AN ACR LGT FO RPT ON ENCOUNTERING MOUNTAIN WAVE CLR AIR TURB ON DSCNT INTO SLC. THE ACFT ALMOST WENT INTO OVERSPD ON THE BARBER POLE.

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ENRTE DSCNT TO FINAL DEST; CTR DIRECTED DSCNT TO FL310. STARTED VNAV (1000 FPM) DSCNT FROM FL390. CTR THEN REQUESTED EXPEDITE THROUGH FL330. CAPT REDUCED THROTTLES TO INCREASE DSCNT RATE WITH VNAV. CRUISE MACH WAS APPROX .81. DURING DSCNT; EXPERIENCED MODERATE MOUNTAIN WAVE WHICH INCREASED IAS TO BARBER POLE (MAX IAS POINTER). CAPT AT THIS POINT DISCONNECTED THE AUTOFLT SYS. THE AIRPLANE'S PITCH DECREASED AND WE ENCOUNTERED SOME POSITIVE G'S (APPROX 1.5 - 2G) AFTER AUTOFLT DISCONNECT. I FELT THAT THE MOUNTAIN WAVE EFFECT; ALONG WITH SOME CHOPPY AIR; AGGRAVATED THE SIT FOLLOWING THE CAPT'S DECISION TO DISCONNECT THE AUTOPLT AND DECREASE THE PITCH ATTITUDE IN ORDER TO SLOW BELOW THE MAX IAS POINTER. THE AIRSPD EXCEEDANCE CLACKER NEVER SOUNDED AND THERE WAS ABSOLUTELY NO DISCOMFORT THROUGHOUT THE MANEUVER. HOWEVER; THE FLT ATTENDANTS IN THE TAIL SECTION WERE AFFECTED BY THE POSITIVE G'S AND SAID THEY WERE SITTING ON THE FLOOR AFTER THIS OCCURRENCE. MOUNTAIN WAVE OCCURRENCES ARE SUBTLE AND OCCUR OVER THE WESTERN UNITED STATES WITH GREAT FREQUENCY. THE VNAV SPD SELECTED WITH AUTOFLT IS UNABLE TO KEEP UP WITH THIS SEVERE MOUNTAIN WAVE. MAYBE A FASTER COMPUTER WITH BETTER TREND UPDATES COULD HOLD THE SELECTED CRUISE SPDS BETTER.