Narrative:

We were on course on the localizer for the ILS runway 35C at dfw. While being vectored for the approach, ATC advised that we were 6 NM in trail of a heavy B747 (air carrier cargo jet). Our TCASII display confirmed the stated separation. Passing the rayma 14.9 DME fix and having been cleared for the approach, we began to descend from 5000 ft to 4000 ft as published. At about 4800 ft, airspeed 180 KTS, autoplt engaged and coupled to the localizer in 'level change' mode. The airplane was suddenly and violently rolled into approximately 30 degrees of left bank. As I was disconnecting the autoplt and autothrottles, and applying back pressure to leveloff and roll wings level, we were just as suddenly and violently rolled into a 20-25 degree right bank. I recovered the airplane to wings level, level flight and resumed a manual ILS approach, keeping the flight path a 'DOT' high above the GS. The approach was uneventful from that point on. A normal, except for being intentionally longer, landing was made. Since we were in IMC conditions, we carefully monitored our separation distance from the B747 on our TCASII. It didn't appear we ever got any closer to it than the 6 mi established by ATC. No passenger or crew were injured as, fortunately, final cabin preparations had just been completed and everyone was seated. However, the event was violent and dramatic enough to unsettle both the crew and passenger. Callback conversation with reporter revealed the following information: the reporter said that the controller told them everything that was required about the preceding traffic and they saw the B747F on their TCASII display at 6 NM and below them. He said that the flight crew felt comfortable about the situation. The onset of the turbulence was a surprise. The initial roll to the left was very abrupt incurring some lateral g-forces that the reporter could not quantify. He said that the next roll was just as abrupt, but now the flight crew was ready for it and it only rolled to 20 or 25 degrees versus the 30 degree initial roll. After the second roll the flight crew found themselves in smooth air once again and they stayed high on the GS for the remainder of the approach. The reporter said that he was a former T38 instructor pilot and he had read about the wake turbulence study in 'callback' and that prompted his report.

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Original NASA ASRS Text

Title: AN ACR B737-200 FLC HAS A WAKE ENCOUNTER BEHIND A B747F AND ROLL BOTH TO THE L AND TO THE R BEFORE ESCAPING THE TURB. NO INJURIES.

Narrative: WE WERE ON COURSE ON THE LOC FOR THE ILS RWY 35C AT DFW. WHILE BEING VECTORED FOR THE APCH, ATC ADVISED THAT WE WERE 6 NM IN TRAIL OF A HVY B747 (ACR CARGO JET). OUR TCASII DISPLAY CONFIRMED THE STATED SEPARATION. PASSING THE RAYMA 14.9 DME FIX AND HAVING BEEN CLRED FOR THE APCH, WE BEGAN TO DSND FROM 5000 FT TO 4000 FT AS PUBLISHED. AT ABOUT 4800 FT, AIRSPD 180 KTS, AUTOPLT ENGAGED AND COUPLED TO THE LOC IN 'LEVEL CHANGE' MODE. THE AIRPLANE WAS SUDDENLY AND VIOLENTLY ROLLED INTO APPROX 30 DEGS OF L BANK. AS I WAS DISCONNECTING THE AUTOPLT AND AUTOTHROTTLES, AND APPLYING BACK PRESSURE TO LEVELOFF AND ROLL WINGS LEVEL, WE WERE JUST AS SUDDENLY AND VIOLENTLY ROLLED INTO A 20-25 DEG R BANK. I RECOVERED THE AIRPLANE TO WINGS LEVEL, LEVEL FLT AND RESUMED A MANUAL ILS APCH, KEEPING THE FLT PATH A 'DOT' HIGH ABOVE THE GS. THE APCH WAS UNEVENTFUL FROM THAT POINT ON. A NORMAL, EXCEPT FOR BEING INTENTIONALLY LONGER, LNDG WAS MADE. SINCE WE WERE IN IMC CONDITIONS, WE CAREFULLY MONITORED OUR SEPARATION DISTANCE FROM THE B747 ON OUR TCASII. IT DIDN'T APPEAR WE EVER GOT ANY CLOSER TO IT THAN THE 6 MI ESTABLISHED BY ATC. NO PAX OR CREW WERE INJURED AS, FORTUNATELY, FINAL CABIN PREPARATIONS HAD JUST BEEN COMPLETED AND EVERYONE WAS SEATED. HOWEVER, THE EVENT WAS VIOLENT AND DRAMATIC ENOUGH TO UNSETTLE BOTH THE CREW AND PAX. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR SAID THAT THE CTLR TOLD THEM EVERYTHING THAT WAS REQUIRED ABOUT THE PRECEDING TFC AND THEY SAW THE B747F ON THEIR TCASII DISPLAY AT 6 NM AND BELOW THEM. HE SAID THAT THE FLC FELT COMFORTABLE ABOUT THE SIT. THE ONSET OF THE TURB WAS A SURPRISE. THE INITIAL ROLL TO THE L WAS VERY ABRUPT INCURRING SOME LATERAL G-FORCES THAT THE RPTR COULD NOT QUANTIFY. HE SAID THAT THE NEXT ROLL WAS JUST AS ABRUPT, BUT NOW THE FLC WAS READY FOR IT AND IT ONLY ROLLED TO 20 OR 25 DEGS VERSUS THE 30 DEG INITIAL ROLL. AFTER THE SECOND ROLL THE FLC FOUND THEMSELVES IN SMOOTH AIR ONCE AGAIN AND THEY STAYED HIGH ON THE GS FOR THE REMAINDER OF THE APCH. THE RPTR SAID THAT HE WAS A FORMER T38 INSTRUCTOR PLT AND HE HAD READ ABOUT THE WAKE TURB STUDY IN 'CALLBACK' AND THAT PROMPTED HIS RPT.

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.