CE525 FLT CREW REPORTS RECEIVING CLEARANCE TO DESCEND TO 5000 FEET FROM ZLC APPROACHING SLC; AND ATTEMPTING TO DESCEND TO THAT ALTITUDE BEFORE BEING QUESTIONED BY ZLC CONTROLLER.

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WHILE FLYING FROM LAS VEGAS; NV; ENRTE TO SALT LAKE CITY; UT; WE WERE GIVEN A 'XING RESTR' TO CROSS 15 MI S OF THE FFU VOR AT ONE-FIVE THOUSAND FT. HOWEVER; MYSELF AND THE OTHER CREW MEMBER HEARD 'CROSS 15 MI S OF FFU AT 5000.' THE OTHER CREW MEMBER READ IT BACK AS SUCH TO ATC AND THEY ACKNOWLEDGED THE READBACK. WE THEN PROGRAMMED THE FMS TO CROSS 15 NM S OF FFU AT 5000 FT AND BEGAN OUR DSCNT. WHEN WE WERE ROUGHLY 25 NM S OF FFU WE WERE ADVISED OF TFC AT 12 O'CLOCK POS AND 10 MI. AT THIS POINT WE HAD THE TFC IN SIGHT AS WE CONTINUED THROUGH 15000 FT. ATC THEN ASKED US WHAT ALT WE WERE AT AND WE ADVISED THEM WE WERE PASSING THROUGH ONE-FOUR THOUSAND FOR FIVE THOUSAND. ATC THEN ADVISED US THAT WE WERE CLRED TO ONE-FIVE THOUSAND AND TO LEVEL AT ONE-THREE THOUSAND. WE HAD A VISUAL ON THE OTHER TFC AND THEY HAD VISUAL ON US AS WELL. ATC ASKED US IF WE WANTED TO HIT A MOUNTAIN. IT IS IMPORTANT TO NOTE THAT WE HAD A VISUAL ON THE TERRAIN AND TFC. BOTH OF US ARE NOT FAMILIAR WITH THE AREA AND WE HAD NOT SET OUR CABIN PRESSURIZATION TO FIELD ELEVATION YET. THIS WOULD HAVE TOLD US THAT 5000 FT WOULD HAVE BEEN TOO LOW FOR THE AREA. FACTORS FOR THIS SITUATION WERE: 1) NOT HEARING THE CORRECT XING RESTR. 2) ATC ACKNOWLEDGING OUR READBACK. 3) PROGRAMMING OF THE FMS. 4) HAVING A VISUAL OF TERRAIN AND TFC AND HAVING AN OPTICAL ILLUSION OF TERRAIN HT. TERRAIN APPEARED LOWER THAN IT ACTUALLY WAS. 5) COPLT PLACING 5000 FT IN THE ALT PRE-SELECT. AS A CORRECTIVE ACTION I WILL BACK UP ALL XING RESTRS WITH FIELD ELEVATION AND OTHER SOURCES AND QUERY ATC AS WELL AS HAVE TAWS ACTIVATED IN ALL DSCNTS.