STALL VANE HEATER NOT REPAIRED PRIOR TO FLT INTO KNOWN ICING.

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AFTER AN IFR FLT WHICH DEPARTED LAS VEGAS; NV; TO GRAND CANYON; ON AN IFR FLT PLAN. THE ENTIRE FLT WAS IMC WITH LIGHT TO MODERATE MIXED ICING. AFTER A NORMAL LNDG AT GCN; THE CAPT AND I WENT OUT TO KNOCK OFF THE REMAINING ICE WITH A BROOM STICK. AS I WAS TAPPING THE ICE OFF THE ACFT (DHC-6) I CAME TO THE STALL WARNING VANES ON THE L WING. I DISCOVERED THAT THE STALL VANE UPPER OF LOWER HAD A SOLID COVERING OF ICE ON THE UPPER STALL VANE. THIS SHOULD NOT HAVE OCCURRED SINCE THE STALL VANES ARE SUPPOSED TO BE HEATED AS PART OF THE PITOT HEAT ANTI-ICE SYS. I BROUGHT THE CAPT OVER AND POINTED IT OUT TO HIM. HE SAID HE WOULD WRITE IT UP TO BE FIXED. I ALSO POINTED IT OUT TO A RAMP PERSON AND HE COMMENTED ON IT. OUR FLT WAS NOT SCHEDULED TO LEAVE FOR 4 HRS AND SINCE WE WERE AT A POINT OF REPAIR I FIGURED THAT THE CAPT MEANT HE WOULD WRITE IT UP AND HAVE IT FIXED BEFORE OUR SCHEDULED RETURN FLT. WE ATE BREAKFAST AND RESTED AND THEN WERE NOTIFIED BY OUR SCHEDULERS THAT WE WOULD BE RETURNING EARLY. I WAS THEN DISTRACTED BY MY PREFLT DUTIES TO NOTICE IF THE STALL VANES HAD BEEN SVCED. WE TOOK OFF FROM GCN UNDER AN IFR FLT PLAN. AT 10000 FT ON A 265 DEG HDG TO INTERCEPT THE LAS 066 DEG RADIAL WE ENCOUNTERED IMC CONDITIONS AND LIGHT TO MODERATE ICING ON THE WAY BACK. I THEN ASKED IF THEY (THE STALL VANES) HAD BEEN REPAIRED. THE CAPT REPLIED IN AN UNCARING VOICE 'NO; WHAT FOR; I'LL WRITE IT UP BACK IN VEGAS.' I FEEL THE CAPT VIOLATED THE FARS AND COMPANY OPS BY NOT FOLLOWING THE MEL WHICH STATES THAT THE STALL VANES CAN BE INOP ONLY IF NOT IN ICING CONDITIONS. THE CAPT WROTE THE STALL VANES UP BACK IN VEGAS AND NOTIFIED DISPATCH THAT HE NEEDED MECHS. THE MECHS THEN SVCED AND FIXED THE INOP STALL VANES. I FEEL THE CAPT WAS NEGLIGENT IN THE SAFETY OF LIVES BY FLYING THAT ACFT WITHOUT REPAIR.