A PRECAUTIONARY CLB TO 5000 FT IN RESPONSE TO A GPWS ALERT FORCES A B727 CARGO FLT TO LEAVE ITS MINIMUM VECTORING ALT AFTER OVERSHOOTING THE E LOC COURSE FOR A NIGHT OP ILS TO RWY 26L AT ONT; CA.

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WE WERE CLRED FOR THE Z1004 3 ARR WITH AN ILS TO RWY 26L AT ONT. AFTER MAJEK; SOCAL APCH BEGAN VECTORING US BEHIND ANOTHER FLT; TURNING US SLIGHTLY INSIDE THE DEPICTED ARR GND TRACK. WE WERE CLRED TO 8000 FT MSL AND THEN 4000 FT MSL. THE AUTOPLT WAS USED TO FLY THE APCH. WX WAS NIGHT VMC ON TOP OF A DECK AT 4500 FT MSL; WITH A CEILING; AND VISIBILITY OF 1000 FT; AND 5 NM BELOW THE DECK. APCH BROUGHT US IN NE OF PETIS NDB ON THE ILS. WE WERE LEVEL AT 4000 FT MSL; AND HAD SLOWED TO 200 KTS. HEADING S. WE WERE APCHING FINAL WHEN WE HEARD SOCAL APCH MAKE A VERY FAINT AND UNREADABLE CALL. SHORTLY THEREAFTER; WE HEARD ANOTHER RADIO CALL AT ABOUT 16 DME; CLRING US DIRECT TO PETIS (12.6 DME); TELLING US TO MAINTAIN 4000 FT MSL UNTIL ESTABLISHED; AND CLRING US FOR THE ILS RWY 26L. WE NOTICED WE WERE FLYING THROUGH FINAL WHILE TURNING R TOWARD PETIS; FINALLY ROLLING OUT ON A 300 FT HDG WITH THE LOC SHOWING FULL SCALE DEFLECTION TO THE R. WE ESTIMATED WE WERE ABOUT 2 NM S OF COURSE; AND WERE STILL LEVEL AT 4000 FT MSL. AT THIS TIME; WE RECEIVED A 'TERRAIN; TERRAIN' CALL FROM THE EGPWS. THE RADAR WAS IN TERRAIN MODE 20 MI SCOPE; BUT SHOWED NO OBSTACLES ALONG OUR FLT PATH. NOBODY NOTICED ANY INDICATIONS OF TERRAIN ON THE RADIO ALTIMETERS. I IMMEDIATELY DISENGAGED THE AUTOPLT; ADDED PWR; AND CLBED TO 5000 FT MSL AND VMC. THE FO NOTIFIED SOCAL APCH; WHO THEN VECTORED US BACK TO FINAL FOR THE ILS. WE CALLED SOCAL AFTER LNDG; AND TALKED TO OUR CTLR. HE SAID HE HAD NO PROB WITH THE WAY WE HANDLED THE SIT. THERE WAS NO TFC IN OUR AREA THAT COULD HAVE BEEN A FACTOR. ACCORDING TO THE CTLR; THE MINIMUM VECTORING ALT FOR OUR POS N OF FINAL AND ON FINAL WAS 4000 FT MSL; ALTHOUGH THE APCH PLATE SHOWS AND ALT OF 4200 FT MSL AT PETIS. MINIMUM VECTORING ALT S OF FINAL IN THE AREA OF OUR OVERSHOOT WAS 4200 FT MSL. THERE IS SOME HIGH TERRAIN ABOUT 5 NM SE OF PETIS REACHING A MAXIMUM OF 3239 FT MSL. MY BEST GUESS IS THAT THE FAINT; UNREADABLE RADIO CALL WAS SUPPOSED TO BE OUR TURN TOWARD PETIS TO INTERCEPT FINAL. BY THE TIME WE FINALLY RECEIVED AND UNDERSTOOD OUR CLRNC TO PETIS; WE OVERSHOT FINAL TO THE S. DURING THE OVERSHOOT; THE HIGH TERRAIN BELOW US AND SE OF PETIS MAY HAVE TRIGGERED THE EGPWS; NOT EXACTLY THE KIND OF EXCITEMENT YOU NEED AT 3 O'CLOCK IN THE MORNING IN THE WX WITH CUMULO GRANITE ALL AROUND YOU. SOCAL TOLD US A COMMUTER FLT HAD THE SAME PROB BEFORE US. IF I HAD IT TO DO OVER AGAIN; I'D STAY ON THE ARR GND TRACK. EVEN THOUGHT SOCAL MAY BE PROVIDING VECTORS AND ALTS; BOTTOM LINE IS; IT'S STILL THE PLT'S JOB TO MAINTAIN GND CLRNC; AND THAT'S TOUGH TO DO IN A NON-FMS ACFT IF YOU'RE NOT ON A PUBLISHED RTING.