BE400 FLT CREW DSNDS BELOW APCH SEGMENT ALT ON LOC BACK COURSE APCH.

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RADAR ALTIMETER INOP (ENTERED IN STATUS BOOK) CAUSED ERRONEOUS RADAR ALT READINGS AND FALSE TAWS PRESENTATION. PF AND PNF BRIEFED APCH PRIOR TO PENETRATION. PNF ADVISED PF HE HAD EXPERIENCED A FALSE TAWS PULL-UP WARNING ON A PREVIOUS APCH TO LUC. FMS SET UP CORRECTLY. HOWEVER; THE 5000 FMS DOESN'T DISPLAY RATE OF DSCNT CUE ON VVI TO REFLECT FMS INPUT LEG ALTS WHEN NAV SOURCED TO LOC. EFIS MAP ALSO DOESN'T SHOW DESIRED ALTS AT FMS NAV POINTS. FACTORS INCREASED TASK LOADING. CINCINNATI APCH REQUESTED WE KEEP UP SPD UNTIL FAF. ARR CHANGED TWICE DURING PENETRATION. IMC WITH LIGHT ICING; LIGHT TO MODERATE TURB. REQUIRED TO FLY WITH BOTH HANDS ON YOKE. FOCUS ON MAINTAINING ACFT CTL. CEILING RAGGED AT ABOUT 1400 FT MSL. COMMENCING APCH; PF ASKED PNF TO CALL OUT AND DIAL IN INST PROC ALTS; PER COMPANY SOP (IE; 'TALK ME THROUGH THE APCH'). INITIAL VECTOR ALT = 3000 FT MSL. FAF ALT = 2100 FT MSL. MDA = 1240 FT MSL. PF CLRED TO INTERCEPT LOC AT 3000 FT; CLRED FOR APCH. ONCE LOC INTERCEPTED; PNF CALLED OUT DSCNT TO 2100 FT. PF UNDERSTOOD THIS TO BE IAF ALT; HANDS/YOKE OBSTRUCTED FMS MAP DISPLAY. SHORTLY THEREAFTER; BUT PRIOR TO FAF; PNF CALLED OUT DSCNT TO 1240 FT AND CHANGED ALT SELECT DISPLAY TO 1240 FT. CREW EXECUTED APCH CHK; CONFIGURED GEAR DOWN; FLAPS 20 DEGS AND BEGAN DSCNT. APCH ACTUAL FAF; AT ABOUT 1500 FT; TAWS 'PULL UP' COMMAND SOUNDED. PNF ADVISED PF TO DISREGARD TAWS COMMAND AS ERRONEOUS. PF NOTED FMS ALT SELECT DISPLAY AND MDA BOTH SHOWED 1240 FT. PF ASKED PNF TO CONFIRM THEY HAD REACHED THE FAF. PNF REALIZED MISTAKE AND SAID 'NO; WE NEED TO BE AT 2100 FT.' PF IMMEDIATELY INITIATED CLB TO 2100 FT. CINCINNATI APCH ADVISED THEIR ALT ALERT HAD SOUNDED. PF LEVELED AT 2100 FT; CONTINUED ON COURSE TO THE FAF. REMAINDER OF APCH SUCCESSFULLY FLOWN WITHOUT INCIDENT. LESSONS LEARNED: 5000 FMS DOESN'T SHOW SAME ALT CUES AS AMS 850. NO ALTS ON MAP; NO VVI CUE WHEN FMS NOT NAV SOURCE. PF NEEDS TO KEEP FMS LEGS PAGE VISIBLE TO XCHK REQUIRED ALTS AT NAV POINTS. FLYING IMC APCHS WITHOUT OPERATING 'RAD ALT' REQUIRES EXTRA ATTN. L SEAT FLYING REQUIRES DIFFERENT XCHK BECAUSE MAP DISPLAY IS OFTEN OBSCURED BY YOKE UNLESS PLT MOVES HEAD.