AN ACR FO REPORTS HIS CAPT FOLLOWS THE LAS RWY 25L ILS GLIDE SLOPE WHICH PUTS THE ACFT BELOW CLASS B AIRSPACE AT PRINO INTXN.

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CLRED VISUAL TO RWY 25L AT LAS AS WE APCHED PRINO. CAPT STARTED TO DSND OUT OF 8000 FT AT ABOUT LAS 22 DME (DON'T RECALL THE SPD; BUT IT WAS WELL ABOVE 210 KIAS). I ADVISED HIM THAT THE CLASS B FLOOR IS 8000 FT UNTIL 20 DME. HE RESPONDED; 'IT'S OK; I'LL JUST FOLLOW THE GS.' I REMINDED HIM AGAIN THAT WE'RE NOT SUPPOSED TO GO BELOW THE FLOOR OF CLASS B AIRSPACE. HE REPEATED HIS PLAN TO FOLLOW THE GS. WITH THE VOR TUNED ON MY SIDE; I COULDN'T SEE THE GS INDICATOR ON HIS ADI; SO I DON'T KNOW HOW CLOSELY HE WAS FLYING THE GS. ATC NEVER SAID ANYTHING TO US; BUT DSNDING 2 MI EARLY OUT OF 8000 FT SEEMS PRETTY EXCESSIVE TO ME. WHEN ONE COMPARES THE STEPDOWNS ON THE RWY 25L ILS (ALT/DME) WITH THE CLASS B AIRSPACE CHART; YOU SEE SOME FAIRLY SIGNIFICANT DIFFERENCES. WHEN ONE FLIES THE RWY 25L GS; ONE FINDS THAT SOME OF THE STEPDOWNS MATCH THE GS FAIRLY CLOSELY; WHILE OTHERS ARE ABOVE THE GS. (IF I RECALL CORRECTLY THE XING RESTR AT SHAND IS VERY HIGH COMPARED TO THE GS; WHICH MAKES FOR A LOT OF ENERGY TO ATTEMPT TO DISSIPATE; PARTICULARLY IF ONE IS ATTEMPTING TO COMPLY WITH THE STANDARD '170 KIAS UNTIL 7 DME' RESTR.) NONE OF WHICH JUSTIFIES NONCOMPLIANCE WITH THE RESTRS ASSOCIATED WITH CLASS B AIRSPACE; BUT IT DOES MAKE OPERATING INTO LAS NEEDLESSLY COMPLICATED. ALIGN LAS CLASS B AIRSPACE AND ILS RWY 25L STEPDOWN FIXES SO THAT IT IS; IN FACT; POSSIBLE TO FLY THE GS TO RWY 25L WITHOUT BUSTING THE STEPDOWNS OR THE CLASS B. AND CONVINCE 'CAN'T BE BOTHERED BECAUSE I'VE NEVER BEEN VIOLATED YET' CAPTS TO PAY ATTN AND BE PROFESSIONAL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE REPORTER STATED THAT FROM AN ENERGY MANAGEMENT STANDPOINT; THE LAS ILS 25L IS AS CHALLENGING AS ANY AIRPORT IN HIS ACR'S SYSTEM. IF THE TYPICAL 170 KTS RESTRICTION IS COMPLIED WITH AND THE ACFT REMAINS IN THE CLASS B THEN DESCENDING AND SLOWING INSIDE OF RELIN INTXN WILL MOST TIMES LEAD TO AN UNSTABILIZED APCH BECAUSE A DESCENT AND SLOW DOWN ARE OCCURRING SIMULTANEOUSLY. MANY CAPTS WILL GO BELOW CLASS B IN ORDER TO HAVE A STABILIZED APCH INSIDE OF THE 7 DME.