COMMERCIAL PLT OF SMA ACFT ON XCOUNTRY ENCOUNTERED SEVERE UP AND DOWNDRAFTS TO THE EXTENT THAT HE LOST THE ABILITY TO MAINTAIN ASSIGNED ALT.

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ON DEP FROM WASHINGTON NATIONAL; I ENCOUNTERED RATHER VIGOROUS UPDRAFTS AND DOWNDRAFTS. BEGAN AT 4000 FT; THEN WAS CLRED TO 6000; THEN GIVEN A BLOCK OF AIRSPACE FROM 6 TO 7. AS WE APCHED EACH MAJOR RIDGE; WE ENCOUNTERED ONE OF THESE DOWNDRAFTS. I IMMEDIATELY ESTABLISHED A FULL RPM AND FULL PWR CLB ATTITUDE AS WE LOST 1500 FT OF ALT EACH TIME. THIS LASTED ABOUT 2 MINS. THEN WE ENCOUNTERED THE UPDRAFT AS WE APCHED THE TOP OF THE RIDGE. I THEN REDUCED PWR TO 15 INCHES AND FOR THE NEXT 1 MIN WE CLBED IN A NOSE DOWN ATTITUDE FROM 6500 FT TO 8000 FT. AS WE APCHED 8000 FT I FURTHER REDUCED PWR TO IDLE TO MAINTAIN MANEUVERING SPD IN THE NOSE DOWN ATTITUDE. THIS 'ROUTINE' CONTINUED FOR ABOUT A DISTANCE OF 40-50 MI AND GRADUALLY DIMINISHED AS WE 'EASED' OUT OF THE MOUNTAINOUS TERRAIN. I CONTACTED DULLES APCH AND CTR A NUMBER OF TIMES TO ADVISE THAT WE WERE UNABLE TO MAINTAIN OUR ASSIGNED ALT. BEING A LITTLE CONCERNED THAT I HAD 'VIOLATED' THE ASSIGNED ALT CLRNC SO MANY TIMES; I THOUGHT IT PRUDENT TO FILE THIS NASA FORM. GND SPD INITIALLY WAS 47 KTS DUE TO HEADWINDS OF 50 KTS OR MORE. EVERY 30 MINS OR SO WE SHOWED AN INCREASED GND SPD OF ABOUT 10 KTS SO THAT BY THE TIME WE REACHED THE IL BORDER; OUR GND SPD INDICATED ABOUT 100 KTS! WE FLEW IN MOSTLY VFR CONDITIONS BELOW THE BASES OF THE BROKEN TO OVCST UNTIL PASSING BEYOND THE MOUNTAINS; AND THEN ON TOP MOST OF THE TIME BACK TO NEW LENOX; IL. MOTHER NATURE WAS DETERMINED TO BE THE CAUSE OF OUR PROBLEM. TO PREVENT A RECURRENCE; I SUGGEST ASSIGNING THE CAUSE OF HIGH WINDS TO SOMEONE OTHER THAN MOTHER NATURE.