Narrative:

I was the captain/PF on the deadhead positioning flight from wichita, ks, to toluca (mmto) airport in mexico city, mexico. The aircraft was a cessna citation V ultra. The aircraft was equipped with the allied signal mk-V1 GPWS. The WX for the toluca airport was received and was good (better than 20000 ft scattered and 5 mi visibility). We were being handled by mexico city approach. Initially, we were flying an airway inbound from the north when we were reclred direct to the mex VOR. I discussed with my copilot (his first trip into mexico) the possibility of being cleared direct to the airport, instead of having to fly the full arrival. We reviewed the terminal chart to make sure that any altitude that approach gave us would compare correctly to our charts (including the minimum radar sector altitudes on the chart). Sure enough, approach cleared us direct to the airport and to maintain 13000 ft. I descended to and leveled at 13000 ft with the autoplt engaged. I have shown our approximately track in red on the enclosed terminal chart. Our track took us over the mountain ridge as shown. I was doing 250 KIAS. As we approached the ridge line, I commented to the other pilot that this would not be prudent at night or in IMC and that we would have adhered to the arrival and subsequent approach. As we neared the ridge line, I wondered to myself if there would be any adverse effect with regard to the GPWS. Sure enough, as the terrain started rising underneath us, the GPWS went off. 'Terrain, terrain' was sounded and the red 'pull up' light illuminated. I was just set to disengage the autoplt and start a pull up maneuver when I realized that we were in the clear to begin with. A momentary glance at the radar altimeter showed approximately 2080 ft and climbing rapidly. We were by then on the far side of the ridge line. My co-captain confirmed that he had felt comfortable with the terrain clearance (he has a military fighter background with extensive low level flying experience), and he verified all indications and warnings. After returning home, I read up again on the GPWS. We obviously received a warning due to an 'excessive closure rate to terrain.' I've enclosed the pertinent pages from our manual. We must have had a momentary spike on the radar altimeter, and being at the higher indicated airspeed, fell into the parameters of the system. Thus, the warning. What a wake-up call, even in the broad daylight and looking at the terrain with the knowledge that we were going to clear. This experience brings me to ask this question. Has any other flight crew ever experienced a similar situation in this area? My recommendation. Mexico city ATC should review their minimum radar sector altitudes for these mountainous areas and should consider raising the altitudes. This would prevent any possible GPWS false alerts to aircrews.

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Original NASA ASRS Text

Title: FLC UNFAMILIAR WITH PARAMETERS OF GPWS, FLEW AT ALT WHICH CAUSED GPWS WARNING. NO ACTION WAS TAKEN BECAUSE THEY WERE IN VISUAL CONDITIONS.

Narrative: I WAS THE CAPT/PF ON THE DEADHEAD POSITIONING FLT FROM WICHITA, KS, TO TOLUCA (MMTO) ARPT IN MEXICO CITY, MEXICO. THE ACFT WAS A CESSNA CITATION V ULTRA. THE ACFT WAS EQUIPPED WITH THE ALLIED SIGNAL MK-V1 GPWS. THE WX FOR THE TOLUCA ARPT WAS RECEIVED AND WAS GOOD (BETTER THAN 20000 FT SCATTERED AND 5 MI VISIBILITY). WE WERE BEING HANDLED BY MEXICO CITY APCH. INITIALLY, WE WERE FLYING AN AIRWAY INBOUND FROM THE N WHEN WE WERE RECLRED DIRECT TO THE MEX VOR. I DISCUSSED WITH MY COPLT (HIS FIRST TRIP INTO MEXICO) THE POSSIBILITY OF BEING CLRED DIRECT TO THE ARPT, INSTEAD OF HAVING TO FLY THE FULL ARR. WE REVIEWED THE TERMINAL CHART TO MAKE SURE THAT ANY ALT THAT APCH GAVE US WOULD COMPARE CORRECTLY TO OUR CHARTS (INCLUDING THE MINIMUM RADAR SECTOR ALTS ON THE CHART). SURE ENOUGH, APCH CLRED US DIRECT TO THE ARPT AND TO MAINTAIN 13000 FT. I DSNDED TO AND LEVELED AT 13000 FT WITH THE AUTOPLT ENGAGED. I HAVE SHOWN OUR APPROX TRACK IN RED ON THE ENCLOSED TERMINAL CHART. OUR TRACK TOOK US OVER THE MOUNTAIN RIDGE AS SHOWN. I WAS DOING 250 KIAS. AS WE APCHED THE RIDGE LINE, I COMMENTED TO THE OTHER PLT THAT THIS WOULD NOT BE PRUDENT AT NIGHT OR IN IMC AND THAT WE WOULD HAVE ADHERED TO THE ARR AND SUBSEQUENT APCH. AS WE NEARED THE RIDGE LINE, I WONDERED TO MYSELF IF THERE WOULD BE ANY ADVERSE EFFECT WITH REGARD TO THE GPWS. SURE ENOUGH, AS THE TERRAIN STARTED RISING UNDERNEATH US, THE GPWS WENT OFF. 'TERRAIN, TERRAIN' WAS SOUNDED AND THE RED 'PULL UP' LIGHT ILLUMINATED. I WAS JUST SET TO DISENGAGE THE AUTOPLT AND START A PULL UP MANEUVER WHEN I REALIZED THAT WE WERE IN THE CLR TO BEGIN WITH. A MOMENTARY GLANCE AT THE RADAR ALTIMETER SHOWED APPROX 2080 FT AND CLBING RAPIDLY. WE WERE BY THEN ON THE FAR SIDE OF THE RIDGE LINE. MY CO-CAPT CONFIRMED THAT HE HAD FELT COMFORTABLE WITH THE TERRAIN CLRNC (HE HAS A MIL FIGHTER BACKGROUND WITH EXTENSIVE LOW LEVEL FLYING EXPERIENCE), AND HE VERIFIED ALL INDICATIONS AND WARNINGS. AFTER RETURNING HOME, I READ UP AGAIN ON THE GPWS. WE OBVIOUSLY RECEIVED A WARNING DUE TO AN 'EXCESSIVE CLOSURE RATE TO TERRAIN.' I'VE ENCLOSED THE PERTINENT PAGES FROM OUR MANUAL. WE MUST HAVE HAD A MOMENTARY SPIKE ON THE RADAR ALTIMETER, AND BEING AT THE HIGHER INDICATED AIRSPD, FELL INTO THE PARAMETERS OF THE SYS. THUS, THE WARNING. WHAT A WAKE-UP CALL, EVEN IN THE BROAD DAYLIGHT AND LOOKING AT THE TERRAIN WITH THE KNOWLEDGE THAT WE WERE GOING TO CLR. THIS EXPERIENCE BRINGS ME TO ASK THIS QUESTION. HAS ANY OTHER FLC EVER EXPERIENCED A SIMILAR SIT IN THIS AREA? MY RECOMMENDATION. MEXICO CITY ATC SHOULD REVIEW THEIR MINIMUM RADAR SECTOR ALTS FOR THESE MOUNTAINOUS AREAS AND SHOULD CONSIDER RAISING THE ALTS. THIS WOULD PREVENT ANY POSSIBLE GPWS FALSE ALERTS TO AIRCREWS.

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.