AN ACR B737-200 STRUCK THE WAKE OF AN L1011 AT APPROX 17000 FT OVER MOD WHILE ON THE MOD 2 STAR. THE B737-200 ROLLED UP TO 20 DEGS L AND R IN SPITE OF AILERON INPUTS BY THE CAPT TO COUNTER THE ROLLS.

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APCHING MOD ON THE MODESTO 2 ARR TO SFO; WE WERE SLOWED TO 250 KTS AND GIVEN A 'CROSS CEDES AT 11000 FT.' WE WERE 10 MI BEHIND A HVY L1011 AND ENCOUNTERED HIS WAKE; CAUSING BANK ANGLES OF 20 DEGS IN EITHER DIRECTION. NEEDLESS TO SAY; I WAS A LITTLE GUN-SHY FROM THAT POINT ON REGARDING WAKE TURB. BAY APCH SLOWED US TO 210 KTS AND DSNDED US; SAYING; 'YOU WILL HAVE PARALLEL TFC ON THE L; A B757; I'LL POINT HIM OUT WHEN HE GETS CLOSER.' I COULD SEE THAT THE WX WAS NOT APPROPRIATE FOR PARALLEL VISUALS; AND I ADVISED THAT WE WERE IMC. APCH THEN GAVE US A HDG OF 280 DEGS TO PROVIDE SEPARATION (I HAD A TARGET ON TCASII THAT APPEARED TO BE 3 MI ABEAM). WE WERE SLOWED TO 180 KTS AND GIVEN A DSCNT TO 2000 FT AND DIRECTED TO 'RPT THE ARPT AND THE TFC IN SIGHT.' SINCE WE WERE STILL IN AND OUT OF CLOUDS; I SAID; 'WE'RE IMC.' APCH THEN GAVE US A HDG OF 260 DEGS TOWARD THE FIELD AND THE UNSEEN TFC AND SAID; 'THE B757 HAS YOU IN SIGHT AND WILL MAINTAIN VISUAL SEPARATION.' WE BROKE OUT AND CALLED THE FIELD; BUT DID NOT CALL THE B757 (WE WERE ON AN INTERCEPT HDG). WE WERE GOING TO BE WINGTIP-TO-WINGTIP OR BEHIND THE B757 AND DOWNWIND OF HIM. IF IT WEREN'T FOR THE SICK PAX; I WOULD HAVE GONE AROUND; BUT INSTEAD; AFTER SWITCHING TO TWR; I ACCELERATED TO 200 KTS TO GET IN FRONT AND THREW EVERYTHING OUT WHILE TURNING; JUST BARELY BEING STABILIZED AT 500 FT. THIS WAS YET ANOTHER CASE OF 'FORCING VISUALS.' THE WX WAS MORE APPROPRIATE FOR THE LDA-DME RWY 28R WITH THE B757 ON THE BAY ILS; BUT THE 4 MI STAGGER WAS NOT APPLIED. APCH WAS ATTEMPTING A JOIN- UP; WITHOUT REGARD FOR WAKE TURB CONSIDERATIONS. THIS IS OBVIOUSLY NOT IN THE BEST INTERESTS OF SAFETY. I AM ALSO CURIOUS HOW THE B757 COULD HAVE ME IN SIGHT WHEN I WAS IN THE CLOUDS (APCH WAS USING 2 DIFFERENT FREQS; SO I COULDN'T HEAR THE B757). THIS WAS A GOOD EXAMPLE OF THE PROBS INHERENT WITH SIMULTANEOUS OFFSET INST APCHS (SOIA) WHERE SEPARATION OF ACFT AND WAKE TURB CRITERIA ARE COMPROMISED FOR CAPACITY ENHANCEMENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THIS RPTR SAID THAT HE WAS AWARE OF PRECEDING TFC BECAUSE OF ITS RADIO XMISSIONS; BUT HE DID NOT KNOW WHAT TYPE IT WAS UNTIL AFTER HE HIT ITS WAKE. HE WAS SURPRISED BY ITS EFFECT. THE B737-200 WAS ROLLED BOTH L AND R TO ABOUT 20 DEGS IN SPITE OF HIS FULL AILERON INPUT THROUGH THE AUTOPLT'S CTL WHEEL STEERING MODE. AFTER THIS HE INQUIRED ABOUT THE TFC AND THEN TOLD THE ARTCC RADAR CTLR ABOUT THE ENCOUNTER.