A B727 FLC EXERCISES AN EVASIVE CLB IN RESPONSE TO A GPWS ALERT WHILE MAKING A NIGHT VISUAL APCH TO TYS RWY 23. THE ACFT WAS DSNDED TO AN MVA OF 2500 FT WHEN THE ALERT OCCURRED. THE MSA IS 8800 FT AND THE INITIAL APCH ALT IS 3600 FT.

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IT WAS OUR 4TH FLT SEGMENT OF THE DAY; INCLUDING A DIVERT FOR WX EARLIER. IT WAS NIGHT (APPROX AB15); SCATTERED CLOUDS WITH VISIBILITY RPTED AT 6 MI WITH HAZE. WE APCHED TYS FROM THE S ON AN IFR FLT PLAN; IN CONTACT WITH APCH CTL. WE WERE TOLD TO EXPECT VECTORS FOR A VISUAL APCH TO RWY 23R; AND WERE VECTORED AROUND THE NE SIDE OF THE FIELD; AND GIVEN DSCNTS DOWN TO 2500 FT MSL. THE COPLT; FLYING THE APCH; HAD BRIEFED A VISUAL APCH AND HAD THE ILS RWY 23R TUNED IN FOR A BACKUP. THE OTHER NAV RADIO WAS STILL ON THE VXV VOR FOR POS ORIENTATION. WE HAD BEEN VECTORED AROUND A L BASE AT APPROX 12-15 MI NE OF THE ARPT; EVENTUALLY GIVEN A 230 DEG VECTOR; TOLD WE WERE 12 MI FROM THE ARPT AND CLRED THE VISUAL APCH. ILS INDICATIONS WERE BELOW GS AND LEFT OF LOC COURSE; AND WE WERE IN LEVEL FLT AT 2500 FT MSL. SOMEWHERE IN BTWN ABOUT 7-11 MI NE OF THE ARPT; THE GPWS SOUNDED 'WHOOP; WHOOP; PULL UP.' WE COULD SEE THE LIGHTS ON THE GND BUT THE HAZY NIGHT CONDITIONS WITH NO DISCERNIBLE HORIZON MADE IT IMPOSSIBLE TO POSITIVELY INSURE THERE WAS NO TERRAIN OR OBSTACLE WHICH MIGHT BE A FACTOR. WE COMMENCED A PULL-UP; AND REACHED APPROX 3600 FT MSL BEFORE THE GPWS SILENCED. WE RPTED OUR CLB TO APCH CTL; AND FROM THAT ALT ACQUIRED A BETTER LOOK AT THE ARPT AND WERE ABLE TO CONTINUE TO A NORMAL LNDG. THE ILS FOR RWY 23R SHOWS AN MSA OF 8800 FT. ALTHOUGH; I HAD BEEN TO TYS MANY TIMES BEFORE AND KNEW THERE IS TERRAIN AROUND THE ARPT; I WASN'T IMMEDIATELY SURE WE WERE CLR OF ALL TERRAIN BECAUSE OF THE MARGINAL VISUAL CONDITIONS. FATIGUE MAY ALSO HAVE BEEN A FACTOR. WE WERE FORTUNATE IN THAT WE DID NOT OVERBOOST ANY ENGS; IF WE HAD BEEN TOTALLY IMC I SUSPECT WE WOULD HAVE. AFTER THE FACT; APCH SAID GPWS WARNINGS LIKE OURS HAD HAPPENED BEFORE. A RIDER ON OUR JUMP SEAT SAID HE HAD SEEN IT HAPPEN BEFORE; TOO. SINCE IT APPEARS OUR EXPERIENCE WAS NOT AN ISOLATED CASE; I SUGGEST A REVIEW TO EITHER ADJUST THE MVA; OR PUBLISH THE FACT THAT A GPWS WARNING IS POSSIBLE ALTHOUGH AT A SAFE VECTORING ALT. IT MIGHT SAVE UNNECESSARY ENG OVERBOOSTS; POSSIBLE TFC CONFLICTS; AND UNNECESSARILY SCARING PAX (AND PLTS!) CALLBACK CONVERSATION WITH FACILITY SPECIALIST REVEALED THE FOLLOWING INFO: FACILITY SPECIALIST ADVISED THAT A 30 DAY EVALUATION IS BEING CONDUCTED WITH ARPT USERS TO OBTAIN INFO IF THERE IS FREQUENT/INFREQUENT GPWS RESOLUTIONS ENCOUNTERED WHEN CONDUCTING AN APCH TO TYS ARPT; RWY 23. THE SURVEY WAS THE RESULT OF INQUIRY INITIATED BY AN ARPT USER. AT THIS POINT IN THE SURVEY; ONLY ONE TYPE OF JET ACFT HAS EXPERIENCED GPWS RESOLUTIONS. INVESTIGATION OF A RECENT USER COMPLAINT DETERMINED ALL CTLR ACTIONS WERE IN ACCORDANCE WITH FACILITY PROCS AND POLICIES. RADAR/DME ARE REQUIRED TO CONDUCT THE RWY 23 APCH. THE MVA HAS BEEN RECENTLY VERIFIED; AS REQUIRED BY FAA REVIEW POLICY. CTLR RADAR INDICATORS DISPLAY REQUIRED OBSTRUCTIONS. THE MSA (8800 FT) SHOULD NOT BE CONFUSED WITH MVA. MSA AND MVA ARE DEVELOPED BY DIFFERING CRITERIA.