Narrative:

I was the captain/PF from ZZZ to lax. I was rested. We planned the runway 6R ILS to lax. On short final we got an enhanced GPWS alert 'too low terrain' followed by whoop, whoop pull up. I initiated a go around. We discussed the situation with maintenance and ourselves. We were configured outside of guppi and our 1000 ft/500 ft checks were normal. We were on profile and should not have gotten the warnings. We reset the circuit breaker for the egpws and inhibited the system. We got vectors back to lax and landed on runway 7L uneventfully. At the gate, we debriefed maintenance. I later remembered that the left FMC had anomalies with distances and that we had a map shift. I called maintenance and told them the additional information. We were where we should have been and we still got the GPWS. This is a major problem my friend, as the egpws is used to assist us in highly mountainous areas. It could have saved air carrier in south america, however, the problem is this: the system uses a database from the FMC. In mountainous terrain, map shifts occur all the time, especially at guatemala city (gua). If a map shift occurs, then you can get wrong warnings (a real problem). They know there are flaws which could stop the use of this system. After a short, day layover, I left back to lax. The flight data recorder showed that we were on GS and our track was on localizer. They don't know why the GPWS went off. I do. I explained it in the prior form. Callback conversation with reporter revealed the following information: all the aircraft recorders were read. It was determined that the crew was on the localizer and GS, on speed and doing just what they were supposed to be doing. The FMC had experienced a map shift of 2 mi west and 1 mi south of aircraft actual position. The FMC is supposed to be updating position from localizer-DME at this time. It failed to do so. The aircraft egpws bulletin calls for the egpws system to be turned off if map shift exists. Of course it took the air carrier flight safety departure, reading the flight recorder, the better part of a day to determine this. This would be exceedingly difficult for the flight crew to accomplish in real time. As the system is now designed it is training the flight crew's to ignore warnings that may prove false.

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Original NASA ASRS Text

Title: B757 CREW HAD A FALSE EGPWS TERRAIN WARNING ON APCH.

Narrative: I WAS THE CAPT/PF FROM ZZZ TO LAX. I WAS RESTED. WE PLANNED THE RWY 6R ILS TO LAX. ON SHORT FINAL WE GOT AN ENHANCED GPWS ALERT 'TOO LOW TERRAIN' FOLLOWED BY WHOOP, WHOOP PULL UP. I INITIATED A GAR. WE DISCUSSED THE SIT WITH MAINT AND OURSELVES. WE WERE CONFIGURED OUTSIDE OF GUPPI AND OUR 1000 FT/500 FT CHKS WERE NORMAL. WE WERE ON PROFILE AND SHOULD NOT HAVE GOTTEN THE WARNINGS. WE RESET THE CIRCUIT BREAKER FOR THE EGPWS AND INHIBITED THE SYS. WE GOT VECTORS BACK TO LAX AND LANDED ON RWY 7L UNEVENTFULLY. AT THE GATE, WE DEBRIEFED MAINT. I LATER REMEMBERED THAT THE L FMC HAD ANOMALIES WITH DISTANCES AND THAT WE HAD A MAP SHIFT. I CALLED MAINT AND TOLD THEM THE ADDITIONAL INFO. WE WERE WHERE WE SHOULD HAVE BEEN AND WE STILL GOT THE GPWS. THIS IS A MAJOR PROB MY FRIEND, AS THE EGPWS IS USED TO ASSIST US IN HIGHLY MOUNTAINOUS AREAS. IT COULD HAVE SAVED ACR IN SOUTH AMERICA, HOWEVER, THE PROB IS THIS: THE SYS USES A DATABASE FROM THE FMC. IN MOUNTAINOUS TERRAIN, MAP SHIFTS OCCUR ALL THE TIME, ESPECIALLY AT GUATEMALA CITY (GUA). IF A MAP SHIFT OCCURS, THEN YOU CAN GET WRONG WARNINGS (A REAL PROB). THEY KNOW THERE ARE FLAWS WHICH COULD STOP THE USE OF THIS SYS. AFTER A SHORT, DAY LAYOVER, I LEFT BACK TO LAX. THE FLT DATA RECORDER SHOWED THAT WE WERE ON GS AND OUR TRACK WAS ON LOC. THEY DON'T KNOW WHY THE GPWS WENT OFF. I DO. I EXPLAINED IT IN THE PRIOR FORM. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: ALL THE ACFT RECORDERS WERE READ. IT WAS DETERMINED THAT THE CREW WAS ON THE LOC AND GS, ON SPD AND DOING JUST WHAT THEY WERE SUPPOSED TO BE DOING. THE FMC HAD EXPERIENCED A MAP SHIFT OF 2 MI W AND 1 MI S OF ACFT ACTUAL POS. THE FMC IS SUPPOSED TO BE UPDATING POS FROM LOC-DME AT THIS TIME. IT FAILED TO DO SO. THE ACFT EGPWS BULLETIN CALLS FOR THE EGPWS SYS TO BE TURNED OFF IF MAP SHIFT EXISTS. OF COURSE IT TOOK THE ACR FLT SAFETY DEP, READING THE FLT RECORDER, THE BETTER PART OF A DAY TO DETERMINE THIS. THIS WOULD BE EXCEEDINGLY DIFFICULT FOR THE FLC TO ACCOMPLISH IN REAL TIME. AS THE SYS IS NOW DESIGNED IT IS TRAINING THE FLC'S TO IGNORE WARNINGS THAT MAY PROVE FALSE.

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.