ACFT MISSED XING ALT AT SPECIFIED INTXN DURING DSCNT.

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ACFT ASSIGNED BEST FORWARD SPD (310 KTS OR GREATER) BY ATC FOR SPACING INTO DTW. A VECTOR OFF COURSE TO INTERCEPT (WAS GIVEN AFTER CLRED TO CROSS POLAR INTXN AT 12000 FT) THE POLAR 1 ARR APPROX 45 MI FROM POLAR. ACFT WAS AT FL230 AT GEIBER INTXN. AT 2500 FT RATE DSCNT WAS ORIGINALLY SELECTED ON THE VERT SPD THUMB WHEEL OF THE AUTOPLT. I THEN LOOKED AWAY TO GET MY APCH CHART OUT FOR RWY 21L AT DTW. I THEN NOTICED THAT THE ACFT WAS ONLY DSNDING AT 1800-1900 FPM. I THEN SELECTED 3000 FT RATE OF DSCNT ON THE THUMB WHEEL. WITH PWR AT IDLE THE ACFT WAS VMO MINUS 20 KTS (320 KTS). THE CAPT RECOMMENDED SPD BRAKES WHICH WERE FULLY DEPLOYED. THE ACFT WAS LEVELED OFF APPROX 2 1/2 NM ON THE OTHER SIDE OF POLAR INTXN. THE ATC CTLR ADVISED US THAT WE HAD NOT MADE OUR XING RESTR. THE 'SET-UP': WITH THE ACFT ASSIGNED 'BEST FORWARD SPD;' ONE OF THE PLTS OPTIONS OF MAKING A XING RESTR (INCREASING RATE OF DSCNT WHICH INCREASES SPD 'WITHOUT THE USE OF SPD BRAKES WHICH CAUSES ACFT VIBRATION WHICH UNNERVES THE PAX'). ALREADY AT VMO MINUS 20 KTS IS TAKEN AWAY WHEN ASSIGNED A VECTOR OFF AN AIRWAY OR ESTABLISHED SID/ STAR TO AN INTXN - THEREBY TAKING AWAY A PRE-PLANNED DSCNT PROFILE AS THE DISTANCE OVER THE GND IS NOW CHANGED (IN OUR CASE SHORTENED) I WAS NOT CAUGHT 'BEHIND THE ACFT' AND WAS UNABLE; AS IT TURNED OUT; TO MEET THE RESTR. I KNEW IT WAS GOING TO BE CLOSE AND A SIMPLE CALL TO THE CTLR WOULD/COULD HAVE RELIEVED THE SIT. NO ONE WANTS TO ADMIT THAT THEIR JUDGEMENT AND PILOTAGE IS OFF EVEN A FRACTION; AND I THOUGHT WE COULD MAKE IT. SOLUTION: CONFESS EARLY IF IT'S GOING TO LOOK CLOSE. ATC SHOULD ADVISE NON RNAV EQUIPPED ACFT WITH THE DISTANCE TO A FIX AT WHICH A RESTR HAS BEEN APPLIED WHEN THE ACFT IS TAKEN OFF OF AN ASSIGNED SID/STAR AND THE FIX IS AN INTXN OR OTHER FORM OF NON-EQUIPPED DME NAVAID AND THE ACFT IS NOT ESTABLISHED ON A RADIAL OR TRANSITION DEFINING THE FIX. SUPPLEMENTAL INFO FROM ACN 302013: THE FO DELAYED STARTING HIS DSCNT UNTIL PAST THE POINT WHERE I WOULD NORMALLY HAVE STARTED DOWN; BUT PRIOR TO THE POINT WHERE I FELT IT NECESSARY TO INSTRUCT HIM. NOW IN THE DSCNT; THE FO STARTED TO BRIEF THE APCH AND MY ATTN WAS MOMENTARILY DIVERTED FROM THE DSCNT. ABOUT HALF WAY THROUGH THE BRIEF; I CHKED THE DSCNT PROGRESS AND REALIZED IT WOULD TAKE 'MAX EFFORT' TO MEET THE RESTR. I ADVISED THE FO AND HE DEPLOYED THE SPD BRAKES (HIS ONLY OPTION TO INCREASE RATE AS WE WERE ALREADY AT MAX AIRSPD AND FLT IDLE PWR). INCLUDE IN CAPT'S BRIEFING AT BEGINNING OF TRIP TO PLAN DSCNTS TO ARRIVE AT XING ALTS 5-10 DME EARLY.