ACR MLG ALT DEVIATION OVERSHOT DURING DESCENT INTO SLC CAUSING LESS THAN STANDARD SEPARATION WITH OUTBOUND COMPANY ACFT.

Narrative

INBND SLC FROM STL WE WERE GIVEN THE STANDARD REPETITIVE CLRNC TO 'CROSS 10 E OF FFU AT 17000' AND 280 KTS.' BEFORE VACATING OUR CRUISE ALT OF FL310; THE CTLR TOLD US TO 'COMMENCE DSCNT TO FL210 TFC.' DSCNT WAS COMMENCED AT A NORMAL RATE; AT APPROX 80 NM FROM FFU; WHICH IS EARLIER THAN NORMAL; BUT TO DUE TO THE WORDING OF OUR LAST CLRNC FROM CENTER; WE BELIEVED THE TFC WAS PROBABLY AT FL310; SO WE STARTED DOWN EARLY. DURING THE DSCNT; WE REMARKED ABOUT WHAT A CLR DAY IT WAS AND THAT A LOWER ALT THAT FAR OUT WOULD BE OK; BECAUSE IT WOULD GIVE OUR PAX A CLOSE LOOK AT THE SNOWCAPPED WASATCH MOUNTAINS. AFTER FL210; WE ASSUMED WE WERE STILL EXPECTED TO CROSS 10 MI E OF FFU AT 17000' MSL AND A 500 FPM CRUISE DSCNT WAS BEGUN. AT APPROX FL208; THE CTLR ASKED US WHAT OUR ALT WAS. WE REPLIED OUT OF FL210 AND WERE TOLD TO CLB IMMEDIATELY TO FL210. WE WERE THEN TOLD OUR TFC WAS AT FL200; 1-2 O'CLOCK; 8-10 MI; EBND. WE IMMEDIATELY PICKED UP OUR TFC VISUALLY ABOUT 1 MINUTE BEFORE PASSING. NO EVASIVE ACTION WAS REQUIRED. THIS SITUATION DEVELOPED DUE TO WRONG ALT CLRNC ASSUMPTIONS BY US; THE FLT CREW AND ALSO; TO A DEGREE AT LEAST; DUE TO AMBIGUOUS ALT AND TFC CALLS FROM ATC. IT IS MY FINDING AS OF 1-2 YRS AGO THAT THE FLT CREW FREQUENTLY HAS TO ASK ATC; 'WHERE IS OUR TFC;' AND QUITE OFTEN I DO. THIS TIME I DIDN'T. HAD WE BEEN TOLD INITIALLY WHEN GIVEN THE 'COMMENCE DSCNT TO FL210' CLRNC THAT WE HAD EBND TFC AT FL200 AND TO EXPECT 10 MI E OF FFU AT 17000' MSL AND 280 KTS; THIS SITUATION WOULD NEVER HAVE DEVELOPED. PROPER WORDING OF CLRNCS GREATLY IMPROVES COMPREHENSION OF SAME. ALSO; BEING TOLD WHERE THE TFC IS ELIMINATES GUESS WORK AND THE CREW'S NEED TO ASK WHERE. NEXT TIME I WILL ASK IF ALL THE INFO NEEDED IS NOT VOLUNTARILY GIVEN BY ATC. SUPPLEMENTAL INFO FROM ACN 84527: AFTER LNDG; WE CONTACTED ATC TO RESOLVE THE MISUNDERSTANDING. HE INDICATED THAT 'IT WORKED OUT FINE; NO PROB.'