Narrative:

8000 ft over smo VOR, wbound on downe 3 STAR, cleared to descend to 6000 ft. At 7500 ft, 250 KTS, 1500 FPM descent, encountered moderate to severe turbulence, heavy rain and got a ground proximity 'terrain' 'pull up' warning. Disconnected the autoplt and began a rapid climb. Warning continued to 10000 ft where I leveled off. We informed lax approach and were cleared to again descend to 6000 ft on a heading of 250 degrees. Approach said highest terrain below us was 4000 ft and there was only rain showers in the area. We began a descent to 6000 ft, this time with speed brakes and 3000 FPM. We were still in moderate turbulence and heavy rain. Suddenly the airspeed jumped plus 30 KTS and the indicated rate of descent increased to 5000 FPM. The nose attitude was normal and had not changed other than small amounts due to turbulence. Again, we got a ground proximity 'terrain' 'pull up' warning at 7500 ft. We immediately pulled up and climbed until it stopped at 10000 ft. Lax approach vectored us out over water and out of the rain showers, where we made a normal descent and an uneventful approach and landing. Lax approach objected to our use of a system that gives false warnings in those conditions. We theorize that a combination of heavy updraft up the west slope of the mountains and very intense rain that was detected as terrain by the radar altimeter caused the false warnings. We have to honor all warnings unless we can see the terrain. Our instruments could be deceiving us and ATC. Supplemental information from acn 233206: captain talked to approach control supervisor who inferred we should have considered GPWS indications as unreliable and disabled it.

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Original NASA ASRS Text

Title: LGT TWICE CLBS ABOVE ASSIGNED ALT WHEN APPARENTLY FALSE GND PROX WARNINGS ARE RECEIVED. ALTDEV ALT UNDERSHOT ALT EXCURSION.

Narrative: 8000 FT OVER SMO VOR, WBOUND ON DOWNE 3 STAR, CLRED TO DSND TO 6000 FT. AT 7500 FT, 250 KTS, 1500 FPM DSCNT, ENCOUNTERED MODERATE TO SEVERE TURB, HVY RAIN AND GOT A GND PROX 'TERRAIN' 'PULL UP' WARNING. DISCONNECTED THE AUTOPLT AND BEGAN A RAPID CLB. WARNING CONTINUED TO 10000 FT WHERE I LEVELED OFF. WE INFORMED LAX APCH AND WERE CLRED TO AGAIN DSND TO 6000 FT ON A HDG OF 250 DEGS. APCH SAID HIGHEST TERRAIN BELOW US WAS 4000 FT AND THERE WAS ONLY RAIN SHOWERS IN THE AREA. WE BEGAN A DSCNT TO 6000 FT, THIS TIME WITH SPD BRAKES AND 3000 FPM. WE WERE STILL IN MODERATE TURB AND HVY RAIN. SUDDENLY THE AIRSPD JUMPED PLUS 30 KTS AND THE INDICATED RATE OF DSCNT INCREASED TO 5000 FPM. THE NOSE ATTITUDE WAS NORMAL AND HAD NOT CHANGED OTHER THAN SMALL AMOUNTS DUE TO TURB. AGAIN, WE GOT A GND PROX 'TERRAIN' 'PULL UP' WARNING AT 7500 FT. WE IMMEDIATELY PULLED UP AND CLBED UNTIL IT STOPPED AT 10000 FT. LAX APCH VECTORED US OUT OVER WATER AND OUT OF THE RAIN SHOWERS, WHERE WE MADE A NORMAL DSCNT AND AN UNEVENTFUL APCH AND LNDG. LAX APCH OBJECTED TO OUR USE OF A SYS THAT GIVES FALSE WARNINGS IN THOSE CONDITIONS. WE THEORIZE THAT A COMBINATION OF HVY UPDRAFT UP THE W SLOPE OF THE MOUNTAINS AND VERY INTENSE RAIN THAT WAS DETECTED AS TERRAIN BY THE RADAR ALTIMETER CAUSED THE FALSE WARNINGS. WE HAVE TO HONOR ALL WARNINGS UNLESS WE CAN SEE THE TERRAIN. OUR INSTS COULD BE DECEIVING US AND ATC. SUPPLEMENTAL INFO FROM ACN 233206: CAPT TALKED TO APCH CTL SUPVR WHO INFERRED WE SHOULD HAVE CONSIDERED GPWS INDICATIONS AS UNRELIABLE AND DISABLED IT.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.