Narrative:

On descent into tucson with scattered thunderstorm and strong up and downdrafts, we received a terrain warning at 13000 ft, while descending at 1500 FPM to 11000 ft. We executed the escape maneuver and I contacted tus approach to inform them and asked what was the minimum vectoring altitude, and was told 11000 ft. The warning ceased and we were given a descent to 8000 ft. At 8500 ft, the terrain warning sounded and we executed the second escape maneuver. I again notified tus approach. We then completed a normal approach and landing. The terrain display during both of these events never displayed red and during the second event, we were 4000 ft above the terrain with a very minimal descent rate. The terrain display again did not display red. There were very strong up and downdrafts in the area -- especially arriving over the high terrain to the east of tucson. Also, the distance to tucson was incorrect on the FMC when we were 110 mi out. The FMC distance indicated 75 mi. We cleared the FMC and reentered the waypoints and this fixed the distance error. The GPWS system was tested normally and had no previous history. My 2 questions are: 1) does strong wind gusts affect the sensors enough to give false warnings, and 2) did the GPWS think it was somewhere else. The navigation displays were right on during this phase of flight and we knew where the terrain was and what the minimum altitudes were.

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Original NASA ASRS Text

Title: B757 FLC EXPERIENCED 2 GPWS WARNINGS DURING DSCNT INTO TUS. THEY EXECUTED THE ESCAPE MANEUVER BOTH TIMES AND FOUND THAT AT THEIR POS OVER THE TERRAIN, SHOULD NOT HAVE TRIGGERED THE WARNINGS.

Narrative: ON DSCNT INTO TUCSON WITH SCATTERED TSTM AND STRONG UP AND DOWNDRAFTS, WE RECEIVED A TERRAIN WARNING AT 13000 FT, WHILE DSNDING AT 1500 FPM TO 11000 FT. WE EXECUTED THE ESCAPE MANEUVER AND I CONTACTED TUS APCH TO INFORM THEM AND ASKED WHAT WAS THE MINIMUM VECTORING ALT, AND WAS TOLD 11000 FT. THE WARNING CEASED AND WE WERE GIVEN A DSCNT TO 8000 FT. AT 8500 FT, THE TERRAIN WARNING SOUNDED AND WE EXECUTED THE SECOND ESCAPE MANEUVER. I AGAIN NOTIFIED TUS APCH. WE THEN COMPLETED A NORMAL APCH AND LNDG. THE TERRAIN DISPLAY DURING BOTH OF THESE EVENTS NEVER DISPLAYED RED AND DURING THE SECOND EVENT, WE WERE 4000 FT ABOVE THE TERRAIN WITH A VERY MINIMAL DSCNT RATE. THE TERRAIN DISPLAY AGAIN DID NOT DISPLAY RED. THERE WERE VERY STRONG UP AND DOWNDRAFTS IN THE AREA -- ESPECIALLY ARRIVING OVER THE HIGH TERRAIN TO THE E OF TUCSON. ALSO, THE DISTANCE TO TUCSON WAS INCORRECT ON THE FMC WHEN WE WERE 110 MI OUT. THE FMC DISTANCE INDICATED 75 MI. WE CLRED THE FMC AND REENTERED THE WAYPOINTS AND THIS FIXED THE DISTANCE ERROR. THE GPWS SYS WAS TESTED NORMALLY AND HAD NO PREVIOUS HISTORY. MY 2 QUESTIONS ARE: 1) DOES STRONG WIND GUSTS AFFECT THE SENSORS ENOUGH TO GIVE FALSE WARNINGS, AND 2) DID THE GPWS THINK IT WAS SOMEWHERE ELSE. THE NAV DISPLAYS WERE RIGHT ON DURING THIS PHASE OF FLT AND WE KNEW WHERE THE TERRAIN WAS AND WHAT THE MINIMUM ALTS WERE.

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.