B727 RECEIVES A GPWS AND PERFORMS AN ALTDEV ALT EXCURSION DURING ITS EVASIVE ACTION CLB. APCH CTLR SAID LATER THAT THIS HAPPENS 10% OF THE TIME IN THAT AREA.

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FLT XYZ; ENRTE FROM GEG TO SLC ON FEB/FRI/97; WAS BEING VECTORED BY SLC APCH CTL FOR AN ILS APCH TO RWY 34L. AT 29 DME IUUH (APPROX 2 1/2-3 MI W OF THE SLC RWY 34L LOC) FROM A HDG OF 155 DEGS AND AT 9000 FT MSL; SLC APCH ISSUED A L TURN TO 360 DEGS; TOLD US TO MAINTAIN 9000 FT MSL UNTIL ESTABLISHED ON THE LOC AND CLRED US FOR THE ILS RWY 34L APCH. AS WE TURNED L TOWARDS THE 360 DEG INTERCEPT HEADING (PASSING THROUGH 060 DEG HDG AND IMC) A 'PULL UP' GPWS WARNING SOUNDED. THE CAPT INITIATED A PULL UP (THE RADIO ALTIMETER INDICATED TERRAIN AT 1000- 1100 FT) AND UPON CLBING THROUGH 9500 FT MSL; THE RADIO ALTIMETER BEGAN INCREASING TO 2000-2500 FT AND THE GPWS WARNING STOPPED. THE FO ADVISED SLC APCH OF THE CLB DUE TO TERRAIN AVOIDANCE AND APCH TOLD US TO DSND TO 9000 FT MSL ASAP DUE TO TFC ABOVE US. WE DSNDED FROM 9600 FT MSL TO 9000 FT MSL AND WERE VECTORED TO THE RWY 34L LOC. THE FLT CONTINUED THE APCH UNEVENTFULLY TO A LNDG ON RWY 34L. IT SHOULD BE NOTED THAT AT NO TIME DURING THE 'PULL UP' MANEUVER DID WE GET ANY RA'S ON TCASII AND THAT OUR ALTDEV DID NOT EXCEED 600 FT ABOVE ASSIGNED ALT. AFTER DISCUSSING THE GPWS 'PULL UP' EVENT WITH SLC APCH; WE WERE ADVISED THAT THE 9000 FT MSL VECTOR ALT IS ROUTINELY USED IN THAT SECTOR AND THAT APPROX 1 FLT OUT OF 10 FLTS EXPERIENCES A GPWS WARNING IN THE AREA WHERE WE GOT OUR WARNING. APCH CTL ALSO CONFIRMED THAT NO TFC CONFLICTS OCCURRED DURING THE 'PULL UP' MANEUVER. SINCE IT HAS BEEN DOCUMENTED BY SLC APCH CTL THAT GPWS WARNINGS OCCUR AT 9000 FT MSL IN THIS SECTOR; WHY HASN'T THIS VECTORING ALT BEEN RAISED TO A HT WHERE THE WARNINGS ARE ELIMINATED? A WARNING SHOULD ALSO BE PUBLISHED IN THE SLC SPECIAL PAGES TO ALERT CREWS OF THE POSSIBILITY OF A GPWS WARNING IN THIS SECTOR AT 9000 FT MSL. CLRLY; THE POTENTIAL FOR A MIDAIR COLLISION EXISTS HERE WHEN AN ACFT HAS TO EXECUTE A GPWS TERRAIN PULL UP AND WHILE DOING SO; CLBS INTO THE PATH OF INBOUND TFC ABOVE. ACTION NEEDS TO BE TAKEN ASAP TO FIX THIS PROB.