ALTDEV ON STAR DSCNT.

Narrative

WE HAD BEEN GIVEN A XING RESTR ON THE ARR INTO LGA BY ZNY. THE RESTR WAS TO CROSS SOMTO INTXN AT 13000 FT. I (BEING THE PNF) HAD GOTTEN THE LCL ATIS WHICH STATED LDA 'A' RWY 22. THE CAPT WAS RELATIVELY NEW (6 MONTHS) ON THE AIRPLANE AND WAS INTERESTED IN THE PROPER WAY TO ENTER THIS RATHER RARELY USED TYPE OF APCH INTO THE MCDU COMPUTER. A 'TRAP' IN THE A-320 IS THAT WHEN AN APCH OR RWY IS ENTERED INTO THE DEST PAGE; IT KNOCKS OUT VERT NAV CONSTRAINTS ENTERED BY THE PLT. THIS IS WHAT HAPPENED. HE HAD PROPERLY ENTERED 13000 FT AT SOMTO INTXN IN THE VERT NAV PORTION OF THE COMPUTER. WHEN I ENTERED THE LNDG RWY IT KNOCKED OUT THE VERT NAV CONSTRAINT AT SOMTO INTXN. HE WAS BUSY WATCHING ME 'BUILD' THE APCH FOR RWY 22. I DIDN'T NOTICE THE CONSTRAINT WAS NO LONGER IN VERT NAV (IT WILL REMAIN UNDER CERTAIN CIRCUMSTANCES). WHILE WE WERE BOTH INVOLVED IN THIS TASK; WE PASSED SOMTO INTXN AT ABOUT 15500 FT. A FEW MI PAST; ATC ASKED OUR ALT. AS I STATED OUR ALT (15100 FT) I REALIZED WE HAD MISSED OUR XING RESTR. I IMMEDIATELY ASKED ATC IF IT WAS A PROB; THAT WE HAD HAD A COMPUTER GLITCH AND HAD MISSED SOMTO INTXN AT 13000 FT. HE SAID IT WAS NO PROB AND TO JUST CONTINUE TO 13000 FT. I FEEL 2 FACTORS CONTRIBUTED TO THIS INCIDENT. ONE IS OBVIOUSLY THE SOFTWARE IN THE PRESENT MCDU COMPUTER ON THE A-320; TO KNOCK OUT THE VERT XING CONSTRAINT AT TIMES. THE OTHER IS THAT THERE WAS A TIME WHEN BOTH OF US WERE TOO INVOLVED INSIDE THE COCKPIT AND NOT MANAGING THE AIRPLANE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR FLIES THE A- 320 DOMESTICALLY FOR ONE OF THE MAJOR UNITED STATES ACRS. SHE HAS NOT HEARD FROM THE FAA ON THIS INCIDENT AND WAS APPROPRIATELY CONTRITE WHEN CAUGHT MISSING THE XING. SHE ENJOYS ALL OF THE 'GEE WHIZ' PARTS OF THE A-320; BUT IS CONCERNED ABOUT HER FLYING SKILLS. SHE MANAGED TO GET HER CAPT CHKED OUT DURING THE MONTH.