B727 GPWS ALERT AT LAS.

Narrative

WE WERE ON RADAR VECTORS FOR A VISUAL APCH TO RWY 19L AT LAS. OUR POS WAS APPROX 8 NM NE OF THE ARPT; ON A HDG OF 250 DEGS; AND AT OUR ASSIGNED ALT OF 5100 FT MSL. THE ARPT WAS IN SIGHT; AS WAS A RIDGELINE IN FRONT OF US THAT APPEARED TO BE NO FACTOR. JUST AS I WAS READING BACK OUR VISUAL APCH CLRNC; WE GOT A GPWS WARNING OF 'WHOOP; WHOOP; PULL UP.' THE RADIO ALTIMETER TAPE MOVED RAPIDLY FROM OFF SCALE TO 1200 FT AGL AND WAS SLOWLY DECREASING. ALTHOUGH I DID NOT FEEL THAT GND CONTACT WAS A FACTOR (SEEING THE RIDGELINE WELL SILHOUETTED AGAINST THE CITY LIGHTS AND MOVING LOWER IN THE WINDSCREEN); THE CAPT (WHO WAS FLYING) ELECTED TO PERFORM THE GPWS ESCAPE MANEUVER. HE WENT TO FIREWALL THRUST AND ROTATED TO ABOUT 20 DEGS NOSE UP. AFTER ABOUT 3-5 SECONDS; THE WARNING CEASED. PWR WAS REDUCED AND THE NOSE LOWERED TO LEVEL FLT. I RPTED THE GPWS WARNING TO THE APCH CTLR; WHO RESPONDED SAYING; 'SOMETIMES THAT HAPPENS OUT THERE. I'M NOT SURE WHY.' JUST A WILD GUESS ON MY PART; BUT MAYBE IT HAS SOMETHING TO DO WITH THE MOUNTAINS AND THE MVA. WE CONTINUED TO AN UNEVENTFUL VISUAL APCH AND LNDG ON RWY 19L. I BELIEVE THAT THE COMBINATION OF THE MVA AND RAPIDLY RISING TERRAIN IS WHAT TRIGGERS THE GPWS; EVEN THOUGH THE ACFT IS WELL CLR OF THE TERRAIN AND NOT DSNDING. THE USE OF FIREWALL THRUST WHILE PERFORMING THE ESCAPE MANEUVER MAY WELL RESULT IN A 3 ENG CHANGE. THIS IS A VERY COSTLY RESULT OF A SIT THAT COULD PROBABLY BE AVOIDED BY ESTABLISHING A HIGHER MVA. RPTR STATES THAT THE INSTALLED GPWS WAS NOT ENHANCED AND THAT THE ESCAPE MANEUVER WAS A 'KNEE JERK' REACTION ON THE PART OF THE CAPT. HE FURTHER STATES THAT IN THE COMPANY SPECIAL PAGES OF COMMERCIAL CHART THAT THE POSSIBILITY OF GPWS ALERTS ARE A POSSIBILITY; HOWEVER; THIS WAS NOT ADDRESSED IN THE DSCNT AND LNDG BRIEFING.