B757-200 CREW DSNDED BELOW THE GS INTERCEPT ALT FOR RWY 21 AT GEG.

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THE DEST WX WAS LOW IFR. CONSEQUENTLY; WE WERE MONITORING ITS WX VIA ACARS WHILE ENRTE; WHICH SHOWED LIGHT WINDS FAVORING RWY 3; AND RPTING RVR FOR THE SAME. THIS ARPT DOES NOT BROADCAST ATIS VIA ACARS. WHILE STILL OUT OF RANGE FOR ATIS VIA RADIO; APCH DISCUSSIONS ENSUED. BASED ON WX RPTED ON ACARS WE ANTICIPATED AND BRIEFED AN ILS APCH FOR RWY 3. THE CAPT WANTED TO PLAN A CAT II APCH. THERE WAS SNOW ON THE RWY; AND BECAUSE OF THIS; HE REASONED THAT WE WOULD HAVE DISCRETION REGARDING THE USE OF AUTO-BRAKES IF WE UTILIZED A CAT II. HE DID NOT WANT TO BE MANDATED TO USE AUTO-BRAKES ON A CONTAMINATED RWY; WHICH WOULD HAVE BEEN THE CASE UTILIZING COMPANY CAT III CRITERIA. MUCH DISCUSSION WAS DEVOTED TO THIS SUBJECT DURING THE INITIAL APCH PHASE. UPON INITIAL CONTACT WITH GEG APCH; WE WERE ASKED IF WE HAD THE CURRENT ATIS. WE DID NOT HAVE IT YET AS WE WERE FINALIZING SETUP AND BRIEFING FOR THE CAT II APCH TO RWY 3. UPON RECEIVING THE ATIS; WE FOUND THE ARPT WAS UTILIZING CAT I APCHS TO RWY 21. THIS WAS A SURPRISE; CONSIDERING THE ACARS RPT ABOVE. I DOUBLECHKED WITH APCH CTL TO ASSURE RWY 21 WAS THE ACTIVE; WHICH WAS AFFIRMED. AT THIS POINT; WE SET UP FOR A CAT I ILS TO RWY 21; BRIEFED THE SAME; AND WERE VECTORED ACCORDINGLY. WHILE LEVEL AT 4000 FT; WE WERE GIVEN THE FOLLOWING FINAL APCH CLRNC: 'ACR X; TURN L HDG 230 DEGS; MAINTAIN 4000 FT UNTIL ESTABLISHED; CLRED THE RWY 21 ILS APCH.' THE TURN CAME SLIGHTLY LATE FOR OUR SPD; AND WE PARTIALLY PASSED THROUGH THE LOC. THIS WAS MERELY A SLIGHT OVERSHOOT; WHERE THE LOC NEEDLE WAS STILL ALIVE. THE CAPT INTERJECTED A XMISSION TO APCH INDICATING WE HAD FLOWN SLIGHTLY THROUGH THE LOC AND WOULD BE CAPTURING FROM THE OTHER SIDE. APCH AFFIRMED THIS; TELLING US 10 DEGS LEFT TO CAPTURE. WHEN THE LOC WAS CTRED; I DEVOTED MY ATTN TO RE-VERIFYING THE ILS IDENT AS IT HAD BEEN WEAK FROM OUR INITIAL APCH ANGLE; AND I WANTED TO REVERIFY IT CONSIDERING THE LOW WX. JUST AFTER THIS; THE CAPT MADE THE FOLLOWING STATEMENT: 'NOW THAT WE WERE ON THE LOC; WE CAN DSND TO 3500 FT; SET 3500 FT IN THE ALT WINDOW.' I WAS ARMING SPEED BRAKES; PRESELECTING THE TWR FREQ; AND MONITORING RAW DATA WHEN HE GAVE ME THIS COMMAND. I DID NOT HAVE TIME TO VERIFY THIS; BECAUSE OF MY HIGH WORKLOAD; AND I COMPLIED WITH HIS INSTRUCTIONS WITHOUT QUESTION. AS IT TURNED OUT; THE ALT HE SPECIFIED WAS INCORRECT. SHORTLY AFTER THE CAPT STARTED DSNDING TO 3500 FT; APCH SAID: 'ACR X IT'S 4000 FT TO THE LOC;' AT WHICH TIME I SAID IN A CONFUSED TONE; 'STANDBY.' I THEN LOOKED DOWN AT MY APCH CHART TO TRY TO QUICKLY SORT IT OUT; AND TOLD THE CAPT; NO LOWER THAN 3600 FT UNTIL THE GS. WE HAD JUST PASSED THROUGH 3600 FT DSNDING WHEN THE CAPT INTERVENED. SHORTLY THEREAFTER; WHILE ESTABLISHING LEVEL FLT AT 3600 FT; WE INTERCEPTED THE GS AND DSNDED ACCORDINGLY. WE WERE INSTRUCTED BY APCH CTL TO CONTACT THE TWR; WERE CLRED TO LAND; AND LANDED NORMALLY. THERE WERE NO FURTHER COMMENTS FROM ATC REGARDING THIS MATTER. BOTH PLTS MADE MISTAKES IN THIS SIT. THE CAPT MISREAD AND MISINTERPED THE CORRECT STEP-DOWN INFO; AND I INCORRECTLY INTERPED THE STEP-DOWN ALT ON XCHK ONCE INFORMED OF OUR IMPROPER DSCNT BY APCH. SEVERAL FACTORS CONTRIBUTED; OR MAY HAVE CONTRIBUTED TO THIS SCENARIO; WHICH FOLLOW: THE DISTR AND ELEVATED WORKLOAD REGARDING THE LOC INTERCEPT VECTOR COMING SLIGHTLY LATE. ACARS WX RPTING ENRTE IMPLIED AND SUGGESTED THE UTILIZATION OF RWY 3; WHICH LED US TO SPEND MOST OF OUR TIME PREPARING ACCORDINGLY. FURTHERMORE; PREPARATIONS WERE UNDERWAY EARLY DUE TO THE LEVEL OF APCH WE WERE ANTICIPATING. I SUGGEST THAT IF AN ARPT IS OPERATING AT OR NEAR APCH MINIMUMS; THE RVR FOR THE RWY IN USE SHOULD BE RPTED ON ACARS; NOT THAT OF A DIFFERENT RWY. THIS LED TO A SOMEWHAT LIMITED TIME TO PREPARE FOR THE ACTUAL APCH IN USE. ANOTHER INFLUENTIAL FACTOR WAS THAT I TRUSTED THIS CAPT; AS WE HAD SAFELY FLOWN TOGETHER FOR 2 MONTHS. HE WAS A FAMILIAR; CAPABLE CAPT; WHO I HAD GROWN TO TRUST. IN RETROSPECT; I SHOULD HAVE SAID SOMETHING LIKE; 'I DO NOT HAVE TIME TO VERIFY THE STEP-DOWN; CAN WE JUST INTERCEPT THE GS AT 4000 FT?' ACCORDINGLY; I BELIEVE IT WOULD BE BENEFICIAL FOR PROCS TO REQUIRE ONE TO STAY AT THE SPECIFIED CLRNC ALT UNTIL GS CAPTURE; AS THERE IS NOTHING TO BE GAINED BY DSNDING. I HAVE FLOWN WITH A FEW PLTS; WHO AS A MATTER OF TECHNIQUE SEEM TO PREFER TO STEP DOWN OUTSIDE THE FAF MUCH LIKE A NON-PRECISION APCH; ONCE ESTABLISHED ON THE LOC; RATHER THAN JUST WAITING ON THE GS TO DSND. UNCOMMON STEP-DOWN CRITERIA. IT IS LIKELY THERE ARE SEVERAL APCHS SYS WIDE WHERE THE STEP-DOWN CRITERIA FOR A LOC APCH DIFFERS FROM THAT OF THE FULL ILS ON THE SAME CHART; BUT IT IS UNUSUAL. FOR THIS PARTICULAR APCH; IF CLRED FOR THE ILS; ONE CAN ONLY DSND TO 4000 FT; BUT FOR THE LOC ONLY APCH; ONE CAN DSND TO 3600 FT. THIS IS CONFUSING. EXACERBATING THIS CONFUSION IS THE FACT THAT THE 3600 FT ALT IS IN BOLD FACE LETTERING; WHICH ONE MAY TEND TO FOCUS ON WHILE RUSHED. LASTLY; THE USE OF ARTIFICIAL LIGHTING AT A LOW LEVEL SO AS TO NOT DISTURB OUR NIGHT VISION FOR THE LOW VISIBILITY APCH MAY ALSO HAVE CONTRIBUTED TO THE MISREADING. IT WAS ALMOST DARK AT THE TIME WE HAD TO LOOK DOWN AT THE APCH PLATE TO CONFIRM THE ABOVE.