Narrative:

While using the VOR DME runway 17 approach into saltillo, mx, we got a GPWS alert. This happened as we leveled off just past the FAF at 7.0 DME (5580 ft - circling MDA). Our radar altimeter initially showed an altitude of 1000 ft AGL and decreased steadily to 500 ft AGL as we were level. We immediately initiated a climb and silenced the warning. We descended back down to the MDA at approximately the 5.0 DME fix. From now on I will know not to descend until 5.0 DME instead of 7.0 DME. Many of my fellow pilots have also gotten into this situation. Callback conversation with reporter revealed the following information: reporter stated aircraft type was metroliner. Reporter had flown this non-precision approach only once before, and the GPWS had not activated because he was 'behind' in getting down to the MDA. On this particular day, he felt that he was doing everything right, and he set off the GPWS! Captain had reservations about returning to this situation, and queried fellow company pilots about the visual geography of saltillo. When questioned about a possible factor of WX observation and altimeter setting accuracy, reporter indicated that this was issued by mexican WX observer. The approach had been briefed by the crew, however, the captain stated that they were lulled by united states standards, and wondered, now, about another countries 'standards.' analyst then contacted a commercial chart publisher and determined that mexican iaps are based upon terps. The terps final approach segment, which extends from the runway to the map (2 NM width at facility to a 7 NM width at 30 NM from the facility), requires an obstacle clearance of only 250 ft. If this final approach was flown as published, this aircraft would have had a minimum of 310 ft of obstacle clearance. This information was forwarded to reporter.

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Original NASA ASRS Text

Title: A PART 135 FLC EXPERIENCED A GPWS ALERT WHILE EXECUTING A NON-PRECISION (VOR) APCH.

Narrative: WHILE USING THE VOR DME RWY 17 APCH INTO SALTILLO, MX, WE GOT A GPWS ALERT. THIS HAPPENED AS WE LEVELED OFF JUST PAST THE FAF AT 7.0 DME (5580 FT - CIRCLING MDA). OUR RADAR ALTIMETER INITIALLY SHOWED AN ALT OF 1000 FT AGL AND DECREASED STEADILY TO 500 FT AGL AS WE WERE LEVEL. WE IMMEDIATELY INITIATED A CLB AND SILENCED THE WARNING. WE DSNDED BACK DOWN TO THE MDA AT APPROX THE 5.0 DME FIX. FROM NOW ON I WILL KNOW NOT TO DSND UNTIL 5.0 DME INSTEAD OF 7.0 DME. MANY OF MY FELLOW PLTS HAVE ALSO GOTTEN INTO THIS SIT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED ACFT TYPE WAS METROLINER. RPTR HAD FLOWN THIS NON-PRECISION APCH ONLY ONCE BEFORE, AND THE GPWS HAD NOT ACTIVATED BECAUSE HE WAS 'BEHIND' IN GETTING DOWN TO THE MDA. ON THIS PARTICULAR DAY, HE FELT THAT HE WAS DOING EVERYTHING RIGHT, AND HE SET OFF THE GPWS! CAPT HAD RESERVATIONS ABOUT RETURNING TO THIS SIT, AND QUERIED FELLOW COMPANY PLTS ABOUT THE VISUAL GEOGRAPHY OF SALTILLO. WHEN QUESTIONED ABOUT A POSSIBLE FACTOR OF WX OBSERVATION AND ALTIMETER SETTING ACCURACY, RPTR INDICATED THAT THIS WAS ISSUED BY MEXICAN WX OBSERVER. THE APCH HAD BEEN BRIEFED BY THE CREW, HOWEVER, THE CAPT STATED THAT THEY WERE LULLED BY UNITED STATES STANDARDS, AND WONDERED, NOW, ABOUT ANOTHER COUNTRIES 'STANDARDS.' ANALYST THEN CONTACTED A COMMERCIAL CHART PUBLISHER AND DETERMINED THAT MEXICAN IAPS ARE BASED UPON TERPS. THE TERPS FINAL APCH SEGMENT, WHICH EXTENDS FROM THE RWY TO THE MAP (2 NM WIDTH AT FACILITY TO A 7 NM WIDTH AT 30 NM FROM THE FACILITY), REQUIRES AN OBSTACLE CLRNC OF ONLY 250 FT. IF THIS FINAL APCH WAS FLOWN AS PUBLISHED, THIS ACFT WOULD HAVE HAD A MINIMUM OF 310 FT OF OBSTACLE CLRNC. THIS INFO WAS FORWARDED TO RPTR.

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.