FLC OF AN MD82 LOST CTL FOR A SHORT TIME AFTER ENCOUNTERING A SEVERE MOUNTAIN WAVE CONDITION AT CRUISE ALT. ATC WAS ADVISED AND A PLT ADVISORY WAS REQUESTED FOR OTHER PLTS ALONG THE RTE OF FLT. THE ACFT CLBED OUT OF CTL AT A RATE HIGHER THAN COULD BE RECORDED ON THE ACFT'S VERT SPD INDICATOR AND THEN QUICKLY DISSIPATED ALLOWING THE ACFT TO FALL THROUGH INTO A DIVE AND LOSE 5000 FT OF ALT.

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AN MD82; WITH A CREW OF 5 AND 44 PAX. I WAS THE COPLT ACTING AS THE PF FOR THE LEG; FLT ON APR/TUE/97. AT APPROX XX00Z; WHILE CRUISING AT MACH .78 AT FL370 WITH A LIGHT ACFT (APPROX 104000 LBS) WE ENCOUNTERED MOUNTAIN WAVE ACTIVITY APCHING RENO; NV. IT BEGAN AS 200-300 FPM VERT DEV ON THE IVSI AND QUICKLY PROGRESSED TO A SERIES OF 500 FPM FLUCTUATIONS; WHICH THE AUTOPLT WAS ABLE TO CTL. APPROX 15 MI SE OF MUSTANG ON J-5 THE WAVE ACTIVITY INCREASED TO 1000-1500 FPM FLUCTUATIONS; THE AUTOPLT WAS STRUGGLING TO MAINTAIN ALT. I ASKED THE CAPT TO GET A LOWER ALT IMMEDIATELY WHEN WE ENCOUNTERED THE MOST EXTREME WAVE ACTIVITY I HAVE SEEN IN 15 YRS OF FLYING JET ACFT. THE ACFT WAS PITCHED UP AND CLBED THROUGH OUR ASSIGNED ALT OF FL370 TO NEARLY FL390 IN A MATTER OF SECONDS. WITH THE AUTOPLT COMMANDING A 3000 FPM VERT DSCNT THE ACFT CONTINUED CLBING. I TOLD THE CAPT I NEEDED HELP WHILE HE CONVEYED OUR PREDICAMENT TO THE CTR CTLR. HE ASSUMED CTL OF THE ACFT; DISCONNECTED THE AUTOPLT AND USED SIGNIFICANT FORCE ON THE ELEVATOR AND MAX PWR TO CTL THE RAPIDLY INCREASING PITCH ATTITUDE. AS WE AGAIN ENTERED THE DSNDING AIRMASS WE RODE THE WAVE DOWN TO APPROX FL340 WHERE CTL WAS REGAINED AND WE LEVELED AT 33000 FT. THE CAPT OFFERED TO DECLARE AN EMER AFTER THE FACT TO ACCOUNT FOR THE ASSIGNED ALTDEVS; HOWEVER; THE ATC SPECIALIST INDICATED THAT WOULD NOT BE NECESSARY CONSIDERING THE FACT THAT WE DID NOT COMPROMISE ANY OTHER ACFT AIRSPACE. LIGHT WAVE ACTIVITY CONTINUED ALONG THE SIERRAS AND THE LNDG AT ONTARIO; CA; WAS UNEVENTFUL. THERE WERE NO INJURIES AND NO ACFT DAMAGE. SUPPLEMENTAL INFO FROM ACN 366794: THE CTLR WAS VERY HELPFUL AND CALM. THERE WAS NO CONFLICTING TFC IN THE AREA. I ASKED CTR TO ISSUE AN ADVISORY FOR THE AREA AND THEY DID. THE ADVISORY WAS FOR SEVERE TURB AT 37000 FT. THERE WERE NOT MANY HUMAN FACTORS EVENTS IN THIS OCCURRENCE. THIS WAS JUST A CHANCE ENCOUNTER WITH A VERY UNUSUAL WX EVENT. THE PERFORMANCE FACTORS BY THE CREW WERE ALL INSTINCTIVE AND PERFORMED UNDER EXTREMELY STRESSFUL CONDITIONS.