Narrative:

While in cruise flight at FL390 crossing the rocky mountains near four corners, we experienced a sudden mountain wave with a resulting deviation from altitude of plus 300 ft followed by a minus 700 ft from cruise FL390. The flight had been smooth and steady with occasional light turbulence. Both pilots were at their flight stations, aircraft was on autoplt in VMC. The aircraft suddenly climbed 300 ft and was heading higher when the captain stopped the climb with the pitch trim on the autoplt. The aircraft then suddenly began losing altitude and was not responding quickly enough to up pitch commands through the autoplt, so the captain disengaged the autoplt and finally stopped the descent at about 700 ft below cruise level FL390. We immediately climbed back to FL390. We never saw any possible traffic conflicts on our TCASII nor did the controller ever mention any. In fact the controller never even questioned us about altitude. It was a brief wave and the flight then continued normally. In retrospect we probably should have expected the wave even though the flight had been fairly smooth. There was an extreme low (winter storm) over the midwest (northeast, ks, sd) moving rapidly eastward, accompanied by strong low level winds. In the future we would also probably disengage the autoplt sooner.

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Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.