THE FLC OF A CORPORATE JET EXPERIENCED MOUNTAIN WAVE ACTIVITY WHILE ENRTE OVER THE ROCKIES. NATIONAL WX CONDITIONS WERE CONDUCIVE TO THE STRONG LOW LEVEL WINDS THAT WERE ENCOUNTERED BY THE FLT. WHEN THE AUTOPLT COMMANDS WERE UNSUCCESSFUL; THE PF REVERTED TO MANUAL FLT.

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WHILE IN CRUISE FLT AT FL390 XING THE ROCKY MOUNTAINS NEAR FOUR CORNERS; WE EXPERIENCED A SUDDEN MOUNTAIN WAVE WITH A RESULTING DEV FROM ALT OF PLUS 300 FT FOLLOWED BY A MINUS 700 FT FROM CRUISE FL390. THE FLT HAD BEEN SMOOTH AND STEADY WITH OCCASIONAL LIGHT TURB. BOTH PLTS WERE AT THEIR FLT STATIONS; ACFT WAS ON AUTOPLT IN VMC. THE ACFT SUDDENLY CLBED 300 FT AND WAS HEADING HIGHER WHEN THE CAPT STOPPED THE CLB WITH THE PITCH TRIM ON THE AUTOPLT. THE ACFT THEN SUDDENLY BEGAN LOSING ALT AND WAS NOT RESPONDING QUICKLY ENOUGH TO UP PITCH COMMANDS THROUGH THE AUTOPLT; SO THE CAPT DISENGAGED THE AUTOPLT AND FINALLY STOPPED THE DSCNT AT ABOUT 700 FT BELOW CRUISE LEVEL FL390. WE IMMEDIATELY CLBED BACK TO FL390. WE NEVER SAW ANY POSSIBLE TFC CONFLICTS ON OUR TCASII NOR DID THE CTLR EVER MENTION ANY. IN FACT THE CTLR NEVER EVEN QUESTIONED US ABOUT ALT. IT WAS A BRIEF WAVE AND THE FLT THEN CONTINUED NORMALLY. IN RETROSPECT WE PROBABLY SHOULD HAVE EXPECTED THE WAVE EVEN THOUGH THE FLT HAD BEEN FAIRLY SMOOTH. THERE WAS AN EXTREME LOW (WINTER STORM) OVER THE MIDWEST (NE; KS; SD) MOVING RAPIDLY EASTWARD; ACCOMPANIED BY STRONG LOW LEVEL WINDS. IN THE FUTURE WE WOULD ALSO PROBABLY DISENGAGE THE AUTOPLT SOONER.