Narrative:

This report is submitted as an altitude deviation due to a severe turbulence encounter. At FL390 and approximately 130 mi away from a WX system; radar (and visual) indicated 3 cells; 2 bunched together (right side) and a third (left side) approximately 30-40 NM from the other two. I felt confident I could transit the gap safely in visual conditions on top and elected to proceed through the gap. I instructed both the passenger and flight attendants to take their seats well in advance. Also; we got minor deviations from mci center if needed. Radar (0-1 degree) indicated little to no activity in this gap. Mins passed and as we transited the gap; I visually noticed the clouds were higher than previously thought and realized it was closing and rising. I would penetrate some clouds very close off the nose that looked 'hard.' I was now not sure how many mi I would have to go through (radar still not painting much) and realized I would not stay above so I turned to the left initially thinking I would do a 180 degree turn and call center (emergency if necessary). We started to encounter moderate turbulence at this time. After approximately 30-40 degrees of turn; I realized I would be in a worse situation as we would be too close to the isolated cell abeam and now behind to my left (still visual); so I turned back right to original course and it was at this time we encountered severe turbulence and the beginning of a 1500 ft loss of altitude. Beginning with the first turn; there really was no adequate time to inform ATC. When clear we informed center of both our altitude deviation and severe turbulence encounter. An aircraft behind me was told to turn and not follow my course. There were no separation issues. This event's duration lasted no more than 1-1/2 mins and we were back in the clear shortly thereafter. Communication with the flight attendants was established; there were no injuries to either them or passenger as all were in seats. It would have been best to avoid system altogether; but after making my decision I wish I had stayed on course to minimize time in turbulence. I am glad that we had all crew and passenger in seats as a precaution.

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Original NASA ASRS Text

Title: B767-400 FLT CREW ENCOUNTERS SEVERE TURB WHILE TRANSITING BTW WX CELLS.

Narrative: THIS RPT IS SUBMITTED AS AN ALTDEV DUE TO A SEVERE TURB ENCOUNTER. AT FL390 AND APPROX 130 MI AWAY FROM A WX SYS; RADAR (AND VISUAL) INDICATED 3 CELLS; 2 BUNCHED TOGETHER (R SIDE) AND A THIRD (L SIDE) APPROX 30-40 NM FROM THE OTHER TWO. I FELT CONFIDENT I COULD TRANSIT THE GAP SAFELY IN VISUAL CONDITIONS ON TOP AND ELECTED TO PROCEED THROUGH THE GAP. I INSTRUCTED BOTH THE PAX AND FLT ATTENDANTS TO TAKE THEIR SEATS WELL IN ADVANCE. ALSO; WE GOT MINOR DEVS FROM MCI CTR IF NEEDED. RADAR (0-1 DEG) INDICATED LITTLE TO NO ACTIVITY IN THIS GAP. MINS PASSED AND AS WE TRANSITED THE GAP; I VISUALLY NOTICED THE CLOUDS WERE HIGHER THAN PREVIOUSLY THOUGHT AND REALIZED IT WAS CLOSING AND RISING. I WOULD PENETRATE SOME CLOUDS VERY CLOSE OFF THE NOSE THAT LOOKED 'HARD.' I WAS NOW NOT SURE HOW MANY MI I WOULD HAVE TO GO THROUGH (RADAR STILL NOT PAINTING MUCH) AND REALIZED I WOULD NOT STAY ABOVE SO I TURNED TO THE L INITIALLY THINKING I WOULD DO A 180 DEG TURN AND CALL CTR (EMER IF NECESSARY). WE STARTED TO ENCOUNTER MODERATE TURB AT THIS TIME. AFTER APPROX 30-40 DEGS OF TURN; I REALIZED I WOULD BE IN A WORSE SITUATION AS WE WOULD BE TOO CLOSE TO THE ISOLATED CELL ABEAM AND NOW BEHIND TO MY L (STILL VISUAL); SO I TURNED BACK R TO ORIGINAL COURSE AND IT WAS AT THIS TIME WE ENCOUNTERED SEVERE TURB AND THE BEGINNING OF A 1500 FT LOSS OF ALT. BEGINNING WITH THE FIRST TURN; THERE REALLY WAS NO ADEQUATE TIME TO INFORM ATC. WHEN CLR WE INFORMED CTR OF BOTH OUR ALTDEV AND SEVERE TURB ENCOUNTER. AN ACFT BEHIND ME WAS TOLD TO TURN AND NOT FOLLOW MY COURSE. THERE WERE NO SEPARATION ISSUES. THIS EVENT'S DURATION LASTED NO MORE THAN 1-1/2 MINS AND WE WERE BACK IN THE CLR SHORTLY THEREAFTER. COM WITH THE FLT ATTENDANTS WAS ESTABLISHED; THERE WERE NO INJURIES TO EITHER THEM OR PAX AS ALL WERE IN SEATS. IT WOULD HAVE BEEN BEST TO AVOID SYS ALTOGETHER; BUT AFTER MAKING MY DECISION I WISH I HAD STAYED ON COURSE TO MINIMIZE TIME IN TURB. I AM GLAD THAT WE HAD ALL CREW AND PAX IN SEATS AS A PRECAUTION.

Data retrieved from NASA's ASRS site as of January 2009 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.