A320 FLT CREW ENCOUNTERS SEVERE MOUNTAIN WAVE DURING CRUISE FLT. SPD CTL SYS SENSED OVERSPD; DISCONNECTED AUTOPLT AND PITCHED NOSE UP TO PREVENT EXCESSIVE OVERSPD. ALT EXCURSION WAS SIGNIFICANT.

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OVER GREAT FALLS; MT; ACFT X EXPERIENCED A SEVERE 'MOUNTAIN WAVE' UPSET. THE ACFT WAS OPERATING AT FL390; MACH .78 (SELECTED) AT A GROSS WT OF 133800 LBS. AUTOPLT #2 (FO WAS PF) WAS ENGAGED AND AUTOTHRUST WAS OPERATIVE. THE RIDE WAS SMOOTH AND NO RPTS OF TURB OF 'MOUNTAIN WAVE' ACTIVITY HAD BEEN RPTED. THE INCIDENT WAS NOT SEVERE AS TO THE LEVEL OF TURB; BUT WAS SEVERE IN THE RAPID CHANGE IN AIRSPD AND ALT. AT THE FIRST FEELING OF THE 'MOUNTAIN WAVE' I SAW A LARGE UPTREND ARROW ON THE AIRSPD INDICATOR AND THE AIRSPD RISING RAPIDLY. I TOOK CTL OF THE ACFT AND PULLED THRUST LEVERS BACK AND DISENGAGED AUTOTHRUST. AS I WAS CONTEMPLATING THE USE OF SPD BRAKES (LESS THAN 2-3 SECONDS) THE ACFT ENCOUNTERED THE 'UPSIDE' OF THE 'WAVE' AND SIMULTANEOUSLY THE AUTOPLT DISENGAGED AND NOSE PITCHED UP RAPIDLY (AS IT IS PROGRAMMED TO) TO AVOID THE IMPENDING OVERSPD SIT. ONCE THE AIRSPD STARTED TO REDUCE; I WAS ABLE TO LOWER THE NOSE AND HAND FLY THE ACFT BACK DOWN TO FL370. THE FO NOTIFIED ATC OF OUR ALTDEV AND RECEIVED A NEW CLRNC FOR FL370. THE COMBINATION OF THE 'WAVE' AND AUTO PITCH UP CAUSED US TO CLB TO FL404; BUT THERE WAS NO OTHER TFC IN THE AREA. WE PASSED THE PIREP TO ATC AND CONFIRMED IT WAS THE FIRST RPT OF ANY PROBS IN THAT AREA. IN CONCLUSION; THE A320 FLT CTL SYS PERFORMED AS PROGRAMMED IN ORDER TO PREVENT THE IMPENDING OVERSPD; BUT IT WOULD HAVE BEEN A MUCH WORSE SIT IF THERE HAD BEEN ANY TFC ABOVE US AT THE TIME. I BELIEVE I WILL IMMEDIATELY COMBINE THRUST REDUCTION WITH USE OF SPD BRAKES NEXT TIME TO AVOID THE AUTOPLT PITCH-UP. ALTHOUGH THIS COULD RESULT IN LOW AIRSPD AT HIGH ALT; I FEEL THAT WOULD BE A BETTER SIT THAN RAPIDLY CHANGING ALT BY ALMOST 1500 FT WITH REDUCED VERT SEPARATION IN THE CONUS. I WILL ALSO GET CLRNC FOR OFFSET ROUTINE ANY TIME CONDITIONS ARE FAVORABLE FOR 'MOUNTAIN WAVE' SITS. SUPPLEMENTAL INFO FROM ACN 385169: NO INJURIES WERE RPTED FROM THE CABIN CREW OR PAX. CREW CONSULTED ACFT MANUALS/CHKLIST AND CONTINUED ON TO DEST.