AN ACR A-320 OVERSHOT ITS ALT AT A XING RESTRICTION.

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ATTEMPTING TO MEET THE ATC XING RESTRICTION; WE USED SPD BRAKES TO INCREASE THE DSCNT RATE. THIS CAUSED SUCH A HIGH RATE OF DSCNT THAT THE AUTOPLT DID NOT ENGAGE IN ALT HOLD. WE DSNDED APPROX 400 FT BELOW OUR ASSIGNED ALT BEFORE MANUALLY CLBING BACK TO OUR ALT. THE A-320 SOFTWARE DOES NOT ALLOW XING ALONG RTES OR TRACKS. A NEW WAYPOINT MUST BE CREATED ALONG THE TRACK IN ORDER TO USE THE VNAV FUNCTION. ATC OFTEN GIVES 'CROSS 30 NM E OF FIX XYZ'; AS A XING CLRNC. THIS IS VERY CUMBERSOME AND POTENTIALLY MISTAKE RIDDEN IN AN A-320. THE VNAV FUNCTION CAUSES SUCH A TIGHT XING THAT THE LAST 4 OR 5 NM'S ARE OFTEN FLOWN USING SPD BRAKES. THIS CAUSES A HIGH RATE OF DSCNT TO MEET THE XING RESTRICTION. THE AUTOPLT DOES NOT CAPTURE AN ALT AT HIGH RATES OF VERT VELOCITY. THIS IS A POTENTIAL ALT BUST AND TCASII RA. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTING CAPT IS NEW TO THE A-320 WHICH IS NEW TO HIS ACR. THIS IS HIS FIRST 'GLASS COCKPIT' ACFT AND ADMITS THAT HE IS LOW ON THE LEARNING CURVE. HE IS AWARE THAT THERE ARE DIFFERENCES IN THE VNAV FEATURES OF OTHER ACFT AND BEMOANS THAT HIS DOES NOT HAVE AN 'ALONG TRACK XING' FEATURE. HIS ACR IS TRYING TO GET THIS FEATURE ADDED TO THEIR ACFT TO HELP PREVENT THIS SIT. IN THE MEANTIME; HE WILL BE A LITTLE BIT MORE ATTENTIVE AND TRY TO PLAN BETTER. HE ALSO IS DISAPPOINTED THAT THE ALT CAPTURE FEATURE LET HIM GO THROUGH HIS SELECTED ALT; BUT ADMITS THAT HE WAS DSNDING AT ABOUT 8000 FPM; A REAL CHALLENGE FOR A DUMB MACHINE. RECOVERY WAS SO FAST THAT APPARENTLY ARTCC PROBABLY DID NOT NOTICE.