AS AN F100 DSNDS TO LOWER ALTS IN MEXICO; THE LACK OF NAVAIDS CAUSES THE FMS NOT TO UPDATE; AND THE ENHANCED GPWS IS LOST JUST WHEN IT IS NEEDED MOST.

Narrative

ATC FACILITY. MEX CTR. LOCATION: BJX/MTY/GDL/PBC. FOR 2 MONTHS I FLEW IN MEXICO TO GDL/MTY/BJX/PBC AND THE SAME SIT PRESENTED ITSELF ON EVERY ARR. WITH THE NEW EGPWS INSTALLED; THE PROC CALLS FOR THE WX RADAR TO BE TURNED ON AND THE SWITCH ON THE FORWARD PANEL TO BE PRESSED TO ACTIVATE THE NEW SYS. WITH THIS DONE; THE NEXT THING TO HAPPEN IS THE ACFT GOES TO IRS ONLY NAV DURING THE APCH AND LNDG. WITH THIS CONDITION THE PROC IN THE F100 AND OTHER FMS ACFT IS TO SWITCH THE SYS TO OVERRIDE WHICH IN ESSENCE DISABLES THE EGPWS PORTION ONLY. THE BASIC GPWS (OLD SYS) STILL WORKS NORMAL. I REALIZE THE EGPWS GETS ITS INFO FROM THE FMS AND IT CANNOT UPDATE ITSELF BUT THIS HAPPENS WHEN YOU NEED IT MOST AND THE MOST INOPPORTUNE TIME (IN THE MOUNTAINS). THIS SYS IS NOT WORKING. EVERY LEG INTO MEXICO A/PS (GDL/MTY/BJX/PBC) WE GOT IRS ONLY NAV WHICH CALLS FOR THE SYS TO BE OVERRIDDEN TO OFF. IS THAT NOT WHEN YOU NEED IT MOST AND BE DISTR THE LEAST. THERE NEEDS TO BE A BETTER WAY TO UPDATE THE FMS POS OR MAYBE THERE NEEDS TO BE SOME NAVAIDS ADDED AT THESE A/PS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR CLAIMS THAT VERY FEW PLTS ARE AWARE OF THIS LIMITATION OF THE ENHANCED GPWS. NOT ALL F100'S HAVE BEEN UPDATED WITH THE NEW ENHANCED GPWS. DURING APCHS INTO MEXICO; FO; THERE IS SO MUCH TERRAIN TO AVOID; THE ENHANCED GPWS LOOKED LIKE THE TOOL TO HAVE. HOWEVER; IT RELIES ON THE FMS TO BE FULLY UPDATED. AS THE ACFT DSNDS ON THE APCHS; THE RADIO SIGNALS FROM ILS LOCS; VORS; AND DME ARE WEAK OR NON EXISTENT. THESE SIGNALS ARE NECESSARY FOR THE ACCURATE UPDATE OF THE FMS. WITH FMS ACCURATELY UPDATED; THE ENHANCED GPWS WILL WORK PROPERLY. IF IT IS NOT UPDATED; A MESSAGE APPEARS INDICATING THE ACFT IS FLYING ACCORDING TO THE LAST KNOWN FMS POS. WITH INACCURATE POSITIONING; THE ENHANCED GPWS HAS TO BE SHUT DOWN. NOW THE POS OF ANY MOUNTAINS OR RAPIDLY CLBING TERRAIN CANNOT BE ESTABLISHED. THIS IS A LOSS OF SAFETY INFO THE FLC COULD USE. OF COURSE THE ACFT STILL HAS THE NORMAL GPWS; BUT THAT TELLS HOW HIGH THE MOUNTAIN TOPS ARE BENEATH THE ACFT. THE REAL LOSS IS THAT THERE IS NO WAY TO KNOW HOW HIGH THE MOUNTAINS ARE IN FRONT OF THE ACFT. THIS IS ESPECIALLY IMPORTANT AS MANY APCHS ARE AT NIGHTTIME IN IMC CONDITIONS. THE ENHANCED GPWS WORKS FINE IN THE UNITED STATES; SUCH AS PLACES LIKE SLC; UT; BECAUSE NAV UPDATING IS PLENTIFUL.