CLOUD SEEDING ACFT DSNDS BELOW FLOOR OF BLOCK ALT DUE TO SEVERE DOWNDRAFT.

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THE DESCRIPTION OF EVENTS THAT WILL FOLLOW ARE NOT UNUSUAL EVENTS IN THE NORMAL OPS OF MAKING CLOUD PENETRATIONS FOR THE PURPOSES OF WX MODIFICATION. HOWEVER; IN THIS PARTICULAR CASE THE CTR CTLR ADMONISHED ME FOR NOT INFORMING HIM OF THE ALTDEV SOONER. WHILE MAKING A CLOUD PENETRATION ON AN EMBEDDED CELL; I FIRST ENCOUNTERED 4500 FPM UPDRAFTS FOLLOWED BY A SHEAR ZONE THEN A SUSTAINED 4000-5000 FPM DOWNDRAFT. I WAS ASSIGNED A BLOCK ALT OF 18000-22000 FT. I WAS AT ABOUT 19000 WHEN I FIRST ENCOUNTERED THE UPDRAFT AND ABOUT 21800 FT WHEN THE SHEAR AND DOWNDRAFT STARTED. THE TURB AND DOWN AIR FROM THE SHEAR CAUSED A RAPID DETERIORATION OF AIRSPD; AND MOMENTARY PWR LOSS OF BOTH ENGS DUE TO FUEL FLOW INTERRUPTION. IN ORDER TO AVOID A STALL IT REQUIRED A PRETTY EXTREME NOSE DOWN ATTITUDE; WHICH PROLONGED THE FUEL INTERRUPTION. HOWEVER THE R ENG RETURNED TO MAKING PWR AFTER ESTIMATED 10 SECONDS; BUT THE L ENG DID NOT RESPOND FOR APPROX ANOTHER 1 MIN AND 30 SECONDS; AT THE SAME TIME I WAS RAPIDLY APCHING THE BOTTOM OF THE BLOCK ALT; 18000 FT. ZKC WAS TALKING TO ANOTHER ACFT BUT AT ABOUT 18300 FT I TRIED TO SQUEEZE MY CALL IN BTWN THE CTR'S XMISSION AND ANOTHER ACFT'S RESPONSE. MY XMISSION WAS; 'ZKC NAVAJO IS GOING TO BE FORCED OUT OF THE BOTTOM OF THE BLOCK;' BUT I BELIEVE MY XMISSION WAS PROBABLY STEPPED ON; UNDERSTANDABLY; BY THE OTHER ACFT. WITH THE STRONG DOWNDRAFT; PROLONGED PWR LOSS ON THE L ENG AND THE AMOUNT OF ACCUMULATED AIRFRAME ICE FROM PREVIOUS 1 HR'S WORTH OF PENETRATIONS; BY THE TIME I GOT ANOTHER CALL INTO CTR I WAS GOING THROUGH 16000 FT. THE DOWNDRAFT WAS STARTING TO SUBSIDE AND THE L ENG WAS SLOWLY STARTING TO REGAIN PWR AND I WAS ABLE TO HOLD ALT AT ABOUT 15500 FT. ZKC'S RESPONSE WAS THAT I SHOULD HAVE NOTIFIED HIM SOONER. I HAVE NOTHING BUT PRAISE FOR ZKC AS I HAVE BEEN FLYING THESE FLTS FOR 5 SUMMER SEASONS; AND THEY ARE GENERALLY VERY HELPFUL AND UNDERSTANDING OF THE CONDITIONS THAT ARE BEING FLOWN IN AND HOW RAPIDLY THINGS CAN OCCUR. ALTDEVS LIKE THIS OCCUR ON OUR FLTS SEVERAL TIMES A SEASON; BUT IN THE 5 YRS OF DOING THIS THEY HAVE NEVER CAUSED A TFC SEPARATION PROB FOR CTR; NOR DID IT THIS TIME; THAT I HAVE BEEN MADE AWARE OF. IT IS AN EXTREMELY RARE CASE; THAT ONCE I HAVE GOTTEN TO THE STORM AREA TO BE WORKED; THAT THERE IS ANOTHER ACFT WITHIN MANY MI OF MY POS. AS TO HOW TO CORRECT THE PROB; MAYBE I SHOULD HAVE TRIED TO NOTIFY THEM SOONER; BUT THE FREQ WAS BUSY BEFORE MY FIRST CALL; AS WAS I. I WAS PROBABLY SATURATED AT LEAST INITIALLY WITH MY SIT. I ALSO KNOW FROM PAST EXPERIENCE CTR DOES NOT LIKE TO BE BOTHERED TOO FAR IN ADVANCE OF A POSSIBLE DEV OR ONE OF LESS THAN 300 FT. IT CAN BE VERY DIFFICULT TRYING TO GUESS IF AN UPDRAFT OR DOWNDRAFT IS GOING TO SUBSIDE IN TIME TO MAINTAIN THE BLOCK ALT OR TO CALL THEM IN ADVANCE OF EVERY TIME WE MIGHT DEV FROM THE BLOCK ALT WOULD FOR SURE ANNOY THEM.