ACR WITH AUTOMATED COCKPIT ON AUTOPLT WITH PMS ENGAGED FOR DSCNT FINDS ACFT CLBING.

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ON THE ARR STAR TO SJC; THE CAPT HAD THE MLG ON AUTOPLT WITH PMS. FMA INDICATED PERF DSCNT (OR CR2) WE WERE LEVEL AT FL200 AND BOTH LOOKED AT OUR APCH CHART FOR SJC TO JUST 1-2 SECONDS AND NOTICED THE ACFT CLBING AT 2000 FPM. PWR WAS BEING ADVANCED BY THE AUTOTHROTTLES. THE FMA SAID PERF DSCNT. WHILE THE ACFT WAS CLBING; THE CAPT DISCONNECTED THE AUTOPLT AT 20700 FT AND WE RESUMED OUR PROFILE. WE ENTERED THIS AUTOPLT MALFUNCTION IN THE ACFT MAINT LOG. WE HAVE HEARD OF OTHER SIMILAR MLG AUTOPLT MALFUNCTIONS. MY RECOMMENDATION TO OTHER MLG CREWS IS TO NOT USE PMS ON DSCNT WITH THE AUTOPLT CONNECTED UNLESS THEY MONITOR THE ACFT 100 PERCENT OF THE TIME; ESPECIALLY WITH XING WAYPOINTS. THE PMS AND AUTOPLT SEEM TO DO FINE IN THE CLB AND LEVEL FLT AND 99.9 PERCENT OF THE TIME IN DSCNT BUT EVERY ONCE IN A WHILE IT WILL CAUSE THE ACFT TO DO SOMETHING WHICH IT IS NOT SUPPOSED TO DO. NOTE: WE HAD BEEN GIVEN A DISCRETIONARY DSCNT CLRNC TO FL200 WITH A FURTHER CLRNC LATER ISSUED TO 12000 FT. THE ALT ARMED IN THE ALT SELECT WAS 12000 'ALT' WAS ARMED IN THE FMA.