AN AIRBUS A319 INTERCEPTING THE GS FROM BELOW L ASSIGNED ALT CLBING 1000 FT WHEN AUTOFLT WAS ARMED FOR THE APCH.

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BEING VECTORED TO FINAL APCH; RWY 12L MSP AT 4000 FT. RECEIVED A FINAL VECTOR TO INTERCEPT RWY 12L LOC AND MAINTAIN 4000 FT UNTIL ESTABLISHED; THEN CLRED FOR THE ILS RWY 12L MSP. CAPT ENGAGED THE ILS GUIDANCE PORTION OF THE ILS BY ARMING THE APCH WITH THE APCH BUTTON. THE ENGS IMMEDIATELY SPOOLED UP AND CLBED TO 5000 FT BEFORE HE COULD TURN OFF THE AUTOPLT AND MANUALLY MANEUVER BACK TO 4000 FT. WE ADVISED ATC THAT THE ACFT HAD MOMENTARILY CLBED TO 5000 FT. APCH CTLR SAID 'NO PROB.' WE DEBRIEFED AFTERWARDS AND CONCLUDED THAT THE FGS WAS TRYING TO CAPTURE THE GS FROM BELOW BY FLYING UP TO IT TO INTERCEPT. WE ARE FURTHER INVESTIGATING TO FIND OUT WHY IT DID THIS. THIS IS NOT NORMAL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR INDICATED THAT THIS IS A KNOWN PROB WITH THE SOFTWARE; AND THAT HIS COMPANY HAS ISSUED A BULLETIN TO CORRECT WITH A NEW PROC. THE PROC RECOMMENDS THAT THE APCH NOT BE ARMED UNTIL INSIDE 18 MI FROM THE RWY. HIS COMPANY IS WORKING WITH THE MANUFACTURER; IN THE MEANTIME; TO SEE IF THE SOFTWARE CAN BE CORRECTED.