Narrative:

While cruising at FL330 with the autoplt engaged in FMS, the vertical speed indicator began oscillating rapidly between 2000 FPM rates of climb and descent. Within a second or two the aircraft rapidly descended 400 ft, then climbed 800 ft before descending another 1800 ft. During this encounter, which only lasted between 10-20 seconds, the autoplt limit flag flickered on and off before the autoplt disengaged. As the aircraft bottomed out at FL316 with the first officer flying, I notified ZME that we had encountered severe turbulence and lost 1400 ft in altitude. Memphis advised us to descend to FL290. At that point the aircraft began climbing rapidly in moderate chop. When I could talk, I advised memphis that we were 'trying.' the first officer managed to level the aircraft at FL322 and began a descent to FL290. Light to moderate turbulence continued at FL290 until we broke out of the clouds about 2 mins later. During the entire brief encounter, the aircraft pitch, roll and airspeed did not change abruptly. The ride was more like being on an express elevator in a tall building. The climbs and dscnts were flat in pitch except that there was a significant amount of chop which made it difficult to communicate with ATC or the other crew members. Fortuitously, knowledge and technological advancements made over the last 20 yrs by ATC and the airline paid off. I had reviewed the radar summary on the computer before preflight. Then I briefed the customer service coordinator that the last hour of the 2 hour 50 min flight might be a little rough and I wanted her to conduct the cabin service as quickly as possible so that she could get the serving carts stowed and the flight attendants seated for the last hour. As we entered the clouds west of memphis, I turned the seat belt light on and asked the so to make sure the flight attendants were seated. They were. The radar was on and I scanned the 180 mi range -- there were no returns. I then selected the 80 NM range and lowered the antenna to paint ground return between 70-80 NM -- still no WX returns -- before or after the event. Shortly after stabilizing at FL290, I asked the so to make sure everybody was ok. There were no injuries. Callback conversation with reporter revealed the following information: the reporting captain is not sure if the first officer disengaged the autoplt or if it disengaged itself. The autoplt limit flag was flashing on/off as fast as it could and the vsi was going between 2000 FPM up to 2000 FPM down as fast as it could go. The air carrier's flight manual calls for the autoplt to be turned off and the aircraft hand flown in vertical windshears such as this to prevent the autoplt trim from getting out of phase with the windshears. The only warning of this situation that the crew had was from the preflight WX briefing. The reporter has confidence that the WX radar was working properly as it tested properly on the ground and painted showers later in the flight.

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Original NASA ASRS Text

Title: AN ACR L1011 CREW EXPERIENCED UNUSUALLY RAPID CLBS AND DIVES IN TURB. THIS TURB OVERPOWERED THE AUTOPLT. WX FACTORS VERT WINDSHEARS OF UNUSUALLY STRONG BUT SMOOTH NATURE THAT CAUSED SEVERAL ALTDEVS.

Narrative: WHILE CRUISING AT FL330 WITH THE AUTOPLT ENGAGED IN FMS, THE VERT SPD INDICATOR BEGAN OSCILLATING RAPIDLY BTWN 2000 FPM RATES OF CLB AND DSCNT. WITHIN A SECOND OR TWO THE ACFT RAPIDLY DSNDED 400 FT, THEN CLBED 800 FT BEFORE DSNDING ANOTHER 1800 FT. DURING THIS ENCOUNTER, WHICH ONLY LASTED BTWN 10-20 SECONDS, THE AUTOPLT LIMIT FLAG FLICKERED ON AND OFF BEFORE THE AUTOPLT DISENGAGED. AS THE ACFT BOTTOMED OUT AT FL316 WITH THE FO FLYING, I NOTIFIED ZME THAT WE HAD ENCOUNTERED SEVERE TURB AND LOST 1400 FT IN ALT. MEMPHIS ADVISED US TO DSND TO FL290. AT THAT POINT THE ACFT BEGAN CLBING RAPIDLY IN MODERATE CHOP. WHEN I COULD TALK, I ADVISED MEMPHIS THAT WE WERE 'TRYING.' THE FO MANAGED TO LEVEL THE ACFT AT FL322 AND BEGAN A DSCNT TO FL290. LIGHT TO MODERATE TURB CONTINUED AT FL290 UNTIL WE BROKE OUT OF THE CLOUDS ABOUT 2 MINS LATER. DURING THE ENTIRE BRIEF ENCOUNTER, THE ACFT PITCH, ROLL AND AIRSPD DID NOT CHANGE ABRUPTLY. THE RIDE WAS MORE LIKE BEING ON AN EXPRESS ELEVATOR IN A TALL BUILDING. THE CLBS AND DSCNTS WERE FLAT IN PITCH EXCEPT THAT THERE WAS A SIGNIFICANT AMOUNT OF CHOP WHICH MADE IT DIFFICULT TO COMMUNICATE WITH ATC OR THE OTHER CREW MEMBERS. FORTUITOUSLY, KNOWLEDGE AND TECHNOLOGICAL ADVANCEMENTS MADE OVER THE LAST 20 YRS BY ATC AND THE AIRLINE PAID OFF. I HAD REVIEWED THE RADAR SUMMARY ON THE COMPUTER BEFORE PREFLT. THEN I BRIEFED THE CUSTOMER SVC COORDINATOR THAT THE LAST HR OF THE 2 HR 50 MIN FLT MIGHT BE A LITTLE ROUGH AND I WANTED HER TO CONDUCT THE CABIN SVC AS QUICKLY AS POSSIBLE SO THAT SHE COULD GET THE SERVING CARTS STOWED AND THE FLT ATTENDANTS SEATED FOR THE LAST HR. AS WE ENTERED THE CLOUDS W OF MEMPHIS, I TURNED THE SEAT BELT LIGHT ON AND ASKED THE SO TO MAKE SURE THE FLT ATTENDANTS WERE SEATED. THEY WERE. THE RADAR WAS ON AND I SCANNED THE 180 MI RANGE -- THERE WERE NO RETURNS. I THEN SELECTED THE 80 NM RANGE AND LOWERED THE ANTENNA TO PAINT GND RETURN BTWN 70-80 NM -- STILL NO WX RETURNS -- BEFORE OR AFTER THE EVENT. SHORTLY AFTER STABILIZING AT FL290, I ASKED THE SO TO MAKE SURE EVERYBODY WAS OK. THERE WERE NO INJURIES. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTING CAPT IS NOT SURE IF THE FO DISENGAGED THE AUTOPLT OR IF IT DISENGAGED ITSELF. THE AUTOPLT LIMIT FLAG WAS FLASHING ON/OFF AS FAST AS IT COULD AND THE VSI WAS GOING BTWN 2000 FPM UP TO 2000 FPM DOWN AS FAST AS IT COULD GO. THE ACR'S FLT MANUAL CALLS FOR THE AUTOPLT TO BE TURNED OFF AND THE ACFT HAND FLOWN IN VERT WINDSHEARS SUCH AS THIS TO PREVENT THE AUTOPLT TRIM FROM GETTING OUT OF PHASE WITH THE WINDSHEARS. THE ONLY WARNING OF THIS SIT THAT THE CREW HAD WAS FROM THE PREFLT WX BRIEFING. THE RPTR HAS CONFIDENCE THAT THE WX RADAR WAS WORKING PROPERLY AS IT TESTED PROPERLY ON THE GND AND PAINTED SHOWERS LATER IN THE FLT.

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.