Narrative:

Best reported altitude according to ATC and PIREPS reported from dispatch was FL330 with light chop. We were at FL330 and about 20 mins west of ALS when ATC told us that there had been PIREPS isolated in the vicinity of ALS of light chop to occasional moderate turbulence with some mountain wave -- again FL330 best reported ride with quite a bit of traffic on the route and not all flts were encountering it. I immediately turned on the seatbelt sign and we adjusted our airspeed to mach .78, which was best turbulence speed, as a precaution. Approaching ALS, there was no change in our ride of occasional light chop. There were no clouds and autothrottles and autoplt were on and working normally. I had looked away from the instrument panel for no more than 20 seconds and in that time the airspeed had gone from mach .78 to mach .86, which was barber pole. The autothrottles were responding by retarding the throttles but not fast enough to keep the overspd from going off, so we pulled them manually all the way to idle. Even then it was about another 10 seconds before the airspeed started to decrease. The autoplt was doing a good job of maintaining our altitude. As the airspeed was slowing through mach .84, I was concerned about it dropping off as drastically as it had increased when we would cross the other side of the mountain wave, so I told the first officer (who was flying) to push up the throttles just out of idle so that the engines would be able to respond more quickly if we suddenly needed the power. Just at about that time we encountered severe turbulence characterized by rapid, violent jolts, with some rolling motion. There was not much in the way of up- and downdrafts associated with the turbulence, which I believe is why none of the flight attendants were injured. They were the only ones not seated at the time. All of the passenger were strapped in -- most had been asleep. As soon as we hit the turbulence, we immediately notified ATC and got a descent to FL290 to try to escape it and after about 20 seconds we were out of it -- back in just occasional light chop. A check of the flight attendants and cabin revealed no injuries, and all the aircraft system and operations appeared normal. I called dispatch right away with our PIREP and made a write-up in the aircraft log regarding the severe turbulence encounter.

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Original NASA ASRS Text

Title: B757 CREW ENCOUNTERS TURB W OF ALS.

Narrative: BEST RPTED ALT ACCORDING TO ATC AND PIREPS RPTED FROM DISPATCH WAS FL330 WITH LIGHT CHOP. WE WERE AT FL330 AND ABOUT 20 MINS W OF ALS WHEN ATC TOLD US THAT THERE HAD BEEN PIREPS ISOLATED IN THE VICINITY OF ALS OF LIGHT CHOP TO OCCASIONAL MODERATE TURB WITH SOME MOUNTAIN WAVE -- AGAIN FL330 BEST RPTED RIDE WITH QUITE A BIT OF TFC ON THE RTE AND NOT ALL FLTS WERE ENCOUNTERING IT. I IMMEDIATELY TURNED ON THE SEATBELT SIGN AND WE ADJUSTED OUR AIRSPD TO MACH .78, WHICH WAS BEST TURB SPD, AS A PRECAUTION. APCHING ALS, THERE WAS NO CHANGE IN OUR RIDE OF OCCASIONAL LIGHT CHOP. THERE WERE NO CLOUDS AND AUTOTHROTTLES AND AUTOPLT WERE ON AND WORKING NORMALLY. I HAD LOOKED AWAY FROM THE INST PANEL FOR NO MORE THAN 20 SECONDS AND IN THAT TIME THE AIRSPD HAD GONE FROM MACH .78 TO MACH .86, WHICH WAS BARBER POLE. THE AUTOTHROTTLES WERE RESPONDING BY RETARDING THE THROTTLES BUT NOT FAST ENOUGH TO KEEP THE OVERSPD FROM GOING OFF, SO WE PULLED THEM MANUALLY ALL THE WAY TO IDLE. EVEN THEN IT WAS ABOUT ANOTHER 10 SECONDS BEFORE THE AIRSPD STARTED TO DECREASE. THE AUTOPLT WAS DOING A GOOD JOB OF MAINTAINING OUR ALT. AS THE AIRSPD WAS SLOWING THROUGH MACH .84, I WAS CONCERNED ABOUT IT DROPPING OFF AS DRASTICALLY AS IT HAD INCREASED WHEN WE WOULD CROSS THE OTHER SIDE OF THE MOUNTAIN WAVE, SO I TOLD THE FO (WHO WAS FLYING) TO PUSH UP THE THROTTLES JUST OUT OF IDLE SO THAT THE ENGS WOULD BE ABLE TO RESPOND MORE QUICKLY IF WE SUDDENLY NEEDED THE PWR. JUST AT ABOUT THAT TIME WE ENCOUNTERED SEVERE TURB CHARACTERIZED BY RAPID, VIOLENT JOLTS, WITH SOME ROLLING MOTION. THERE WAS NOT MUCH IN THE WAY OF UP- AND DOWNDRAFTS ASSOCIATED WITH THE TURB, WHICH I BELIEVE IS WHY NONE OF THE FLT ATTENDANTS WERE INJURED. THEY WERE THE ONLY ONES NOT SEATED AT THE TIME. ALL OF THE PAX WERE STRAPPED IN -- MOST HAD BEEN ASLEEP. AS SOON AS WE HIT THE TURB, WE IMMEDIATELY NOTIFIED ATC AND GOT A DSCNT TO FL290 TO TRY TO ESCAPE IT AND AFTER ABOUT 20 SECONDS WE WERE OUT OF IT -- BACK IN JUST OCCASIONAL LIGHT CHOP. A CHK OF THE FLT ATTENDANTS AND CABIN REVEALED NO INJURIES, AND ALL THE ACFT SYS AND OPS APPEARED NORMAL. I CALLED DISPATCH RIGHT AWAY WITH OUR PIREP AND MADE A WRITE-UP IN THE ACFT LOG REGARDING THE SEVERE TURB ENCOUNTER.

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.