A320 FLT CREW NOTES FMS EXHIBITS PROBLEMS WITH 'TERRAIN' FEATURE; CLAIMING THAT TERRAIN MAPPING DISAPPEARS WITHOUT EXPLANATION.

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THIS IS MORE OF A GENERAL SAFETY CONCERN THAT I HAVE ABOUT THE A320 EGPWS. ON ACFT THAT HAVE THE TERRAIN SWITCH BELOW THE PFD; THE TERRAIN MAPPING TURNS ITSELF OFF AT VARIOUS TIMES. USUALLY IT TURNS OFF IMMEDIATELY AFTER TAKEOFF AND SOMETIMES WHILE ON APCH. ON TODAY'S APCH INTO LAS; I NOTICED THAT THE TERRAIN MAPPING WAS OFF AFTER LNDG. THIS RAISES A SAFETY CONCERN BECAUSE THERE IS NO INDICATION THAT THIS HAS HAPPENED OTHER THAN THE (VERY DIM DURING THE DAY) TERRAIN ANNUNCIATION ON THE ND EXTINGUISHING. I HAVE NOTICED THIS SYSTEM WEAKNESS SINCE JOINING THE FLEET TWO YEARS AGO AND HAVE BEEN TOLD BY LCA'S AND PI'S THAT 'THAT'S JUST THE WAY IT IS.' THIS IS UNACCEPTABLE AND UNSAFE. LET'S GET THIS FIXED.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT THIS ANOMALY ONLY OCCURS ON ACFT WITH THE TERRAIN ON/OFF SWITCH LOCATED BELOW THE PFD FOR EACH PLT. SOME ACFT HAVE A DIFFERENT TYPE SWITCH (WITH A POSITIVE CLICK WHEN ACTIVATED) IN A DIFFERENT LOCATION; AND THIS FEATURE OPERATES PROPERLY ON THESE ACFT. THE DISAPPEARANCE OF TERRAIN MAPPING IS SPORADIC; BUT HAPPENS MORE FREQUENTLY DURING TKOF OR APCH.