CARJ CREW GOT A GPWS TERRAIN WARNING AFTER THE CAPT THOUGHT IT WAS OK TO DSND TO THE APCH PLATE CHARTED ALT; WHEN NOT ON THE CHARTED RADIAL.

Narrative

ON A R BASE FOR A VISUAL APCH TO RWY 17 AT MHT WE HAD A GPWS 'TERRAIN; TERRAIN' ALERT. A NEW FO WAS FLYING. WE WERE AT 2000 FT AND CLRED FOR THE VISUAL BY APCH. AS WE WERE BACKING UP THE APCHS WITH THE VOR (ILS INOP) THE CHART SHOWS THAT BEYOND 15 DME FROM CON YOU MAY DSND TO 1200 FT. I SET THIS ACTIVE IN THE PRE-SELECTOR AFTER HAVING BEEN CLRED FOR THE VISUAL. I SET IT WITH THE INTENTION OF WHEN THE FO WAS 'ESTABLISHED' INBOUND HE COULD THEN DSND TO THAT ALT SAFELY. WHILE I WAS SETTING THE REF SPD CARD; THE FO STARTED A DSCNT. AS WE APPEARED MUCH CLOSER TO THE RWY THAN WE WERE; I WAS NOT CONCERNED. AT ABOUT 1500-1800 FT WE GOT A GPWS 'TERRAIN; TERRAIN' MESSAGE. I IMMEDIATELY DIRECTED THE FO TO CLB BACK TO 2000 FT. (ATC ALSO NOTICED THAT WE APPEARED LOW AND QUERIED US.) THE ALARM WENT AWAY; A STABILIZED APCH WAS ACCOMPLISHED ONCE ON FINAL AND AN UNEVENTFUL LNDG WAS COMPLETED. AS VISIBILITY WAS BETTER THAN 10 MI AND ALL DOCUMENTED TWRS WERE IN SIGHT; THE SAFE OUTCOME OF THIS FLT WAS NEVER IN QUESTION. FROM NOW ON I WILL NOT SET A LOWER THAN MINIMUM VECTORING ALT AFTER BEING CLRED FOR A VISUAL UNLESS ESTABLISHED ON FINAL!