GPWS.

Narrative

DURING VECTORS TO THE ILS APCH RWY 19R AT FAIRBANKS; AS; AT A POS APPROX 15 MI NE OF THE ARPT WE EXPERIENCED A GPWS 'TERRAIN TERRAIN' ALERT. AT THE TIME WE WERE LEVEL AT 4000 FT ON A BASE LEG VECTOR; AUTOPLT ENGAGED IN HDG AND ALT. COMPANY PROC IN RESPONSE TO THIS ALERT IN IMC; CORRECTLY CALLS FOR IMMEDIATE PRECAUTIONARY EVASIVE CLB ACTION. THIS WAS TAKEN AND THE ALERT CEASED AT 4800 FT. WE INFORMED ATC AS WE INITIATED THIS MANEUVER AND THEY AUTH US TO LEVEL AT 5000 FT. BY THIS STAGE WE HAD GONE THROUGH THE LOC AND WE WERE VECTORED BACK TO IT FROM THE NW SIDE AND CLRED FOR THE ILS TO INTERCEPT AT 3800 FT OR ABOVE 'CLRED FOR THE APCH.' THE AUTOPLT; WHICH HAD BEEN DISENGAGED DURING THE PREVIOUS COMMOTION; WAS REENGAGED; AND THE LOC CAPTURED. THE GS CAPTURED ALMOST SIMULTANEOUSLY AT APPROX 4000 FT. THERE FOLLOWED AN INCREASING DIVERGENT HUNTING OR PHUGOID OSCILLATION ON THE GS; NECESSITATING DISENGAGEMENT OF THE AUTOPLT. THE APCH WAS STABILIZED MANUALLY; THE AUTOPLT RE-ENGAGED; AND THE APCH WAS CONTINUED TO AN UNEVENTFUL CONCLUSION. AFTER LNDG; WE DECIDED TO VISIT THE RADAR CTLR TO DISCUSS THE EVENT. THIS PROVED VERY ENLIGHTENING. THEY INFORMED US THAT ANOTHER CARRIER HAD HAD SEVERAL CASES OF A SIMILAR WARNING AT THAT POS! ALSO THEY POINTED OUT THAT THE GS WAS CHARTED AS UNUSABLE ABOVE 2500 FT; WHICH; EVEN IF WE HAD BRIEFED; I AM SURE IN THE HEAT OF THE EVENT WE WOULD NOT HAVE RECALLED. RECOMMENDATIONS: 1) PUT OUT AN ALERT THAT A GPWS WARNING MAY OCCUR AT 4000 FT VECTORING ALT ON THIS APCH. 2) ON 10-0 PAGES RE-EMPHASIZE THAT THE GS IS UNUSABLE ABOVE 2500 FT.