Narrative:

Flight encountered severe turbulence just prior to akero intersection on route A461. Cruising at FL390 this turbulence caused excessive downdraft and the aircraft to lose about 19000 ft before recovery could be established. The aircraft was at FL390, mach .83, autoplt B engaged, altitude hold and INS mode selected. WX radar showed no significant activities directly ahead. Aircraft was in and out of thin layer of high cirrus prior to encounter. Approximately 30 mi prior to akero at FL390 encountered light to moderate turbulence. Radar showed some returns 30 degrees left, 50 mi ahead and another 100 mi straight ahead. The turbulence quickly progressed from moderate to severe. Aircraft pitched slightly up then excessively down to about 15 degrees entering uncontrollable rapid descent of more than 6000 FPM vsi. The sudden pitch down caused the aircraft's manuals, loose papers and loose articles to experience zero G's. These paper and articles in the cockpit made some instrumentation readings near impossible. The autoplt was disengaged and thrust levers, which were already being reduced to maintain mach .82 were retarded to idle immediately. In an attempt to keep from overstressing the aircraft, altitude was traded for smooth recovery at FL200. The aircraft quickly regained altitude to FL310 where we notified hong kong control of our encounter with severe turbulence with location and our intention of maintaining present flight level which was approved. The entire episode took place within a time frame of less than 2 mins from FL390 down to FL200 and then back to FL310. During the whole event, IAS limitation was never exceeded. This severe turbulence encounter was reported to vice president of operation upon landing. A company incident report was generated in addition. The encounter was also entered into the aircraft's log. I believe we encountered what is known as a jet upset situation. We could only recover from, not avoid, this encounter. The crew handled the event in a professional manner, each focusing on his responsibilities. We were lucky to have god as our passenger that night.

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Original NASA ASRS Text

Title: B747 FREIGHTER EXPERIENCES A 'JET UPSET' AFTER ENCOUNTERING SEVERE TURB 200 MI SE OF HONG KONG. ACFT WAS OUT OF CTL FROM FL390 TO FL200. FO STATES THAT THE ACFT NEVER EXCEEDED IAS LIMITATIONS.

Narrative: FLT ENCOUNTERED SEVERE TURB JUST PRIOR TO AKERO INTXN ON RTE A461. CRUISING AT FL390 THIS TURB CAUSED EXCESSIVE DOWNDRAFT AND THE ACFT TO LOSE ABOUT 19000 FT BEFORE RECOVERY COULD BE ESTABLISHED. THE ACFT WAS AT FL390, MACH .83, AUTOPLT B ENGAGED, ALT HOLD AND INS MODE SELECTED. WX RADAR SHOWED NO SIGNIFICANT ACTIVITIES DIRECTLY AHEAD. ACFT WAS IN AND OUT OF THIN LAYER OF HIGH CIRRUS PRIOR TO ENCOUNTER. APPROX 30 MI PRIOR TO AKERO AT FL390 ENCOUNTERED LIGHT TO MODERATE TURB. RADAR SHOWED SOME RETURNS 30 DEGS L, 50 MI AHEAD AND ANOTHER 100 MI STRAIGHT AHEAD. THE TURB QUICKLY PROGRESSED FROM MODERATE TO SEVERE. ACFT PITCHED SLIGHTLY UP THEN EXCESSIVELY DOWN TO ABOUT 15 DEGS ENTERING UNCTLABLE RAPID DSCNT OF MORE THAN 6000 FPM VSI. THE SUDDEN PITCH DOWN CAUSED THE ACFT'S MANUALS, LOOSE PAPERS AND LOOSE ARTICLES TO EXPERIENCE ZERO G'S. THESE PAPER AND ARTICLES IN THE COCKPIT MADE SOME INSTRUMENTATION READINGS NEAR IMPOSSIBLE. THE AUTOPLT WAS DISENGAGED AND THRUST LEVERS, WHICH WERE ALREADY BEING REDUCED TO MAINTAIN MACH .82 WERE RETARDED TO IDLE IMMEDIATELY. IN AN ATTEMPT TO KEEP FROM OVERSTRESSING THE ACFT, ALT WAS TRADED FOR SMOOTH RECOVERY AT FL200. THE ACFT QUICKLY REGAINED ALT TO FL310 WHERE WE NOTIFIED HONG KONG CTL OF OUR ENCOUNTER WITH SEVERE TURB WITH LOCATION AND OUR INTENTION OF MAINTAINING PRESENT FLT LEVEL WHICH WAS APPROVED. THE ENTIRE EPISODE TOOK PLACE WITHIN A TIME FRAME OF LESS THAN 2 MINS FROM FL390 DOWN TO FL200 AND THEN BACK TO FL310. DURING THE WHOLE EVENT, IAS LIMITATION WAS NEVER EXCEEDED. THIS SEVERE TURB ENCOUNTER WAS RPTED TO VICE PRESIDENT OF OP UPON LNDG. A COMPANY INCIDENT RPT WAS GENERATED IN ADDITION. THE ENCOUNTER WAS ALSO ENTERED INTO THE ACFT'S LOG. I BELIEVE WE ENCOUNTERED WHAT IS KNOWN AS A JET UPSET SIT. WE COULD ONLY RECOVER FROM, NOT AVOID, THIS ENCOUNTER. THE CREW HANDLED THE EVENT IN A PROFESSIONAL MANNER, EACH FOCUSING ON HIS RESPONSIBILITIES. WE WERE LUCKY TO HAVE GOD AS OUR PAX THAT NIGHT.

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.