AT A VERY LOW RATE.) IT TOOK ME SEVERAL MINS TO COMPLETE MY COMS BUSINESS. WHEN I LOOKED UP; I WAS STARTLED TO SEE THAT WE WERE ONLY 2 MI OR SO FROM KERNO; 2000 FT OR SO HIGH; AND DSNDING AT 300 FPM IN VERT SPD MODE. THE CAPT WAS ABSORBED IN FILLING OUT HIS LOGBOOK FOR THE TRIP. I TOLD ATC WE WOULD NOT MAKE OUR XING RESTR; AND WE WERE RECLRED TO A LOWER ALT; AND THE REST OF THE FLT WAS UNEVENTFUL. I THINK THIS SIT CLRLY RESULTED FROM THE DISTR OF BOTH CREW MEMBERS AT THE SAME TIME. THE PF NEEDS TO BE EXTRA VIGILANT WHEN THE PNF IS TEMPORARILY OUT OF THE LOOP. IN RETROSPECT; THIS WAS A VERY POOR TIME TO START WORKING ON A TRIP LOG. AS A CONTRIBUTING FACTOR; THE DSCNT WAS MADE IN VERT SPD MODE. THIS TOOK AWAY THE PROTECTION OF XING RESTRS WHICH WAS ALREADY PROGRAMMED INTO THE FMC IF VNAV WERE USED. I AM NOT SURE WHY PF SELECTED VERT SPD FOR THIS DSCNT (THOUGH NONE OF US USE VNAV EXCLUSIVELY). IN LATER DISCUSSION WE AGREED IT WAS NOT THE BEST CHOICE IN THIS CASE. I REALLY TRY HARD TO STAY UP WITH THE SIT; EVEN WHEN COPYING ATIS; ETC. IT IS NOT ALWAYS EASY; BUT I WILL BE RE-DOUBLING MY EFFORTS.