A LIGHT TRANSPORT PLT RPTS MISREADING THE HLN RWY 27 ILS COMMERCIAL APCH CHART AND RECEIVING AN EGPWS 'TERRAIN' WARNING AFTER COMMENCING AN EARLY DSCNT FROM 10000 FT.

Narrative

ON A FLT FROM GTF TO HLN ON V21; WE RECEIVED CLRNC TO INTERCEPT HLN 15 MILE ARC (336 DEG R); AND MAINTAIN 10000 FT UNTIL ESTABLISHED. WE HAD BRIEFED THE APCH (ALSO ON GND AT GTF); COMPLETED APPROPRIATE CHKLISTS AND SET UP THE FMS TO INTERCEPT THE ARC INSIDE THE 336 DEG RADIAL; BETWEEN THE STEP-DOWN FIX AT THE 053 DEG RADIAL. AUTOPLT WAS ON. LOOKING AT THE CHART; ONCE WE RPTED TO TWR WE WERE ON THE ARC AT 10000 FT; WE WERE CLRED APCH. I WAS PLT FLYING; ASKED FOR 8100 FT ON ALERTER. BEGAN DSCNT TO 8100 FT BEFORE THE 053 DEG RADIAL. SECONDS LATER; EGPWS 'TERRAIN; TERRAIN' -- NO ACTION TAKEN BECAUSE NO VISIBLE TERRAIN ON OUR DISPLAY. WE ONLY RECEIVED ONE WARNING AND SHORTLY THEREAFTER REACHED THE 053 DEG RADIAL WHERE 8100 FT WAS THE MINIMUM ALT. WE BOTH MISSED THE '10000 NOPT' WAY UP ON THE TOP OF THE COMMERCIAL PLATE. CAPT HAD BEEN FLYING TYPE/MODEL FOR 10 YRS WITH NUMEROUS TRIPS INTO HLN IN VMC/IMC. I HAVE BEEN IN TYPE/MODEL FOR 2 YRS ALSO WITH NUMEROUS TRIPS TO HLN ON THIS APCH IN IMC. I FEEL THIS CHART NEEDS TO BE MORE CLEAR. 10000 FT MIN ALT IS EASY TO MISS; VERY SHORT SEGMENT ON CHART. NON RADAR ENVIRONMENT ALSO DIDN'T HELP. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THE APCH CHART USED WAS THE HLN ILS RWY 27. HE ALSO STATED THAT HIS ACR TOLD HIM THAT THEY CONTACTED THE CHART MANUFACTURER ABOUT THIS ISSUE. THE RPTR ALSO REITERATED THAT BOTH HE AND THE CAPT HAVE FLOWN THIS APCH NUMEROUS TIMES AND HAD MISSED THE DSND CONSTRAINT.