Narrative:

The description of events that will follow are not unusual events in the normal operations of making cloud penetrations for the purposes of WX modification. However, in this particular case the center controller admonished me for not informing him of the altitude deviation 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 altitude 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 turbulence and down air from the shear caused a rapid deterioration of airspeed, and momentary power loss of both engines 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 right engine returned to making power after estimated 10 seconds, but the left engine did not respond for approximately another 1 min and 30 seconds, at the same time I was rapidly approaching the bottom of the block altitude, 18000 ft. ZKC was talking to another aircraft but at about 18300 ft I tried to squeeze my call in between the center's transmission and another aircraft's response. My transmission was, 'ZKC navajo is going to be forced out of the bottom of the block,' but I believe my transmission was probably stepped on, understandably, by the other aircraft. With the strong downdraft, prolonged power loss on the left engine and the amount of accumulated airframe ice from previous 1 hour's worth of penetrations, by the time I got another call into center I was going through 16000 ft. The downdraft was starting to subside and the left engine was slowly starting to regain power and I was able to hold altitude 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 traffic separation problem for center, 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 aircraft within many mi of my position. As to how to correct the problem, maybe I should have tried to notify them sooner, but the frequency was busy before my first call, as was I. I was probably saturated at least initially with my situation. I also know from past experience center does not like to be bothered too far in advance of a possible deviation 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 altitude or to call them in advance of every time we might deviation from the block altitude would for sure annoy them.

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Original NASA ASRS Text

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

Narrative: 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.

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.