FLC OF B767-200 ENCOUNTERS MOUNTAIN WAVE AND IS UNABLE TO MAINTAIN ASSIGNED ALT.

Narrative

WHEN IN THE VICINITY OF TOBE VOR; WBOUND AT FL390; IN CLR SMOOTH AIR; THE ACFT ENCOUNTERED A LARGE STRONG MOUNTAIN WAVE ON LEE SIDE OF ROCKY MOUNTAINS. THERE WAS NO WARNING. THE AIRSPD INCREASED RAPIDLY ABOVE BARBER POLE AND ACFT CLBED ON AUTOPLT TO APPROX FL405. I DISCONNECTED AUTOTHROTTLES AND THEN THE AUTOPLT. I REDUCED THRUST AND LOWERED NOSE TO RETURN TO ASSIGNED ALT. WHILE THIS WAS HAPPENING ZAB CALLED ASKING ABOUT OUR ALT. WE INFORMED CTR THAT WE WERE IN A STRONG MOUNTAIN WAVE AND ATTEMPTING TO RETURN TO ASSIGNED ALT. THE CTR CTLR SAID THAT THERE WAS NO PROB. WHEN WE FLEW OUT OF THE UPWIND SIDE OF THE MOUNTAIN WAVE THE ALT AND AIRSPD DECREASED RAPIDLY. MAX CONTINUOUS PWR WAS APPLIED YET AIRSPD DECREASED TO CLEAN SPD PLUS 10 KTS AND ACFT DSNDED TO ABOUT FL382. WHEN AIRSPD STARTED INCREASING I SLOWLY EASED ACFT TO ASSIGNED ALT AND RE-ENGAGED AUTOPLT AND THROTTLES. I DON'T KNOW OF ANY WAY THIS OCCURRENCE COULD HAVE BEEN PREVENTED. DURING THE RECOVERY PHASE I CTLED THE ACFT ATTITUDE WHILE THE COPLT CTLED THE THRUST SETTING FOR ME WHICH WORKED WELL.