HAD ALTDEV EXCURSION; B727 FLC BELOW MSA; HAS GPWS ACTIVATE.

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I WAS A FE ON A B727 SCHEDULED CARRIER FLT ON SEPT XX INTO EUGENE; OR. SCATTERED CLOUDS AT APPROX 5500 FT MSL BASES. APCH CTL CLRED US TO DSND AND MAINTAIN 4300 FT MSL AND FLY A RADAR VECTOR OF 265 DEGS WHICH WAS MEANT AS A L BASE VECTOR TO RWY 16. AS WE DSNDED ON THAT VECTOR TO THE CLRED ALT OF 4300 FT (WE DID RESPOND TO APCH WITH 'DSND AND MAINTAIN 4300 FT' AS WE ARE REQUIRED). WE WERE ABLE TO GET BELOW THOSE SCATTERED CLOUDS WITH BASES AT 5500 FT MSL AND LEVEL OFF AT 4300 FT MSL IN VMC AND WITH 20 PLUS MI OF VISIBILITY. LEVEL AT 4300 FT MSL WE COULD SEE THAT APCH CTL HAD CLRED US TO AN ALT THAT WAS BELOW THE 5100 FT MINIMUM SAFE ALT FOR THAT SECTOR BUT SURELY WAS AT THEIR MINIMUM VECTORING ALT (OF COURSE PLTS DO NOT HAVE ACCESS TO MVA'S). THIS 4300 FT MSL CLRED ALT WAS NEAR A RIDGE OF HILLS NW OF THE EUGENE ARPT. KNOWING WE WERE AT AN MVA ALT AND ALSO COULD VISUALLY SEE THAT WE WERE NOT DANGEROUSLY NEAR TERRAIN WE REMAINED AT 4300 FT AND 265 DEG HDG AS CLRED BY ATC. OVER THE RIDGE OF HILLS WE WERE STARTLED TO HEAR OUR GPWS ANNOUNCE 'TERRAIN; TERRAIN' THIS STARTLED US SINCE AS I MENTIONED EARLIER WE WERE AT AN ATC CLRED ALT AND HDG AND COULD VISUALLY SEE THAT WE WERE IN NO DANGER OF BEING NEAR TERRAIN. OF COURSE THE CAPT REACTED TO THE WARNING IMMEDIATELY AS WE ARE TRAINED TO DO. OUR FLT MANUAL STATES VERBATIM; 'IF TERRAIN INDUCED WARNINGS ARE ENCOUNTERED UNDER DAY VISUAL CONDITIONS WITH TERRAIN AND OBSTACLES CLRLY IN SIGHT: IMMEDIATELY TAKE POSITIVE CORRECTIVE ACTION UNTIL WARNING CEASES OR TERRAIN CLRNC IS ASSURED.' THE GPWS ONLY ANNOUNCED 'TERRAIN; TERRAIN' ONCE AS BEST AS I CAN REMEMBER. WITH EXCELLENT KNOWLEDGE OF THE FLT MANUAL AND GOOD JUDGEMENT THE CAPT REACTED IMMEDIATELY TO THE WARNING. UNFORTUNATELY IN HIS STARTLED STATE WHILE ATTEMPTING TO ADD PWR AND INCREASE PITCH HE INITIALLY LOST 200 FT FROM 4300 FT (BY THIS TIME WE WERE PAST THE RIDGE AND IN NO DANGER OF TERRAIN AND THE GPWS HAD CEASED ITS ANNOUNCEMENT) BUT IMMEDIATELY RECOGNIZED THIS AND MADE AN ADJUSTMENT TO BE ASSURED OF TERRAIN CLRNC (AS STATED IN THE FLT MANUAL) BY CLBING TO 500 FT ABOVE 4300 FT TO 4800 FT. REALIZING RIGHT AWAY THAT WE WERE CLR OF CONFLICT (ASSURED OF TERRAIN CLRNC AND NO GPWS ANNOUNCEMENT) HE IMMEDIATELY WENT BACK TO THE CLRED ALT OF 4300 FT. THE GPWS WARNING AND REQUIRED REACTION DEFINITELY OVERRODE THE NECESSITY TO HOLD 4300 FT BUT AS A PLT WITH A HEALTHY RESPECT FOR RULES AND SAFETY NOT ONLY DO I FEEL OUR ACTION WAS APPROPRIATE BUT THAT I HAVE A MORAL OBLIGATION TO RPT OUR DEV OF ALT. APCH CTL DID NOT QUERY US IN ANY RESPECT AND WE PERHAPS SHOULD HAVE OFFERED THE INFO. IN RETROSPECT AS A CREW WE DISCUSSED THAT IT WOULD HAVE BEEN IN THE BEST INTEREST FOR EVERYONE TO HAVE SHARED OUR EVENT. THAT WAY APCH CTL WOULD HAVE UNDERSTOOD OUR NECESSITY TO DEVIATE FROM 4300 FT. ALSO; THEY MAY ADJUST THEIR MVA OR USE A DIFFERENT VECTORING TECHNIQUE TO ALLEVIATE TERRAIN ANNOUNCEMENT BY AN ACFT'S GPWS. THIS EVENT ALSO TEACHES ME TO QUERY A CTLR ABOUT MVA IN THE FUTURE IF I CAN VISUALLY SEE THAT IT IS UNUSUALLY CLOSE TO TERRAIN. SUPPLEMENTAL INFO FROM ACN 317287: THE PF WAS CLRED TO 4300 FT AND WENT TO 4000 FT BACK UP AND THEN DOWN TO 4000 FT AGAIN. I REMINDED HIM BOTH TIMES OF HIS CLRED ALT AS THE DEV WAS HAPPENING. ALSO HE HAD A 1000 FT CALL. WE GOT 'TOO LOW; TERRAIN' HE WENT TO 4500 FT WHEN I SAID WE'RE TOO LOW AND TRIED TO SOUND URGENT. EXPLAINING BEFORE WE STARTED DOWN THAT IN 5 MONTHS OF FLYING INTO EUGENE I'VE NEVER BEEN THAT LOW OVER THAT HILL. TURNING FINAL WE WERE TOLD ABOUT TFC AND COULDN'T SEE THEM OUTSIDE. ON TCASII; 100 FT BELOW AND 2 MI AHEAD. WE GOT WARNED (TCASII) TO CLB AND DIDN'T. WE NEVER SAW THE TFC.